Adding upstream version 2.4.1.upstream/2.4.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

13 files changed, 16130 insertions, 0 deletions

diff --git a/src/hooks/dhcp/high_availability/tests/Makefile.am b/src/hooks/dhcp/high_availability/tests/Makefile.am
new file mode 100644
index 0000000..f99f8eb
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/Makefile.am
@@ -0,0 +1,70 @@
+SUBDIRS = .
+
+AM_CPPFLAGS = -I$(top_builddir)/src/lib -I$(top_srcdir)/src/lib
+AM_CPPFLAGS += -I$(top_builddir)/src/hooks/dhcp/high_availability -I$(top_srcdir)/src/hooks/dhcp/high_availability
+AM_CPPFLAGS += $(BOOST_INCLUDES) $(CRYPTO_CFLAGS) $(CRYPTO_INCLUDES)
+AM_CPPFLAGS += -DLIBDHCP_HA_SO=\"$(abs_top_builddir)/src/hooks/dhcp/high_availability/.libs/libdhcp_ha.so\"
+AM_CPPFLAGS += -DINSTALL_PROG=\"$(abs_top_srcdir)/install-sh\"
+TEST_CA_DIR = $(abs_top_srcdir)/src/lib/asiolink/testutils/ca
+AM_CPPFLAGS += -DTEST_CA_DIR=\"$(TEST_CA_DIR)\"
+TEST_HTTP_DIR = $(abs_top_srcdir)/src/lib/http/tests/testdata
+AM_CPPFLAGS += -DTEST_HTTP_DIR=\"$(TEST_HTTP_DIR)\"
+
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+
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+
+TESTS =
+if HAVE_GTEST
+TESTS += ha_unittests
+
+ha_unittests_SOURCES  = command_creator_unittest.cc
+ha_unittests_SOURCES += communication_state_unittest.cc
+ha_unittests_SOURCES += ha_config_unittest.cc
+ha_unittests_SOURCES += ha_impl_unittest.cc
+ha_unittests_SOURCES += ha_service_unittest.cc
+ha_unittests_SOURCES += ha_test.cc ha_test.h
+ha_unittests_SOURCES += ha_mt_unittest.cc
+ha_unittests_SOURCES += lease_update_backlog_unittest.cc
+ha_unittests_SOURCES += query_filter_unittest.cc
+ha_unittests_SOURCES += run_unittests.cc
+
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+
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+
+ha_unittests_CXXFLAGS = $(AM_CXXFLAGS)
+
+ha_unittests_LDADD  = $(top_builddir)/src/hooks/dhcp/high_availability/libha.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/dhcpsrv/libkea-dhcpsrv.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/process/libkea-process.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/eval/libkea-eval.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/dhcp_ddns/libkea-dhcp_ddns.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/stats/libkea-stats.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/config/libkea-cfgclient.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/http/libkea-http.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/dhcp/libkea-dhcp++.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/hooks/libkea-hooks.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/database/libkea-database.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/cc/libkea-cc.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/asiolink/libkea-asiolink.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/dns/libkea-dns++.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/cryptolink/libkea-cryptolink.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/log/libkea-log.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/util/libkea-util.la
+ha_unittests_LDADD += $(top_builddir)/src/lib/exceptions/libkea-exceptions.la
+ha_unittests_LDADD += $(LOG4CPLUS_LIBS)
+ha_unittests_LDADD += $(CRYPTO_LIBS)
+ha_unittests_LDADD += $(BOOST_LIBS)
+ha_unittests_LDADD += $(GTEST_LDADD)
+endif
+noinst_PROGRAMS = $(TESTS)
diff --git a/src/hooks/dhcp/high_availability/tests/Makefile.in b/src/hooks/dhcp/high_availability/tests/Makefile.in
new file mode 100644
index 0000000..a346891
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/Makefile.in
@@ -0,0 +1,1217 @@
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diff --git a/src/hooks/dhcp/high_availability/tests/command_creator_unittest.cc b/src/hooks/dhcp/high_availability/tests/command_creator_unittest.cc
new file mode 100644
index 0000000..8966b4c
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/command_creator_unittest.cc
@@ -0,0 +1,530 @@
+// Copyright (C) 2018-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <ha_server_type.h>
+#include <lease_update_backlog.h>
+#include <command_creator.h>
+#include <asiolink/io_address.h>
+#include <cc/data.h>
+#include <exceptions/exceptions.h>
+#include <dhcp/hwaddr.h>
+#include <dhcpsrv/lease.h>
+#include <boost/pointer_cast.hpp>
+#include <gtest/gtest.h>
+#include <vector>
+
+using namespace isc;
+using namespace isc::asiolink;
+using namespace isc::data;
+using namespace isc::dhcp;
+using namespace isc::ha;
+
+namespace {
+
+/// @brief Creates lease instance used in tests.
+///
+/// It creates a lease with an address of 192.1.2.3 and for the HW address
+/// of 0b:0b:0b:0b:0b:0b.
+///
+/// @return Pointer to the created lease.
+Lease4Ptr createLease4() {
+    HWAddrPtr hwaddr(new HWAddr(std::vector<uint8_t>(6, 11), HTYPE_ETHER));
+    Lease4Ptr lease4(new Lease4(IOAddress("192.1.2.3"), hwaddr,
+                                static_cast<const uint8_t*>(0), 0,
+                                60, 0, 1));
+    return (lease4);
+}
+
+/// @brief Creates IPv6 lease instance used in tests.
+///
+/// It creates a lease with an address of 2001:db8:1::cafe and for the
+/// DUID of 02:02:02:02:02:02:02:02.
+///
+/// @return Pointer to the created lease.
+Lease6Ptr createLease6() {
+    DuidPtr duid(new DUID(std::vector<uint8_t>(8, 02)));
+    Lease6Ptr lease6(new Lease6(Lease::TYPE_NA, IOAddress("2001:db8:1::cafe"),
+                                duid, 1234, 50, 60, 1));
+    return (lease6);
+}
+
+/// @brief Returns JSON representation of the lease.
+///
+/// @param lease_ptr Pointer to the lease.
+/// @return Pointer to the JSON representation of the lease.
+ElementPtr leaseAsJson(const LeasePtr& lease_ptr) {
+    ElementPtr lease = boost::const_pointer_cast<Element>(lease_ptr->toElement());
+    // Replace cltt with expiration time as this is what the lease_cmds hooks
+    // library expects.
+    int64_t cltt = lease->get("cltt")->intValue();
+    int64_t valid_lifetime = lease->get("valid-lft")->intValue();
+    int64_t expire = cltt + valid_lifetime;
+    lease->set("expire", Element::create(expire));
+    lease->remove("cltt");
+    return (lease);
+}
+
+/// @brief Performs basic checks on the command.
+///
+/// It tests whether the command name is correct, if it contains arguments
+/// and if the arguments are held in a map.
+///
+/// @param command Pointer to the command to be tested.
+/// @param expected_command Expected command name.
+/// @param expected_service Expected service name.
+/// @param [out] arguments Pointer to the arguments map extracted from the
+/// provided command.
+void
+testCommandBasics(const ConstElementPtr& command,
+                  const std::string& expected_command,
+                  const std::string& expected_service,
+                  ConstElementPtr& arguments) {
+    ASSERT_TRUE(command);
+    ASSERT_EQ(Element::map, command->getType());
+    ConstElementPtr command_name = command->get("command");
+    ASSERT_TRUE(command_name);
+
+    // Make sure the command is a string.
+    ASSERT_EQ(Element::string, command_name->getType());
+    // Verify the command name.
+    ASSERT_EQ(expected_command, command_name->stringValue());
+
+    // Make sure that the service is present and includes a single
+    // entry.
+    ConstElementPtr service = command->get("service");
+    ASSERT_TRUE(service);
+    ASSERT_EQ(Element::list, service->getType());
+    ASSERT_EQ(1, service->size());
+    ASSERT_EQ(Element::string, service->get(0)->getType());
+    EXPECT_EQ(expected_service, service->get(0)->stringValue());
+
+    // Make sure that arguments are present.
+    ConstElementPtr command_arguments = command->get("arguments");
+    ASSERT_TRUE(command_arguments);
+    // Make sure that arguments are held in a map.
+    ASSERT_EQ(Element::map, command_arguments->getType());
+
+    // Return extracted arguments.
+    arguments = command_arguments;
+}
+
+/// @brief Performs basic checks on the command.
+///
+/// This variant of the function expects no arguments to be provided.
+///
+/// @param command Pointer to the command to be tested.
+/// @param expected_command Expected command name.
+/// @param expected_service Expected service name.
+void
+testCommandBasics(const ConstElementPtr& command,
+                  const std::string& expected_command,
+                  const std::string& expected_service) {
+    ASSERT_TRUE(command);
+    ASSERT_EQ(Element::map, command->getType());
+    ConstElementPtr command_name = command->get("command");
+    ASSERT_TRUE(command_name);
+
+    // Make sure the command is a string.
+    ASSERT_EQ(Element::string, command_name->getType());
+    // Verify the command name.
+    ASSERT_EQ(expected_command, command_name->stringValue());
+
+    // Make sure that the service is present and includes a single
+    // entry.
+    ConstElementPtr service = command->get("service");
+    ASSERT_TRUE(service);
+    ASSERT_EQ(Element::list, service->getType());
+    ASSERT_EQ(1, service->size());
+    ASSERT_EQ(Element::string, service->get(0)->getType());
+    EXPECT_EQ(expected_service, service->get(0)->stringValue());
+
+    ConstElementPtr command_arguments = command->get("arguments");
+    ASSERT_FALSE(command_arguments);
+}
+
+// This test verifies that the dhcp-disable command is correct.
+TEST(CommandCreatorTest, createDHCPDisable4) {
+    // Create command with max-period value set to 20.
+    ConstElementPtr command = CommandCreator::createDHCPDisable(20, HAServerType::DHCPv4);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "dhcp-disable", "dhcp4",
+                                              arguments));
+    ASSERT_EQ(2, arguments->size());
+    ConstElementPtr max_period = arguments->get("max-period");
+    ASSERT_TRUE(max_period);
+    ASSERT_EQ(Element::integer, max_period->getType());
+    EXPECT_EQ(20, max_period->intValue());
+    ConstElementPtr origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+
+    // Repeat the test but this time the max-period is not specified.
+    command = CommandCreator::createDHCPDisable(0, HAServerType::DHCPv4);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "dhcp-disable", "dhcp4",
+                                              arguments));
+    ASSERT_EQ(1, arguments->size());
+    origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+}
+
+// This test verifies that the dhcp-enable command is correct.
+TEST(CommandCreatorTest, createDHCPEnable4) {
+    ConstElementPtr arguments;
+    ConstElementPtr command = CommandCreator::createDHCPEnable(HAServerType::DHCPv4);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "dhcp-enable", "dhcp4",
+                                              arguments));
+    ASSERT_EQ(1, arguments->size());
+    ConstElementPtr origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+}
+
+// This test verifies that the ha-reset command sent to DHCPv4 server is correct.
+TEST(CommandCreatorTest, createHAReset4) {
+    ConstElementPtr command = CommandCreator::createHAReset(HAServerType::DHCPv4);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-reset", "dhcp4"));
+}
+
+// This test verifies that the ha-heartbeat command is correct.
+TEST(CommandCreatorTest, createHeartbeat4) {
+    ConstElementPtr command = CommandCreator::createHeartbeat(HAServerType::DHCPv4);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-heartbeat", "dhcp4"));
+}
+
+// This test verifies that the command generated for the lease update
+// is correct.
+TEST(CommandCreatorTest, createLease4Update) {
+    ConstElementPtr command = CommandCreator::createLease4Update(*createLease4());
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease4-update", "dhcp4",
+                                              arguments));
+    ElementPtr lease_as_json = leaseAsJson(createLease4());
+    // The lease update must contain the "force-create" parameter indicating that
+    // the lease must be created if it doesn't exist.
+    lease_as_json->set("force-create", Element::create(true));
+    lease_as_json->set("origin", Element::create("ha-partner"));
+    EXPECT_EQ(lease_as_json->str(), arguments->str());
+}
+
+// This test verifies that the command generated for the lease deletion
+// is correct.
+TEST(CommandCreatorTest, createLease4Delete) {
+    ConstElementPtr command = CommandCreator::createLease4Delete(*createLease4());
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease4-del", "dhcp4",
+                                              arguments));
+    ElementPtr lease_as_json = leaseAsJson(createLease4());
+    lease_as_json->set("origin", Element::create("ha-partner"));
+    EXPECT_EQ(lease_as_json->str(), arguments->str());
+}
+
+// This test verifies that the lease4-get-all command is correct.
+TEST(CommandCreatorTest, createLease4GetAll) {
+    ConstElementPtr command = CommandCreator::createLease4GetAll();
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease4-get-all", "dhcp4"));
+}
+
+// This test verifies that the lease4-get-page command is correct when
+// first page is fetched.
+TEST(CommandCreatorTest, createLease4GetPageStart) {
+    Lease4Ptr lease4;
+    ConstElementPtr command = CommandCreator::createLease4GetPage(lease4, 10);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease4-get-page", "dhcp4",
+                                              arguments));
+
+    ConstElementPtr from = arguments->get("from");
+    ASSERT_TRUE(from);
+    EXPECT_EQ(Element::string, from->getType());
+    EXPECT_EQ("start", from->stringValue());
+
+    ConstElementPtr limit = arguments->get("limit");
+    ASSERT_TRUE(limit);
+    ASSERT_EQ(Element::integer, limit->getType());
+    EXPECT_EQ(10, limit->intValue());
+}
+
+// This test verifies that the lease4-get-page command is correct when next
+// page is fetched.
+TEST(CommandCreatorTest, createLease4GetPageAddress) {
+    Lease4Ptr lease4(new Lease4());
+    lease4->addr_ = IOAddress("1.2.3.4");
+
+    ConstElementPtr command = CommandCreator::createLease4GetPage(lease4, 15);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease4-get-page", "dhcp4",
+                                              arguments));
+
+    ConstElementPtr from = arguments->get("from");
+    ASSERT_TRUE(from);
+    EXPECT_EQ(Element::string, from->getType());
+    EXPECT_EQ("1.2.3.4", from->stringValue());
+
+    ConstElementPtr limit = arguments->get("limit");
+    ASSERT_TRUE(limit);
+    ASSERT_EQ(Element::integer, limit->getType());
+    EXPECT_EQ(15, limit->intValue());
+}
+
+// This test verifies that exception is thrown if limit is set to 0 while
+// creating lease4-get-page command.
+TEST(CommandCreatorTest, createLease4GetPageZeroLimit) {
+    Lease4Ptr lease4;
+    EXPECT_THROW(CommandCreator::createLease4GetPage(lease4, 0), BadValue);
+}
+
+// This test verifies that the dhcp-disable command (DHCPv6 case) is
+// correct.
+TEST(CommandCreatorTest, createDHCPDisable6) {
+    // Create command with max-period value set to 20.
+    ConstElementPtr command = CommandCreator::createDHCPDisable(20, HAServerType::DHCPv6);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "dhcp-disable", "dhcp6",
+                                              arguments));
+    ASSERT_EQ(2, arguments->size());
+    ConstElementPtr max_period = arguments->get("max-period");
+    ASSERT_TRUE(max_period);
+    ASSERT_EQ(Element::integer, max_period->getType());
+    EXPECT_EQ(20, max_period->intValue());
+    ConstElementPtr origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+
+    // Repeat the test but this time the max-period is not specified.
+    command = CommandCreator::createDHCPDisable(0, HAServerType::DHCPv6);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "dhcp-disable", "dhcp6",
+                                              arguments));
+    ASSERT_EQ(1, arguments->size());
+    origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+}
+
+// This test verifies that the dhcp-enable command (DHCPv6 case) is
+// correct.
+TEST(CommandCreatorTest, createDHCPEnable6) {
+    ConstElementPtr arguments;
+    ConstElementPtr command = CommandCreator::createDHCPEnable(HAServerType::DHCPv6);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "dhcp-enable", "dhcp6",
+                                              arguments));
+    ASSERT_EQ(1, arguments->size());
+    ConstElementPtr origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+}
+
+// This test verifies that the ha-reset command sent to DHCPv6 server is correct.
+TEST(CommandCreatorTest, createHAReset6) {
+    ConstElementPtr command = CommandCreator::createHAReset(HAServerType::DHCPv6);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-reset", "dhcp6"));
+}
+
+// This test verifies that the command generated for the lease update
+// is correct.
+TEST(CommandCreatorTest, createLease6Update) {
+    ConstElementPtr command = CommandCreator::createLease6Update(*createLease6());
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-update", "dhcp6",
+                                              arguments));
+    ElementPtr lease_as_json = leaseAsJson(createLease6());
+    // The lease update must contain the "force-create" parameter indicating that
+    // the lease must be created if it doesn't exist.
+    lease_as_json->set("force-create", Element::create(true));
+    lease_as_json->set("origin", Element::create("ha-partner"));
+    EXPECT_EQ(lease_as_json->str(), arguments->str());
+}
+
+// This test verifies that the command generated for the lease deletion
+// is correct.
+TEST(CommandCreatorTest, createLease6Delete) {
+    ConstElementPtr command = CommandCreator::createLease6Delete(*createLease6());
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-del", "dhcp6",
+                                              arguments));
+    ElementPtr lease_as_json = leaseAsJson(createLease6());
+    lease_as_json->set("origin", Element::create("ha-partner"));
+    EXPECT_EQ(lease_as_json->str(), arguments->str());
+}
+
+// This test verifies that the lease6-bulk-apply command is correct.
+TEST(CommandCreatorTest, createLease6BulkApply) {
+    Lease6Ptr lease = createLease6();
+    Lease6Ptr deleted_lease = createLease6();
+
+    Lease6CollectionPtr leases(new Lease6Collection());
+    Lease6CollectionPtr deleted_leases(new Lease6Collection());
+
+    leases->push_back(lease);
+    deleted_leases->push_back(lease);
+
+    ConstElementPtr command = CommandCreator::createLease6BulkApply(leases, deleted_leases);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-bulk-apply",
+                                              "dhcp6", arguments));
+
+    ConstElementPtr origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+
+    // Verify deleted-leases.
+    auto deleted_leases_json = arguments->get("deleted-leases");
+    ASSERT_TRUE(deleted_leases_json);
+    ASSERT_EQ(Element::list, deleted_leases_json->getType());
+    ASSERT_EQ(1, deleted_leases_json->size());
+    auto lease_as_json = deleted_leases_json->get(0);
+    EXPECT_EQ(leaseAsJson(createLease6())->str(), lease_as_json->str());
+
+    // Verify leases.
+    auto leases_json = arguments->get("leases");
+    ASSERT_TRUE(leases_json);
+    ASSERT_EQ(Element::list, leases_json->getType());
+    ASSERT_EQ(1, leases_json->size());
+    lease_as_json = leases_json->get(0);
+    EXPECT_EQ(leaseAsJson(createLease6())->str(), lease_as_json->str());
+}
+
+// This test verifies that the lease6-bulk-apply command can be created
+// from DHCPv6 leases backlog.
+TEST(CommandCreatorTest, createLease6BulkApplyFromBacklog) {
+    Lease6Ptr lease = createLease6();
+    Lease6Ptr deleted_lease = createLease6();
+
+    LeaseUpdateBacklog backlog(100);
+    backlog.push(LeaseUpdateBacklog::ADD, lease);
+    backlog.push(LeaseUpdateBacklog::DELETE, deleted_lease);
+
+    ConstElementPtr command = CommandCreator::createLease6BulkApply(backlog);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-bulk-apply",
+                                              "dhcp6", arguments));
+
+    ConstElementPtr origin = arguments->get("origin");
+    ASSERT_TRUE(origin);
+    ASSERT_EQ("ha-partner", origin->stringValue());
+
+    // Verify deleted-leases.
+    auto deleted_leases_json = arguments->get("deleted-leases");
+    ASSERT_TRUE(deleted_leases_json);
+    ASSERT_EQ(Element::list, deleted_leases_json->getType());
+    ASSERT_EQ(1, deleted_leases_json->size());
+    auto lease_as_json = deleted_leases_json->get(0);
+    EXPECT_EQ(leaseAsJson(createLease6())->str(), lease_as_json->str());
+
+    // Verify leases.
+    auto leases_json = arguments->get("leases");
+    ASSERT_TRUE(leases_json);
+    ASSERT_EQ(Element::list, leases_json->getType());
+    ASSERT_EQ(1, leases_json->size());
+    lease_as_json = leases_json->get(0);
+    EXPECT_EQ(leaseAsJson(createLease6())->str(), lease_as_json->str());
+
+    // Make sure the backlog is now empty.
+    EXPECT_EQ(0, backlog.size());
+}
+
+// This test verifies that the lease6-get-all command is correct.
+TEST(CommandCreatorTest, createLease6GetAll) {
+    ConstElementPtr command = CommandCreator::createLease6GetAll();
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-get-all", "dhcp6"));
+}
+
+
+// This test verifies that the lease6-get-page command is correct when
+// first page is fetched.
+TEST(CommandCreatorTest, createLease6GetPageStart) {
+    Lease6Ptr lease6;
+    ConstElementPtr command = CommandCreator::createLease6GetPage(lease6, 10);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-get-page", "dhcp6",
+                                              arguments));
+
+    ConstElementPtr from = arguments->get("from");
+    ASSERT_TRUE(from);
+    EXPECT_EQ(Element::string, from->getType());
+    EXPECT_EQ("start", from->stringValue());
+
+    ConstElementPtr limit = arguments->get("limit");
+    ASSERT_TRUE(limit);
+    ASSERT_EQ(Element::integer, limit->getType());
+    EXPECT_EQ(10, limit->intValue());
+}
+
+// This test verifies that the lease6-get-page command is correct when next
+// page is fetched.
+TEST(CommandCreatorTest, createLease6GetPageAddress) {
+    Lease6Ptr lease6(new Lease6());
+    lease6->addr_ = IOAddress("2001:db8:1::1");
+
+    ConstElementPtr command = CommandCreator::createLease6GetPage(lease6, 15);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "lease6-get-page", "dhcp6",
+                                              arguments));
+
+    ConstElementPtr from = arguments->get("from");
+    ASSERT_TRUE(from);
+    EXPECT_EQ(Element::string, from->getType());
+    EXPECT_EQ("2001:db8:1::1", from->stringValue());
+
+    ConstElementPtr limit = arguments->get("limit");
+    ASSERT_TRUE(limit);
+    ASSERT_EQ(Element::integer, limit->getType());
+    EXPECT_EQ(15, limit->intValue());
+}
+
+// This test verifies that exception is thrown if limit is set to 0 while
+// creating lease6-get-page command.
+TEST(CommandCreatorTest, createLease6GetPageZeroLimit) {
+    Lease6Ptr lease6;
+    EXPECT_THROW(CommandCreator::createLease6GetPage(lease6, 0), BadValue);
+}
+
+// This test verifies that the ha-maintenance-notify command is correct
+// while being sent to the DHCPv4 server.
+TEST(CommandCreatorTest, createMaintenanceNotify4) {
+    ConstElementPtr command = CommandCreator::createMaintenanceNotify(true, HAServerType::DHCPv4);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-maintenance-notify", "dhcp4",
+                                              arguments));
+
+    auto cancel = arguments->get("cancel");
+    ASSERT_TRUE(cancel);
+    ASSERT_EQ(Element::boolean, cancel->getType());
+    EXPECT_TRUE(cancel->boolValue());
+}
+
+// This test verifies that the ha-maintenance-notify command is correct
+// while being sent to the DHCPv6 server.
+TEST(CommandCreatorTest, createMaintenanceNotify6) {
+    ConstElementPtr command = CommandCreator::createMaintenanceNotify(false, HAServerType::DHCPv6);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-maintenance-notify", "dhcp6",
+                                              arguments));
+
+    auto cancel = arguments->get("cancel");
+    ASSERT_TRUE(cancel);
+    ASSERT_EQ(Element::boolean, cancel->getType());
+    EXPECT_FALSE(cancel->boolValue());
+}
+
+// This test verifies that the ha-sync-complete-notify command sent to a
+// DHCPv4 server is correct.
+TEST(CommandCreatorTest, createSyncCompleteNotify4) {
+    ConstElementPtr command = CommandCreator::createSyncCompleteNotify(HAServerType::DHCPv4);
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-sync-complete-notify", "dhcp4"));
+}
+
+// This test verifies that the ha-sync-complete-notify command sent to a
+// DHCPv4 server is correct.
+TEST(CommandCreatorTest, createSyncCompleteNotify6) {
+    ConstElementPtr command = CommandCreator::createSyncCompleteNotify(HAServerType::DHCPv6);
+    ConstElementPtr arguments;
+    ASSERT_NO_FATAL_FAILURE(testCommandBasics(command, "ha-sync-complete-notify", "dhcp6"));
+}
+
+}
diff --git a/src/hooks/dhcp/high_availability/tests/communication_state_unittest.cc b/src/hooks/dhcp/high_availability/tests/communication_state_unittest.cc
new file mode 100644
index 0000000..8c33c09
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/communication_state_unittest.cc
@@ -0,0 +1,1201 @@
+// Copyright (C) 2018-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <ha_test.h>
+#include <asiolink/asio_wrapper.h>
+#include <communication_state.h>
+#include <asiolink/interval_timer.h>
+#include <asiolink/io_service.h>
+#include <dhcp/dhcp4.h>
+#include <dhcp/dhcp6.h>
+#include <exceptions/exceptions.h>
+#include <http/date_time.h>
+#include <util/multi_threading_mgr.h>
+
+#include <boost/date_time/posix_time/posix_time.hpp>
+#include <gtest/gtest.h>
+#include <functional>
+#include <limits>
+#include <sstream>
+
+using namespace isc;
+using namespace isc::asiolink;
+using namespace isc::data;
+using namespace isc::dhcp;
+using namespace isc::ha;
+using namespace isc::ha::test;
+using namespace isc::http;
+using namespace isc::util;
+
+using namespace boost::posix_time;
+using namespace boost::gregorian;
+
+
+namespace {
+
+
+/// @brief Test fixture class for @c CommunicationState class.
+class CommunicationStateTest : public HATest {
+public:
+
+    /// @brief Constructor.
+    CommunicationStateTest()
+        : state_(io_service_, createValidConfiguration()),
+          state6_(io_service_, createValidConfiguration()) {
+        MultiThreadingMgr::instance().setMode(false);
+    }
+
+    /// @brief Destructor.
+    ~CommunicationStateTest() {
+        MultiThreadingMgr::instance().setMode(false);
+        io_service_->poll();
+    }
+
+    /// @brief Verifies that the partner state is set and retrieved correctly.
+    void partnerStateTest();
+
+    /// @brief Verifies that the partner state is set to unavailable.
+    ///
+    /// Whether or not the function under test also resets the clock skew is
+    /// tested in a different test case.
+    void partnerStateUnavailableTest();
+
+    /// @brief Verifies that the partner's scopes are set and retrieved correctly.
+    void partnerScopesTest();
+
+    /// @brief Verifies that the object is poked right after construction.
+    void initialDurationTest();
+
+    /// @brief Verifies that poking the state updates the returned duration.
+    void pokeTest();
+
+    /// @brief Test that heartbeat function is triggered.
+    void heartbeatTest();
+
+    /// @brief Test that invalid values provided to startHeartbeat are rejected.
+    void startHeartbeatInvalidValuesTest();
+
+    /// @brief Test that failure detection works properly for DHCPv4 case.
+    void detectFailureV4Test();
+
+    /// @brief This test verifies that it is possible to disable analysis of the DHCPv4
+    /// packets in which case the partner's failure is assumed when there is
+    /// no connection over the control channel.
+    void failureDetectionDisabled4Test();
+
+    /// @brief Test that failure detection works properly for DHCPv6 case.
+    void detectFailureV6Test();
+
+    /// @brief This test verifies that it is possible to disable analysis of the DHCPv6
+    /// packets in which case the partner's failure is assumed when there is
+    /// no connection over the control channel.
+    void failureDetectionDisabled6Test();
+
+    /// @brief This test verifies that the clock skew is checked properly by the
+    /// clockSkewShouldWarn and clockSkewShouldTerminate functions.
+    void clockSkewTest();
+
+    /// @brief This test verifies that the clock skew calculations take into
+    /// account whether or not the partner is available.
+    void clockSkewPartnerUnavailableTest();
+
+    /// @brief This test verifies that the clock skew value is formatted correctly
+    /// for logging.
+    void logFormatClockSkewTest();
+
+    /// @brief This test verifies that too many rejected lease updates cause
+    /// the service termination.
+    void rejectedLeaseUpdatesTerminateTest();
+
+    /// @brief Tests that the communication state report is correct.
+    void getReportTest();
+
+    /// @brief Tests unusual values used to create the report.
+    void getReportDefaultValuesTest();
+
+    /// @brief Tests that unsent updates count can be incremented and fetched.
+    void getUnsentUpdateCountTest();
+
+    /// @brief Tests that unsent updates count from partner can be set and
+    /// a difference from previous value detected.
+    void hasPartnerNewUnsentUpdatesTest();
+
+    /// @brief Test that gathering rejected leases works fine in DHCPv4 case.
+    void reportRejectedLeasesV4Test();
+
+    /// @brief Test that rejected leases are cleared after reporting respective
+    /// successful leases.
+    void reportSuccessfulLeasesV4Test();
+
+    /// @brief Test that invalid values are not accepted when reporting
+    /// rejected leases.
+    void reportRejectedLeasesV4InvalidValuesTest();
+
+    /// @brief Test that gathering rejected leases works fine in the DHCPv6 case.
+    void reportRejectedLeasesV6Test();
+
+    /// @brief Test that rejected leases are cleared after reporting respective
+    /// successful leases.
+    void reportSuccessfulLeasesV6Test();
+
+    /// @brief Test that invalid values are not accepted when reporting
+    /// rejected leases.
+    void reportRejectedLeasesV6InvalidValuesTest();
+
+    /// @brief Test that old rejected lease updates are discarded while
+    /// getting the rejected lease updates count.
+    void getRejectedLeaseUpdatesCountFromContainerTest();
+
+    /// @brief Returns test heartbeat implementation.
+    ///
+    /// @return Pointer to heartbeat implementation function under test.
+    std::function<void()> getHeartbeatImpl() {
+        return (std::bind(&CommunicationStateTest::heartbeatImpl, this));
+    }
+
+    /// @brief Test heartbeat implementation.
+    ///
+    /// It simply pokes the communication state object. Note that the real
+    /// implementation would send an actual heartbeat command prior to
+    /// poking the state.
+    void heartbeatImpl() {
+        state_.poke();
+    }
+
+    /// @brief Communication state object used throughout the tests.
+    NakedCommunicationState4 state_;
+
+    /// @brief Communication state for IPv6 used throughout the tests.
+    NakedCommunicationState6 state6_;
+};
+
+// Verifies that the partner state is set and retrieved correctly.
+void
+CommunicationStateTest::partnerStateTest() {
+    // Initially the state is unknown.
+    EXPECT_LT(state_.getPartnerState(), 0);
+
+    state_.setPartnerState("hot-standby");
+    EXPECT_EQ(HA_HOT_STANDBY_ST, state_.getPartnerState());
+
+    state_.setPartnerState("load-balancing");
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, state_.getPartnerState());
+
+    state_.setPartnerState("partner-down");
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, state_.getPartnerState());
+
+    state_.setPartnerState("ready");
+    EXPECT_EQ(HA_READY_ST, state_.getPartnerState());
+
+    state_.setPartnerState("syncing");
+    EXPECT_EQ(HA_SYNCING_ST, state_.getPartnerState());
+
+    state_.setPartnerState("terminated");
+    EXPECT_EQ(HA_TERMINATED_ST, state_.getPartnerState());
+
+    state_.setPartnerState("waiting");
+    EXPECT_EQ(HA_WAITING_ST, state_.getPartnerState());
+
+    state_.setPartnerState("unavailable");
+    EXPECT_EQ(HA_UNAVAILABLE_ST, state_.getPartnerState());
+
+    // An attempt to set unsupported value should result in exception.
+    EXPECT_THROW(state_.setPartnerState("unsupported"), BadValue);
+}
+
+// Verifies that the partner state is set to unavailable.
+void
+CommunicationStateTest::partnerStateUnavailableTest() {
+    // Initially the state is unknown.
+    EXPECT_LT(state_.getPartnerState(), 0);
+
+    // Set a valid state initially.
+    state_.setPartnerState("hot-standby");
+    EXPECT_EQ(HA_HOT_STANDBY_ST, state_.getPartnerState());
+
+    state_.setPartnerUnavailable();
+    EXPECT_EQ(HA_UNAVAILABLE_ST, state_.getPartnerState());
+}
+
+// Verifies that the partner's scopes are set and retrieved correctly.
+void
+CommunicationStateTest::partnerScopesTest() {
+    // Initially, the scopes should be empty.
+    ASSERT_TRUE(state_.getPartnerScopes().empty());
+
+    // Set new partner scopes.
+    ASSERT_NO_THROW(
+        state_.setPartnerScopes(Element::fromJSON("[ \"server1\", \"server2\" ]"))
+    );
+
+    // Get them back.
+    auto returned = state_.getPartnerScopes();
+    EXPECT_EQ(2, returned.size());
+    EXPECT_EQ(1, returned.count("server1"));
+    EXPECT_EQ(1, returned.count("server2"));
+
+    // Override the scopes.
+    ASSERT_NO_THROW(
+        state_.setPartnerScopes(Element::fromJSON("[ \"server1\" ]"))
+    );
+    returned = state_.getPartnerScopes();
+    EXPECT_EQ(1, returned.size());
+    EXPECT_EQ(1, returned.count("server1"));
+
+    // Clear the scopes.
+    ASSERT_NO_THROW(
+        state_.setPartnerScopes(Element::fromJSON("[ ]"))
+    );
+    returned = state_.getPartnerScopes();
+    EXPECT_TRUE(returned.empty());
+
+    // An attempt to set invalid JSON should fail.
+    EXPECT_THROW(state_.setPartnerScopes(Element::fromJSON("{ \"not-a-list\": 1 }")),
+                 BadValue);
+}
+
+// Verifies that the object is poked right after construction.
+void
+CommunicationStateTest::initialDurationTest() {
+    EXPECT_TRUE(state_.isPoked());
+}
+
+// Verifies that  poking the state updates the returned duration.
+void
+CommunicationStateTest::pokeTest() {
+    state_.modifyPokeTime(-30);
+    ASSERT_GE(state_.getDurationInMillisecs(), 30000);
+    ASSERT_TRUE(state_.isCommunicationInterrupted());
+    ASSERT_NO_THROW(state_.poke());
+    EXPECT_TRUE(state_.isPoked());
+    EXPECT_FALSE(state_.isCommunicationInterrupted());
+}
+
+// Test that heartbeat function is triggered.
+void
+CommunicationStateTest::heartbeatTest() {
+    // Set poke time to the past and expect that the object is considered
+    // not poked.
+    state_.modifyPokeTime(-30);
+    EXPECT_FALSE(state_.isPoked());
+
+    // Run heartbeat every 1 second.
+    ASSERT_NO_THROW(state_.startHeartbeat(1, getHeartbeatImpl()));
+    runIOService(1200);
+
+    // After > than 1 second the state should have been poked.
+    EXPECT_TRUE(state_.isPoked());
+
+    // Repeat the test.
+    state_.modifyPokeTime(-30);
+    EXPECT_FALSE(state_.isPoked());
+    ASSERT_NO_THROW(state_.startHeartbeat(1, getHeartbeatImpl()));
+    runIOService(1200);
+    EXPECT_TRUE(state_.isPoked());
+}
+
+// Test that invalid values provided to startHeartbeat are rejected.
+void
+CommunicationStateTest::startHeartbeatInvalidValuesTest() {
+    EXPECT_THROW(state_.startHeartbeat(-1, getHeartbeatImpl()), BadValue);
+    EXPECT_THROW(state_.startHeartbeat(0, getHeartbeatImpl()), BadValue);
+    EXPECT_THROW(state_.startHeartbeat(1, 0), BadValue);
+}
+
+// Test that failure detection works properly for DHCPv4 case.
+void
+CommunicationStateTest::detectFailureV4Test() {
+    // Initially, there should be no unacked clients recorded.
+    ASSERT_FALSE(state_.failureDetected());
+    EXPECT_EQ(0, state_.getUnackedClientsCount());
+    EXPECT_EQ(0, state_.getConnectingClientsCount());
+    EXPECT_EQ(0, state_.getAnalyzedMessagesCount());
+
+    // The maximum number of unacked clients is 10. Let's provide 10
+    // DHCPDISCOVER messages with the "secs" value of 15 which exceeds
+    // the threshold of 10. All these clients should be recorded as
+    // unacked.
+    for (uint8_t i = 0; i < 10; ++i) {
+        // Some of the requests have no client identifier to test that
+        // we don't fall over if the client identifier is null.
+        const uint8_t client_id_seed = (i < 5 ? i : 0);
+        ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, i,
+                                                             client_id_seed,
+                                                             15)));
+        // We don't exceed the maximum of number of unacked clients so the
+        // partner failure shouldn't be reported.
+        ASSERT_FALSE(state_.failureDetected())
+            << "failure detected for the request number "
+            << static_cast<int>(i);
+    }
+    EXPECT_EQ(10, state_.getUnackedClientsCount());
+    EXPECT_EQ(10, state_.getConnectingClientsCount());
+    EXPECT_EQ(10, state_.getAnalyzedMessagesCount());
+
+    // Let's provide similar set of requests but this time the "secs" field is
+    // below the threshold. They should not be counted as failures. Also,
+    // all of these requests have client identifier.
+    for (uint8_t i = 0; i < 10; ++i) {
+        ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, i, i,
+                                                             9)));
+        ASSERT_FALSE(state_.failureDetected())
+            << "failure detected for the request number "
+            << static_cast<int>(i);
+    }
+    EXPECT_EQ(10, state_.getUnackedClientsCount());
+    EXPECT_EQ(15, state_.getConnectingClientsCount());
+    EXPECT_EQ(20, state_.getAnalyzedMessagesCount());
+
+    // Let's create a message from a new (not recorded yet) client with the
+    // "secs" field value below the threshold. It should not be counted as failure.
+    ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, 10, 10, 6)));
+
+    // Still no failure.
+    ASSERT_FALSE(state_.failureDetected());
+    EXPECT_EQ(10, state_.getUnackedClientsCount());
+    EXPECT_EQ(16, state_.getConnectingClientsCount());
+    EXPECT_EQ(21, state_.getAnalyzedMessagesCount());
+
+    // Let's repeat one of the requests which already have been recorded as
+    // unacked but with a greater value of "secs" field. This should not
+    // be counted because only new clients count.
+    ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, 3, 3, 20)));
+    ASSERT_FALSE(state_.failureDetected());
+    EXPECT_EQ(10, state_.getUnackedClientsCount());
+    EXPECT_EQ(16, state_.getConnectingClientsCount());
+    EXPECT_EQ(22, state_.getAnalyzedMessagesCount());
+
+    // This time let's simulate a client with a MAC address already recorded but
+    // with a client identifier. This should be counted as a new unacked request.
+    ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, 7, 7, 15)));
+    ASSERT_TRUE(state_.failureDetected());
+    EXPECT_EQ(11, state_.getUnackedClientsCount());
+    EXPECT_EQ(16, state_.getConnectingClientsCount());
+    EXPECT_EQ(23, state_.getAnalyzedMessagesCount());
+
+    // Poking should cause all counters to reset as it is an indication that the
+    // control connection has been re-established.
+    ASSERT_NO_THROW(state_.poke());
+
+    // We're back to no failure state.
+    EXPECT_FALSE(state_.failureDetected());
+    EXPECT_EQ(0, state_.getUnackedClientsCount());
+    EXPECT_EQ(0, state_.getConnectingClientsCount());
+    EXPECT_EQ(0, state_.getAnalyzedMessagesCount());
+
+    // Send 11 DHCPDISCOVER messages with the "secs" field bytes swapped. Swapping
+    // bytes was reported for some misbehaving Windows clients. The server should
+    // detect bytes swapping when second byte is 0 and the first byte is non-zero.
+    // However, the first byte is equal to 5 which is below our threshold so none
+    // of the requests below should count as unacked.
+    for (uint8_t i = 0; i < 11; ++i) {
+        ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, i, i,
+                                                             0x0500)));
+        ASSERT_FALSE(state_.failureDetected())
+            << "failure detected for the request number "
+            << static_cast<int>(i)
+            << " when testing swapped secs field bytes";
+    }
+    EXPECT_EQ(0, state_.getUnackedClientsCount());
+    EXPECT_EQ(11, state_.getConnectingClientsCount());
+    EXPECT_EQ(11, state_.getAnalyzedMessagesCount());
+
+    // Repeat the same test, but this time either the first byte exceeds the
+    // secs threshold or the second byte is non-zero. All should be counted
+    // as unacked.
+    for (uint8_t i = 0; i < 10; ++i) {
+        uint16_t secs = (i % 2 == 0 ? 0x0F00 : 0x0501);
+        ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, i, i,
+                                                             secs)));
+        ASSERT_FALSE(state_.failureDetected())
+            << "failure detected for the request number "
+            << static_cast<int>(i)
+            << " when testing swapped secs field bytes";
+    }
+
+    // This last message should cause the failure state.
+    ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, 11, 11,
+                                                         0x30)));
+    EXPECT_TRUE(state_.failureDetected());
+    EXPECT_EQ(11, state_.getUnackedClientsCount());
+    EXPECT_EQ(12, state_.getConnectingClientsCount());
+    EXPECT_EQ(22, state_.getAnalyzedMessagesCount());
+}
+
+// This test verifies that it is possible to disable analysis of the DHCPv4
+// packets in which case the partner's failure is assumed when there is
+// no connection over the control channel.
+void
+CommunicationStateTest::failureDetectionDisabled4Test() {
+    state_.config_->setMaxUnackedClients(0);
+    EXPECT_TRUE(state_.failureDetected());
+}
+
+// Test that failure detection works properly for DHCPv6 case.
+void
+CommunicationStateTest::detectFailureV6Test() {
+    // Initially, there should be no unacked clients recorded.
+    ASSERT_FALSE(state6_.failureDetected());
+    EXPECT_EQ(0, state6_.getUnackedClientsCount());
+    EXPECT_EQ(0, state6_.getConnectingClientsCount());
+    EXPECT_EQ(0, state6_.getAnalyzedMessagesCount());
+
+    // The maximum number of unacked clients is 10. Let's provide 10
+    // Solicit messages with the "elapsed time" value of 1500 which exceeds
+    // the threshold of 10000ms. Note that the elapsed time value is provided
+    // in 1/100s of 1 second. All these clients should be recorded as
+    // unacked.
+    for (uint8_t i = 0; i < 10; ++i) {
+        ASSERT_NO_THROW(state6_.analyzeMessage(createMessage6(DHCPV6_SOLICIT, i,
+                                                              1500)));
+        // We don't exceed the maximum number of unacked clients so the
+        // partner failure shouldn't be reported.
+        ASSERT_FALSE(state6_.failureDetected())
+            << "failure detected for the request number "
+            << static_cast<int>(i);
+    }
+    EXPECT_EQ(10, state6_.getUnackedClientsCount());
+    EXPECT_EQ(10, state6_.getConnectingClientsCount());
+    EXPECT_EQ(10, state6_.getAnalyzedMessagesCount());
+
+    // Let's provide similar set of requests but this time the "elapsed time" is
+    // below the threshold. This should not reduce the number of unacked or new
+    // clients.
+    for (uint8_t i = 0; i < 10; ++i) {
+        ASSERT_NO_THROW(state6_.analyzeMessage(createMessage6(DHCPV6_SOLICIT, i,
+                                                             900)));
+        ASSERT_FALSE(state6_.failureDetected())
+            << "failure detected for the request number "
+            << static_cast<int>(i);
+    }
+    EXPECT_EQ(10, state6_.getUnackedClientsCount());
+    EXPECT_EQ(10, state6_.getConnectingClientsCount());
+    EXPECT_EQ(20, state6_.getAnalyzedMessagesCount());
+
+    // Let's create a message from a new (not recorded yet) client with the
+    // "elapsed time" value below the threshold. It should not count as failure.
+    ASSERT_NO_THROW(state6_.analyzeMessage(createMessage6(DHCPV6_SOLICIT, 10, 600)));
+
+    // Still no failure.
+    ASSERT_FALSE(state6_.failureDetected());
+    EXPECT_EQ(10, state6_.getUnackedClientsCount());
+    EXPECT_EQ(11, state6_.getConnectingClientsCount());
+    EXPECT_EQ(21, state6_.getAnalyzedMessagesCount());
+
+    // Let's repeat one of the requests which already have been recorded as
+    // unacked but with a greater value of "elapsed time". This should not
+    // be counted because only new clients count.
+    ASSERT_NO_THROW(state6_.analyzeMessage(createMessage6(DHCPV6_SOLICIT, 3, 2000)));
+    ASSERT_FALSE(state6_.failureDetected());
+    EXPECT_EQ(10, state6_.getUnackedClientsCount());
+    EXPECT_EQ(11, state6_.getConnectingClientsCount());
+    EXPECT_EQ(22, state6_.getAnalyzedMessagesCount());
+
+    // New unacked client should cause failure to be detected.
+    ASSERT_NO_THROW(state6_.analyzeMessage(createMessage6(DHCPV6_SOLICIT, 11, 1500)));
+    ASSERT_TRUE(state6_.failureDetected());
+    EXPECT_EQ(11, state6_.getUnackedClientsCount());
+    EXPECT_EQ(12, state6_.getConnectingClientsCount());
+    EXPECT_EQ(23, state6_.getAnalyzedMessagesCount());
+
+    // Poking should cause all counters to reset as it is an indication that the
+    // control connection has been re-established.
+    ASSERT_NO_THROW(state6_.poke());
+
+    // We're back to no failure state.
+    EXPECT_FALSE(state6_.failureDetected());
+    EXPECT_EQ(0, state6_.getUnackedClientsCount());
+    EXPECT_EQ(0, state6_.getConnectingClientsCount());
+    EXPECT_EQ(0, state6_.getAnalyzedMessagesCount());
+}
+
+// This test verifies that it is possible to disable analysis of the DHCPv6
+// packets in which case the partner's failure is assumed when there is
+// no connection over the control channel.
+void
+CommunicationStateTest::failureDetectionDisabled6Test() {
+    state6_.config_->setMaxUnackedClients(0);
+    EXPECT_TRUE(state6_.failureDetected());
+}
+
+// This test verifies that the clock skew is checked properly by the
+// clockSkewShouldWarn and clockSkewShouldTerminate functions.
+void
+CommunicationStateTest::clockSkewTest() {
+    // Default clock skew is 0.
+    EXPECT_FALSE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+
+    // Partner time is ahead by 15s (no warning).
+    state_.clock_skew_ += boost::posix_time::time_duration(0, 0, 15);
+    EXPECT_FALSE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+
+    // Partner time is behind by 15s (no warning).
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+    state_.clock_skew_ -= boost::posix_time::time_duration(0, 0, 15);
+    EXPECT_FALSE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+
+    // Partner time is ahead by 35s (warning).
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+    state_.clock_skew_ += boost::posix_time::time_duration(0, 0, 35);
+    EXPECT_TRUE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+
+    // Partner time is behind by 35s (warning).
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+    state_.clock_skew_ -= boost::posix_time::time_duration(0, 0, 35);
+    state_.last_clock_skew_warn_ = boost::posix_time::ptime();
+    EXPECT_TRUE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+
+    // Due to the gating mechanism this should not return true the second
+    // time.
+    EXPECT_FALSE(state_.clockSkewShouldWarn());
+
+    // But should warn if the warning was issued more than 60 seconds ago.
+    state_.last_clock_skew_warn_ -= boost::posix_time::time_duration(0, 1, 30);
+    EXPECT_TRUE(state_.clockSkewShouldWarn());
+
+    // Partner time is ahead by 65s (warning and terminate).
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+    state_.clock_skew_ += boost::posix_time::time_duration(0, 1, 5);
+    state_.last_clock_skew_warn_ = boost::posix_time::ptime();
+    EXPECT_TRUE(state_.clockSkewShouldWarn());
+    EXPECT_TRUE(state_.clockSkewShouldTerminate());
+
+    // Partner time is behind by 65s (warning and terminate).
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+    state_.clock_skew_ -= boost::posix_time::time_duration(0, 1, 5);
+    state_.last_clock_skew_warn_ = boost::posix_time::ptime();
+    EXPECT_TRUE(state_.clockSkewShouldWarn());
+    EXPECT_TRUE(state_.clockSkewShouldTerminate());
+}
+
+// This test verifies that the clock skew calculations take into
+// account whether or not the partner is available.
+void
+CommunicationStateTest::clockSkewPartnerUnavailableTest() {
+    // Default clock skew is 0.
+    EXPECT_FALSE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+
+    // Move the clock skew beyond the 60s limit. The alarms about the
+    // too high clock skew should be activated.
+    state_.setPartnerTime(HttpDateTime().rfc1123Format());
+    state_.clock_skew_ += boost::posix_time::time_duration(0, 1, 5);
+    EXPECT_TRUE(state_.clockSkewShouldWarn());
+    EXPECT_TRUE(state_.clockSkewShouldTerminate());
+
+    // Mark the partner as unavailable. It should reset the clock skew
+    // because we don't know the actual partner's time at the moment.
+    state_.setPartnerUnavailable();
+    EXPECT_FALSE(state_.clockSkewShouldWarn());
+    EXPECT_FALSE(state_.clockSkewShouldTerminate());
+}
+
+// This test verifies that the clock skew value is formatted correctly
+// for logging.
+void
+CommunicationStateTest::logFormatClockSkewTest() {
+    // Make sure logFormatClockSkew() does not throw if called prior
+    // the first call to setPartnerTime().
+    std::string log;
+    ASSERT_NO_THROW(log = state_.logFormatClockSkew());
+    EXPECT_EQ(std::string("skew not initialized"), log);
+
+    // Get current time.
+    boost::posix_time::ptime now = HttpDateTime().getPtime();
+
+    // Partner time is ahead by 15s.
+    boost::posix_time::time_duration offset(0,0,15);
+    state_.setPartnerTime(HttpDateTime(now + offset).rfc1123Format());
+    ASSERT_NO_THROW(log = state_.logFormatClockSkew());
+
+    // The logFormatClockSkew uses the clock_skew_ value which is computed
+    // at the time when setPartnerTime() is called. Therefore, we can't
+    // just assume that it is 15s because it may be already slightly off.
+    // Let's compare the output with the actual clock_skew_ value remembered
+    // in the state_ instance.
+    ASSERT_FALSE(state_.clock_skew_.is_special());
+    ASSERT_FALSE(state_.clock_skew_.is_negative());
+    std::ostringstream s;
+    s << state_.clock_skew_.seconds() << "s ahead";
+    EXPECT_TRUE(log.find(s.str()) != std::string::npos) <<
+                " log content wrong: " << log;
+
+    // Partner time is behind by 15s.
+    state_.setPartnerTime(HttpDateTime(now - offset).rfc1123Format());
+    ASSERT_NO_THROW(log = state_.logFormatClockSkew());
+
+    // Again, extract the actual clock skew remembered in the state_ instance.
+    ASSERT_FALSE(state_.clock_skew_.is_special());
+    auto skew = state_.clock_skew_;
+
+    // It must be negative this time.
+    ASSERT_TRUE(skew.is_negative());
+    // Convert it to positive value so we can use to to build the expected string.
+    skew = -skew;
+    std::ostringstream s2;
+    s2 << skew.seconds() << "s behind";
+    EXPECT_TRUE(log.find(s2.str()) != std::string::npos) <<
+                " log content wrong: " << log;
+
+    offset = hours(18) + minutes(37) + seconds(15);
+    ptime mytime(date(2019, Jul, 23), offset);
+
+    state_.my_time_at_skew_ = mytime;
+    state_.partner_time_at_skew_ = mytime + seconds(25);
+    state_.clock_skew_ = seconds(25);
+    ASSERT_NO_THROW(log = state_.logFormatClockSkew());
+    std::string expected("my time: 2019-07-23 18:37:15, "
+                         "partner's time: 2019-07-23 18:37:40, "
+                         "partner's clock is 25s ahead");
+    EXPECT_EQ(expected, log);
+}
+
+void
+CommunicationStateTest::rejectedLeaseUpdatesTerminateTest() {
+    EXPECT_FALSE(state_.rejectedLeaseUpdatesShouldTerminate());
+    // Reject several lease updates but do not exceed the limit.
+    for (auto i = 0; i < 9; ++i) {
+        ASSERT_NO_THROW(state_.reportRejectedLeaseUpdate(createMessage4(DHCPDISCOVER, i, i, 0)));
+    }
+    EXPECT_FALSE(state_.rejectedLeaseUpdatesShouldTerminate());
+    // Add one more. It should exceed the limit.
+    ASSERT_NO_THROW(state_.reportRejectedLeaseUpdate(createMessage4(DHCPDISCOVER, 9, 9, 0)));
+    EXPECT_TRUE(state_.rejectedLeaseUpdatesShouldTerminate());
+}
+
+// Tests that the communication state report is correct.
+void
+CommunicationStateTest::getReportTest() {
+    state_.setPartnerState("waiting");
+
+    auto scopes = Element::createList();
+    scopes->add(Element::create("server1"));
+    state_.setPartnerScopes(scopes);
+
+    state_.poke();
+
+    // Simulate the communications interrupted state.
+    state_.modifyPokeTime(-100);
+
+    // Send two DHCP packets of which one has secs value beyond the threshold and
+    // the other one lower than the threshold.
+    ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, 0, 0, 5)));
+    ASSERT_NO_THROW(state_.analyzeMessage(createMessage4(DHCPDISCOVER, 1, 0, 15)));
+
+    // Get the report.
+    auto report = state_.getReport();
+    ASSERT_TRUE(report);
+
+    // Compare with the expected output.
+    std::string expected = "{"
+        "    \"age\": 100,"
+        "    \"in-touch\": true,"
+        "    \"last-scopes\": [ \"server1\" ],"
+        "    \"last-state\": \"waiting\","
+        "    \"communication-interrupted\": true,"
+        "    \"connecting-clients\": 2,"
+        "    \"unacked-clients\": 1,"
+        "    \"unacked-clients-left\": 10,"
+        "    \"analyzed-packets\": 2"
+        "}";
+    EXPECT_TRUE(isEquivalent(Element::fromJSON(expected), report));
+}
+
+// Tests unusual values used to create the report.
+void
+CommunicationStateTest::getReportDefaultValuesTest() {
+    auto report = state_.getReport();
+    ASSERT_TRUE(report);
+
+    // Compare with the expected output.
+    std::string expected = "{"
+        "    \"age\": 0,"
+        "    \"in-touch\": false,"
+        "    \"last-scopes\": [ ],"
+        "    \"last-state\": \"\","
+        "    \"communication-interrupted\": false,"
+        "    \"connecting-clients\": 0,"
+        "    \"unacked-clients\": 0,"
+        "    \"unacked-clients-left\": 0,"
+        "    \"analyzed-packets\": 0"
+        "}";
+    EXPECT_TRUE(isEquivalent(Element::fromJSON(expected), report));
+}
+
+void
+CommunicationStateTest::getUnsentUpdateCountTest() {
+    // Initially the count should be 0.
+    EXPECT_EQ(0, state_.getUnsentUpdateCount());
+
+    // Increasing the value by 1 several times.
+    EXPECT_NO_THROW(state_.increaseUnsentUpdateCount());
+    EXPECT_EQ(1, state_.getUnsentUpdateCount());
+    EXPECT_NO_THROW(state_.increaseUnsentUpdateCount());
+    EXPECT_EQ(2, state_.getUnsentUpdateCount());
+    EXPECT_NO_THROW(state_.increaseUnsentUpdateCount());
+    EXPECT_EQ(3, state_.getUnsentUpdateCount());
+
+    // Test that the method under test protects against an overflow
+    // resetting the value to 0.
+    state_.unsent_update_count_ = std::numeric_limits<uint64_t>::max();
+    EXPECT_NO_THROW(state_.increaseUnsentUpdateCount());
+    EXPECT_EQ(1, state_.getUnsentUpdateCount());
+}
+
+void
+CommunicationStateTest::hasPartnerNewUnsentUpdatesTest() {
+    // Initially the counts should be 0.
+    EXPECT_FALSE(state_.hasPartnerNewUnsentUpdates());
+
+    // Set a positive value. It should be noticed.
+    EXPECT_NO_THROW(state_.setPartnerUnsentUpdateCount(5));
+    EXPECT_TRUE(state_.hasPartnerNewUnsentUpdates());
+
+    // No change, no new unsent updates.
+    EXPECT_NO_THROW(state_.setPartnerUnsentUpdateCount(5));
+    EXPECT_FALSE(state_.hasPartnerNewUnsentUpdates());
+
+    // Change it again. New updates.
+    EXPECT_NO_THROW(state_.setPartnerUnsentUpdateCount(10));
+    EXPECT_TRUE(state_.hasPartnerNewUnsentUpdates());
+
+    // Set it to 0 to simulate restart. No updates.
+    EXPECT_NO_THROW(state_.setPartnerUnsentUpdateCount(0));
+    EXPECT_FALSE(state_.hasPartnerNewUnsentUpdates());
+}
+
+void
+CommunicationStateTest::reportRejectedLeasesV4Test() {
+    // Initially, there should be no rejected leases.
+    EXPECT_EQ(0, state_.getRejectedLeaseUpdatesCount());
+
+    // Reject lease update.
+    auto msg = createMessage4(DHCPREQUEST, 1, 0, 0);
+    state_.reportRejectedLeaseUpdate(msg);
+    EXPECT_EQ(1, state_.getRejectedLeaseUpdatesCount());
+
+    // Reject another lease update.
+    msg = createMessage4(DHCPREQUEST, 2, 0, 0);
+    state_.reportRejectedLeaseUpdate(msg);
+    EXPECT_EQ(2, state_.getRejectedLeaseUpdatesCount());
+
+    // Reject a lease with a short (zero) lease lifetime.
+    // This lease should be discarded when we call the
+    // getRejectedLeaseUpdatesCount().
+    msg = createMessage4(DHCPREQUEST, 3, 0, 0);
+    state_.reportRejectedLeaseUpdate(msg, 0);
+    EXPECT_EQ(2, state_.getRejectedLeaseUpdatesCount());
+
+    // Reject lease update for a client using the same MAC
+    // address but different client identifier. It should
+    // be treated as a different lease.
+    msg = createMessage4(DHCPREQUEST, 2, 1, 0);
+    state_.reportRejectedLeaseUpdate(msg);
+    EXPECT_EQ(3, state_.getRejectedLeaseUpdatesCount());
+
+    // Clear rejected leases and make sure the counter
+    // is now 0.
+    state_.clearRejectedLeaseUpdates();
+    EXPECT_EQ(0, state_.getRejectedLeaseUpdatesCount());
+}
+
+void
+CommunicationStateTest::reportSuccessfulLeasesV4Test() {
+    // Initially, there should be no rejected leases.
+    EXPECT_EQ(0, state_.getRejectedLeaseUpdatesCount());
+    auto msg0 = createMessage4(DHCPREQUEST, 1, 0, 0);
+    // Reject lease update.
+    state_.reportRejectedLeaseUpdate(msg0);
+    EXPECT_EQ(1, state_.getRejectedLeaseUpdatesCount());
+    // Reject another lease update.
+    auto msg1 = createMessage4(DHCPREQUEST, 2, 0, 0);
+    state_.reportRejectedLeaseUpdate(msg1);
+    EXPECT_EQ(2, state_.getRejectedLeaseUpdatesCount());
+    // Report successful lease for the first message.
+    // It should reduce the number of rejected lease
+    // updates.
+    EXPECT_TRUE(state_.reportSuccessfulLeaseUpdate(msg0));
+    EXPECT_EQ(1, state_.getRejectedLeaseUpdatesCount());
+    // Report successful lease update for another message.
+    auto msg2 = createMessage4(DHCPREQUEST, 1, 1, 0);
+    EXPECT_FALSE(state_.reportSuccessfulLeaseUpdate(msg2));
+    EXPECT_EQ(1, state_.getRejectedLeaseUpdatesCount());
+    // There should be no rejected lease updates.
+    EXPECT_TRUE(state_.reportSuccessfulLeaseUpdate(msg1));
+    EXPECT_EQ(0, state_.getRejectedLeaseUpdatesCount());
+}
+
+void
+CommunicationStateTest::reportRejectedLeasesV4InvalidValuesTest() {
+    // Populate one valid update. Without it our functions under test
+    // would return early.
+    state_.reportRejectedLeaseUpdate(createMessage4(DHCPREQUEST, 1, 0, 0));
+    // Using DHCPv6 message in the DHCPv4 context is a programming
+    // error and deserves an exception.
+    auto msg = createMessage6(DHCPV6_REQUEST, 1, 0);
+    EXPECT_THROW(state_.reportRejectedLeaseUpdate(msg), BadValue);
+    EXPECT_THROW(state_.reportSuccessfulLeaseUpdate(msg), BadValue);
+}
+
+void
+CommunicationStateTest::reportRejectedLeasesV6Test() {
+    // Initially, there should be no rejected leases.
+    EXPECT_EQ(0, state6_.getRejectedLeaseUpdatesCount());
+
+    // Reject lease update.
+    auto msg = createMessage6(DHCPV6_REQUEST, 1, 0);
+    state6_.reportRejectedLeaseUpdate(msg);
+    EXPECT_EQ(1, state6_.getRejectedLeaseUpdatesCount());
+
+    // Reject another lease update.
+    msg = createMessage6(DHCPV6_REQUEST, 2, 0);
+    state6_.reportRejectedLeaseUpdate(msg);
+    EXPECT_EQ(2, state6_.getRejectedLeaseUpdatesCount());
+
+    // Reject a lease with a short (zero) lease lifetime.
+    // This lease should be discarded when we call the
+    // getRejectedLeaseUpdatesCount().
+    msg = createMessage6(DHCPV6_REQUEST, 3, 0);
+    state6_.reportRejectedLeaseUpdate(msg, 0);
+    EXPECT_EQ(2, state6_.getRejectedLeaseUpdatesCount());
+
+    // Reject it again. It should not affect the counter.
+    msg = createMessage6(DHCPV6_REQUEST, 2, 0);
+    state6_.reportRejectedLeaseUpdate(msg);
+    EXPECT_EQ(2, state6_.getRejectedLeaseUpdatesCount());
+
+    // Clear rejected lease updates and make sure they
+    // are now 0.
+    state6_.clearRejectedLeaseUpdates();
+    EXPECT_EQ(0, state6_.getRejectedLeaseUpdatesCount());
+}
+
+void
+CommunicationStateTest::reportSuccessfulLeasesV6Test() {
+    // Initially, there should be no rejected leases.
+    EXPECT_EQ(0, state6_.getRejectedLeaseUpdatesCount());
+    // Reject lease update.
+    auto msg0 = createMessage6(DHCPV6_SOLICIT, 1, 0);
+    EXPECT_TRUE(state6_.reportRejectedLeaseUpdate(msg0));
+    EXPECT_EQ(1, state6_.getRejectedLeaseUpdatesCount());
+    // Reject another lease update.
+    auto msg1 = createMessage6(DHCPV6_SOLICIT, 2, 0);
+    EXPECT_TRUE(state6_.reportRejectedLeaseUpdate(msg1));
+    EXPECT_EQ(2, state6_.getRejectedLeaseUpdatesCount());
+    // Report successful lease for the first message.
+    // It should reduce the number of rejected lease
+    // updates.
+    EXPECT_TRUE(state6_.reportSuccessfulLeaseUpdate(msg0));
+    EXPECT_EQ(1, state6_.getRejectedLeaseUpdatesCount());
+    // Report successful lease update for a lease that wasn't
+    // rejected. It should not affect the counter.
+    auto msg2 = createMessage6(DHCPV6_SOLICIT, 3, 0);
+    EXPECT_FALSE(state6_.reportSuccessfulLeaseUpdate(msg2));
+    EXPECT_EQ(1, state6_.getRejectedLeaseUpdatesCount());
+    // Report successful lease update for the last lease.
+    // The counter should now be 0.
+    EXPECT_TRUE(state6_.reportSuccessfulLeaseUpdate(msg1));
+    EXPECT_EQ(0, state6_.getRejectedLeaseUpdatesCount());
+}
+
+void
+CommunicationStateTest::reportRejectedLeasesV6InvalidValuesTest() {
+    // Populate one valid update. Without it our functions under test
+    // would return early.
+    state6_.reportRejectedLeaseUpdate(createMessage6(DHCPV6_REQUEST, 1, 0));
+    // Using DHCPv4 message in the DHCPv6 context is a programming
+    // error and deserves an exception.
+    auto msg0 = createMessage4(DHCPREQUEST, 1, 1, 0);
+    EXPECT_THROW(state6_.reportRejectedLeaseUpdate(msg0), BadValue);
+    EXPECT_THROW(state6_.reportSuccessfulLeaseUpdate(msg0), BadValue);
+
+    auto msg1 = createMessage6(DHCPV6_SOLICIT, 1, 0);
+    msg1->delOption(D6O_CLIENTID);
+    EXPECT_FALSE(state6_.reportRejectedLeaseUpdate(msg1));
+    EXPECT_FALSE(state6_.reportSuccessfulLeaseUpdate(msg1));
+}
+
+void
+CommunicationStateTest::getRejectedLeaseUpdatesCountFromContainerTest() {
+    // Create a simple multi index container with two indexes. The
+    // first index is on the ordinal number to distinguish between
+    // different entries. The second index is on the expire_ field
+    // that is identical to the expire_ field in the RejectedClients4
+    // and RejectedClients6 containers.
+    struct Entry {
+        int64_t ordinal_;
+        int64_t expire_;
+    };
+    typedef boost::multi_index_container<
+        Entry,
+        boost::multi_index::indexed_by<
+            boost::multi_index::ordered_unique<
+                boost::multi_index::member<Entry, int64_t,
+                                           &Entry::ordinal_>
+            >,
+            boost::multi_index::ordered_non_unique<
+                boost::multi_index::member<Entry, int64_t,
+                                           &Entry::expire_>
+            >
+        >
+    > Entries;
+    // Add many entries to the container. Odd entries have lifetime
+    // expiring in the future. Even entries have lifetimes expiring in
+    // the past.
+    Entries entries;
+    for (auto i = 0; i < 1000; i++) {
+        entries.insert({i, time(NULL) + (i % 2 ? 100 + i : -1 - i)});
+    }
+    // Get the count of valid entries. It should remove the expiring
+    // entries.
+    auto valid_entries_count = state_.getRejectedLeaseUpdatesCountFromContainer(entries);
+    EXPECT_EQ(500, valid_entries_count);
+    EXPECT_EQ(500, entries.size());
+
+    // Validate that we removed expired entries, not the valid ones.
+    for (auto entry : entries) {
+        EXPECT_EQ(1, entry.ordinal_ % 2);
+    }
+}
+
+TEST_F(CommunicationStateTest, partnerStateTest) {
+    partnerStateTest();
+}
+
+TEST_F(CommunicationStateTest, partnerStateTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    partnerStateTest();
+}
+
+TEST_F(CommunicationStateTest, partnerStateUnavailableTest) {
+    partnerStateUnavailableTest();
+}
+
+TEST_F(CommunicationStateTest, partnerStateUnavailableTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    partnerStateUnavailableTest();
+}
+
+TEST_F(CommunicationStateTest, partnerScopesTest) {
+    partnerScopesTest();
+}
+
+TEST_F(CommunicationStateTest, partnerScopesTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    partnerScopesTest();
+}
+
+TEST_F(CommunicationStateTest, initialDurationTest) {
+    initialDurationTest();
+}
+
+TEST_F(CommunicationStateTest, initialDurationTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    initialDurationTest();
+}
+
+TEST_F(CommunicationStateTest, pokeTest) {
+    pokeTest();
+}
+
+TEST_F(CommunicationStateTest, pokeTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    pokeTest();
+}
+
+TEST_F(CommunicationStateTest, heartbeatTest) {
+    heartbeatTest();
+}
+
+TEST_F(CommunicationStateTest, heartbeatTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    heartbeatTest();
+}
+
+TEST_F(CommunicationStateTest, startHeartbeatInvalidValuesTest) {
+    startHeartbeatInvalidValuesTest();
+}
+
+TEST_F(CommunicationStateTest, startHeartbeatInvalidValuesTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    startHeartbeatInvalidValuesTest();
+}
+
+TEST_F(CommunicationStateTest, detectFailureV4Test) {
+    detectFailureV4Test();
+}
+
+TEST_F(CommunicationStateTest, detectFailureV4TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    detectFailureV4Test();
+}
+
+TEST_F(CommunicationStateTest, failureDetectionDisabled4Test) {
+    failureDetectionDisabled4Test();
+}
+
+TEST_F(CommunicationStateTest, failureDetectionDisabled4TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    failureDetectionDisabled4Test();
+}
+
+TEST_F(CommunicationStateTest, detectFailureV6Test) {
+    detectFailureV6Test();
+}
+
+TEST_F(CommunicationStateTest, detectFailureV6TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    detectFailureV6Test();
+}
+
+TEST_F(CommunicationStateTest, failureDetectionDisabled6Test) {
+    failureDetectionDisabled6Test();
+}
+
+TEST_F(CommunicationStateTest, failureDetectionDisabled6TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    failureDetectionDisabled6Test();
+}
+
+TEST_F(CommunicationStateTest, clockSkewTest) {
+    clockSkewTest();
+}
+
+TEST_F(CommunicationStateTest, clockSkewTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    clockSkewTest();
+}
+
+TEST_F(CommunicationStateTest, clockSkewPartnerUnavailableTest) {
+    clockSkewPartnerUnavailableTest();
+}
+
+TEST_F(CommunicationStateTest, clockSkewPartnerUnavailableTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    clockSkewPartnerUnavailableTest();
+}
+
+TEST_F(CommunicationStateTest, rejectedLeaseUpdatesTerminateTest) {
+    rejectedLeaseUpdatesTerminateTest();
+}
+
+TEST_F(CommunicationStateTest, rejectedLeaseUpdatesTerminateTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    rejectedLeaseUpdatesTerminateTest();
+}
+
+TEST_F(CommunicationStateTest, logFormatClockSkewTest) {
+    logFormatClockSkewTest();
+}
+
+TEST_F(CommunicationStateTest, logFormatClockSkewTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    logFormatClockSkewTest();
+}
+
+TEST_F(CommunicationStateTest, getReportTest) {
+    getReportTest();
+}
+
+TEST_F(CommunicationStateTest, getReportTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    getReportTest();
+}
+
+TEST_F(CommunicationStateTest, getReportDefaultValuesTest) {
+    getReportDefaultValuesTest();
+}
+
+TEST_F(CommunicationStateTest, getReportDefaultValuesTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    getReportDefaultValuesTest();
+}
+
+TEST_F(CommunicationStateTest, getUnsentUpdateCountTest) {
+    getUnsentUpdateCountTest();
+}
+
+TEST_F(CommunicationStateTest, getUnsentUpdateCountTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    getUnsentUpdateCountTest();
+}
+
+TEST_F(CommunicationStateTest, hasPartnerNewUnsentUpdatesTest) {
+    hasPartnerNewUnsentUpdatesTest();
+}
+
+TEST_F(CommunicationStateTest, hasPartnerNewUnsentUpdatesTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hasPartnerNewUnsentUpdatesTest();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV4Test) {
+    reportRejectedLeasesV4Test();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV4TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    reportRejectedLeasesV4Test();
+}
+
+TEST_F(CommunicationStateTest, reportSuccessfulLeasesV4Test) {
+    reportSuccessfulLeasesV4Test();
+}
+
+TEST_F(CommunicationStateTest, reportSuccessfulLeasesV4TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    reportSuccessfulLeasesV4Test();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV4InvalidValuesTest) {
+    reportRejectedLeasesV4InvalidValuesTest();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV4InvalidValuesTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    reportRejectedLeasesV4InvalidValuesTest();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV6Test) {
+    reportRejectedLeasesV6Test();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV6TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    reportRejectedLeasesV6Test();
+}
+
+TEST_F(CommunicationStateTest, reportSuccessfulLeasesV6Test) {
+    reportSuccessfulLeasesV6Test();
+}
+
+TEST_F(CommunicationStateTest, reportSuccessfulLeasesV6TestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    reportSuccessfulLeasesV6Test();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV6InvalidValuesTest) {
+    reportRejectedLeasesV6InvalidValuesTest();
+}
+
+TEST_F(CommunicationStateTest, reportRejectedLeasesV6InvalidValuesTestMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    reportRejectedLeasesV6InvalidValuesTest();
+}
+
+TEST_F(CommunicationStateTest, getRejectedLeaseUpdatesCountFromContainerTest) {
+    getRejectedLeaseUpdatesCountFromContainerTest();
+}
+
+}
diff --git a/src/hooks/dhcp/high_availability/tests/ha_config_unittest.cc b/src/hooks/dhcp/high_availability/tests/ha_config_unittest.cc
new file mode 100644
index 0000000..f8e6940
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/ha_config_unittest.cc
@@ -0,0 +1,1879 @@
+// Copyright (C) 2017-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <ha_impl.h>
+#include <ha_service_states.h>
+#include <ha_test.h>
+#include <cc/command_interpreter.h>
+#include <cc/data.h>
+#include <cc/dhcp_config_error.h>
+#include <config/command_mgr.h>
+#include <util/state_model.h>
+#include <util/multi_threading_mgr.h>
+#include <testutils/gtest_utils.h>
+#include <string>
+
+using namespace isc;
+using namespace isc::config;
+using namespace isc::data;
+using namespace isc::ha;
+using namespace isc::hooks;
+using namespace isc::ha::test;
+using namespace isc::util;
+
+namespace {
+
+/// @brief Test fixture class for testing HA hooks library
+/// configuration.
+class HAConfigTest : public HATest {
+public:
+    /// @brief Constructor.
+    HAConfigTest() : HATest(), hardware_threads_(MultiThreadingMgr::detectThreadCount()) {
+    }
+
+    /// @brief Verifies if an exception is thrown if provided HA
+    /// configuration is invalid.
+    ///
+    /// @param invalid_config Configuration to be tested.
+    /// @param expected_error Expected error message.
+    void testInvalidConfig(const std::string& invalid_config,
+                           const std::string& expected_error) {
+        HAImplPtr impl(new HAImpl());
+        try {
+            impl->configure(Element::fromJSON(invalid_config));
+            ADD_FAILURE() << "expected ConfigError exception, thrown no exception";
+
+        } catch (const ConfigError& ex) {
+            // Expect the error to be contained in the exception message.
+            std::string const exception(ex.what());
+            EXPECT_NE(exception.find(expected_error), std::string::npos);
+        } catch (...) {
+            ADD_FAILURE() << "expected ConfigError exception, thrown different"
+                " exception type";
+        }
+    }
+
+    /// @brief number of threads the system reports as supported
+    uint32_t hardware_threads_;
+};
+
+// Verifies that load balancing configuration is parsed correctly.
+TEST_F(HAConfigTest, configureLoadBalancing) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"send-lease-updates\": false,"
+        "        \"sync-leases\": false,"
+        "        \"sync-timeout\": 20000,"
+        "        \"sync-page-limit\": 3,"
+        "        \"delayed-updates-limit\": 111,"
+        "        \"heartbeat-delay\": 8,"
+        "        \"max-response-delay\": 11,"
+        "        \"max-ack-delay\": 5,"
+        "        \"max-unacked-clients\": 20,"
+        "        \"max-rejected-lease-updates\": 9,"
+        "        \"wait-backup-ack\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"basic-auth-password\": \"1234\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"basic-auth-user\": \"\","
+        "                \"role\": \"secondary\""
+        "            },"
+        "            {"
+        "                \"name\": \"server3\","
+        "                \"url\": \"http://127.0.0.1:8082/\","
+        "                \"basic-auth-user\": \"foo\","
+        "                \"basic-auth-password\": \"bar\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": false"
+        "            }"
+        "        ],"
+        "        \"state-machine\": {"
+        "            \"states\": ["
+        "                {"
+        "                    \"state\": \"waiting\","
+        "                    \"pause\": \"once\""
+        "                },"
+        "                {"
+        "                    \"state\": \"ready\","
+        "                    \"pause\": \"always\""
+        "                },"
+        "                {"
+        "                    \"state\": \"partner-down\","
+        "                    \"pause\": \"never\""
+        "                }"
+        "            ]"
+        "        }"
+        "    }"
+        "]";
+
+    HAImplPtr impl(new HAImpl());
+    ASSERT_NO_THROW(impl->configure(Element::fromJSON(ha_config)));
+    EXPECT_EQ("server1", impl->getConfig()->getThisServerName());
+    EXPECT_EQ(HAConfig::LOAD_BALANCING, impl->getConfig()->getHAMode());
+    EXPECT_FALSE(impl->getConfig()->amSendingLeaseUpdates());
+    EXPECT_FALSE(impl->getConfig()->amSyncingLeases());
+    EXPECT_EQ(20000, impl->getConfig()->getSyncTimeout());
+    EXPECT_EQ(3, impl->getConfig()->getSyncPageLimit());
+    EXPECT_EQ(111, impl->getConfig()->getDelayedUpdatesLimit());
+    EXPECT_TRUE(impl->getConfig()->amAllowingCommRecovery());
+    EXPECT_EQ(8, impl->getConfig()->getHeartbeatDelay());
+    EXPECT_EQ(11, impl->getConfig()->getMaxResponseDelay());
+    EXPECT_EQ(5, impl->getConfig()->getMaxAckDelay());
+    EXPECT_EQ(20, impl->getConfig()->getMaxUnackedClients());
+    EXPECT_EQ(9, impl->getConfig()->getMaxRejectedLeaseUpdates());
+    EXPECT_FALSE(impl->getConfig()->amWaitingBackupAck());
+
+    HAConfig::PeerConfigPtr cfg = impl->getConfig()->getThisServerConfig();
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server1", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8080/", cfg->getUrl().toText());
+    EXPECT_EQ(cfg->getLogLabel(), "server1 (http://127.0.0.1:8080/)");
+    EXPECT_EQ(HAConfig::PeerConfig::PRIMARY, cfg->getRole());
+    EXPECT_FALSE(cfg->isAutoFailover());
+    EXPECT_FALSE(cfg->getBasicAuth());
+
+    cfg = impl->getConfig()->getPeerConfig("server2");
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server2", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8081/", cfg->getUrl().toText());
+    EXPECT_EQ(cfg->getLogLabel(), "server2 (http://127.0.0.1:8081/)");
+    EXPECT_EQ(HAConfig::PeerConfig::SECONDARY, cfg->getRole());
+    EXPECT_TRUE(cfg->isAutoFailover());
+    EXPECT_FALSE(cfg->getBasicAuth());
+
+    cfg = impl->getConfig()->getPeerConfig("server3");
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server3", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8082/", cfg->getUrl().toText());
+    EXPECT_EQ(cfg->getLogLabel(), "server3 (http://127.0.0.1:8082/)");
+    EXPECT_EQ(HAConfig::PeerConfig::BACKUP, cfg->getRole());
+    EXPECT_FALSE(cfg->isAutoFailover());
+    ASSERT_TRUE(cfg->getBasicAuth());
+    EXPECT_EQ("foo:bar", cfg->getBasicAuth()->getSecret());
+
+    // Verify that per-state configuration is correct.
+
+    HAConfig::StateConfigPtr state_cfg;
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_BACKUP_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_COMMUNICATION_RECOVERY_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_LOAD_BALANCING_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_PARTNER_DOWN_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_READY_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_ALWAYS, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_SYNCING_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_TERMINATED_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_WAITING_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_ONCE, state_cfg->getPausing());
+
+    // Verify multi-threading default values. Default is 0 for the listener and client threads, but
+    // after MT is applied, HAImpl resolves them to the auto-detected values.
+    EXPECT_TRUE(impl->getConfig()->getEnableMultiThreading());
+    EXPECT_TRUE(impl->getConfig()->getHttpDedicatedListener());
+    EXPECT_EQ(hardware_threads_, impl->getConfig()->getHttpListenerThreads());
+    EXPECT_EQ(hardware_threads_, impl->getConfig()->getHttpClientThreads());
+}
+
+// Verifies that hot standby configuration is parsed correctly.
+TEST_F(HAConfigTest, configureHotStandby) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"basic-auth-user\": \"admin\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            },"
+        "            {"
+        "                \"name\": \"server3\","
+        "                \"url\": \"http://127.0.0.1:8082/\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": false"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+
+    HAImplPtr impl(new HAImpl());
+    ASSERT_NO_THROW(impl->configure(Element::fromJSON(ha_config)));
+    EXPECT_EQ("server1", impl->getConfig()->getThisServerName());
+    EXPECT_EQ(HAConfig::HOT_STANDBY, impl->getConfig()->getHAMode());
+    EXPECT_TRUE(impl->getConfig()->amSendingLeaseUpdates());
+    EXPECT_TRUE(impl->getConfig()->amSyncingLeases());
+    EXPECT_EQ(60000, impl->getConfig()->getSyncTimeout());
+    EXPECT_EQ(10000, impl->getConfig()->getSyncPageLimit());
+    EXPECT_EQ(0, impl->getConfig()->getDelayedUpdatesLimit());
+    EXPECT_FALSE(impl->getConfig()->amAllowingCommRecovery());
+    EXPECT_EQ(10000, impl->getConfig()->getHeartbeatDelay());
+    EXPECT_EQ(10000, impl->getConfig()->getMaxAckDelay());
+    EXPECT_EQ(10, impl->getConfig()->getMaxUnackedClients());
+    EXPECT_EQ(10, impl->getConfig()->getMaxRejectedLeaseUpdates());
+    EXPECT_FALSE(impl->getConfig()->amWaitingBackupAck());
+
+    HAConfig::PeerConfigPtr cfg = impl->getConfig()->getThisServerConfig();
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server1", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8080/", cfg->getUrl().toText());
+    EXPECT_EQ(HAConfig::PeerConfig::PRIMARY, cfg->getRole());
+    EXPECT_FALSE(cfg->isAutoFailover());
+    ASSERT_TRUE(cfg->getBasicAuth());
+    EXPECT_EQ("admin:", cfg->getBasicAuth()->getSecret());
+
+    cfg = impl->getConfig()->getPeerConfig("server2");
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server2", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8081/", cfg->getUrl().toText());
+    EXPECT_EQ(HAConfig::PeerConfig::STANDBY, cfg->getRole());
+    EXPECT_TRUE(cfg->isAutoFailover());
+    EXPECT_FALSE(cfg->getBasicAuth());
+
+    cfg = impl->getConfig()->getPeerConfig("server3");
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server3", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8082/", cfg->getUrl().toText());
+    EXPECT_EQ(HAConfig::PeerConfig::BACKUP, cfg->getRole());
+    EXPECT_FALSE(cfg->isAutoFailover());
+    EXPECT_FALSE(cfg->getBasicAuth());
+
+    HAConfig::StateConfigPtr state_cfg;
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_BACKUP_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_HOT_STANDBY_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_PARTNER_DOWN_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_READY_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_SYNCING_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_TERMINATED_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    ASSERT_NO_THROW(state_cfg = impl->getConfig()->getStateMachineConfig()->
+                    getStateConfig(HA_WAITING_ST));
+    ASSERT_TRUE(state_cfg);
+    EXPECT_EQ(STATE_PAUSE_NEVER, state_cfg->getPausing());
+
+    // Verify multi-threading default values. Default is 0 for the listener and client threads, but
+    // after MT is applied, HAImpl resolves them to the auto-detected values.
+    EXPECT_TRUE(impl->getConfig()->getEnableMultiThreading());
+    EXPECT_TRUE(impl->getConfig()->getHttpDedicatedListener());
+    EXPECT_EQ(hardware_threads_, impl->getConfig()->getHttpListenerThreads());
+    EXPECT_EQ(hardware_threads_, impl->getConfig()->getHttpClientThreads());
+}
+
+// Verifies that passive-backup configuration is parsed correctly.
+TEST_F(HAConfigTest, configurePassiveBackup) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"wait-backup-ack\": true,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"backup\""
+        "            },"
+        "            {"
+        "                \"name\": \"server3\","
+        "                \"url\": \"http://127.0.0.1:8082/\","
+        "                \"basic-auth-user\": \"keatest\","
+        "                \"basic-auth-password-file\": \""
+                             TEST_HTTP_DIR "/hiddenp\","
+        "                \"role\": \"backup\""
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+
+    HAImplPtr impl(new HAImpl());
+    ASSERT_NO_THROW(impl->configure(Element::fromJSON(ha_config)));
+    EXPECT_EQ("server1", impl->getConfig()->getThisServerName());
+    EXPECT_EQ(HAConfig::PASSIVE_BACKUP, impl->getConfig()->getHAMode());
+    EXPECT_TRUE(impl->getConfig()->amSendingLeaseUpdates());
+    EXPECT_TRUE(impl->getConfig()->amWaitingBackupAck());
+
+    HAConfig::PeerConfigPtr cfg = impl->getConfig()->getThisServerConfig();
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server1", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8080/", cfg->getUrl().toText());
+    EXPECT_EQ(HAConfig::PeerConfig::PRIMARY, cfg->getRole());
+    EXPECT_FALSE(cfg->getBasicAuth());
+
+    cfg = impl->getConfig()->getPeerConfig("server2");
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server2", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8081/", cfg->getUrl().toText());
+    EXPECT_EQ(HAConfig::PeerConfig::BACKUP, cfg->getRole());
+    EXPECT_FALSE(cfg->getBasicAuth());
+
+    cfg = impl->getConfig()->getPeerConfig("server3");
+    ASSERT_TRUE(cfg);
+    EXPECT_EQ("server3", cfg->getName());
+    EXPECT_EQ("http://127.0.0.1:8082/", cfg->getUrl().toText());
+    EXPECT_EQ(HAConfig::PeerConfig::BACKUP, cfg->getRole());
+    ASSERT_TRUE(cfg->getBasicAuth());
+    EXPECT_EQ("a2VhdGVzdDpLZWFUZXN0", cfg->getBasicAuth()->getCredential());
+
+    // Verify multi-threading default values. Default is 0 for the listener and client threads, but
+    // after MT is applied, HAImpl resolves them to the auto-detected values.
+    EXPECT_TRUE(impl->getConfig()->getEnableMultiThreading());
+    EXPECT_TRUE(impl->getConfig()->getHttpDedicatedListener());
+    EXPECT_EQ(hardware_threads_, impl->getConfig()->getHttpListenerThreads());
+    EXPECT_EQ(hardware_threads_, impl->getConfig()->getHttpClientThreads());
+}
+
+// This server name must not be empty.
+TEST_F(HAConfigTest, emptyServerName) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'this-server-name' value must not be empty");
+}
+
+// There must be a configuration provided for this server.
+TEST_F(HAConfigTest, nonMatchingServerName) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"foo\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "no peer configuration specified for the 'foo'");
+}
+
+// Error should be returned when mode is invalid.
+TEST_F(HAConfigTest, unsupportedMode) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"unsupported-mode\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "unsupported value 'unsupported-mode' for mode parameter");
+}
+
+// Error should be returned when heartbeat-delay is negative.
+TEST_F(HAConfigTest, negativeHeartbeatDelay) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"heartbeat-delay\": -1,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'heartbeat-delay' must not be negative");
+}
+
+// Error should be returned when heartbeat-delay is too large.
+TEST_F(HAConfigTest, largeHeartbeatDelay) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"heartbeat-delay\": 65536,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'heartbeat-delay' must not be greater than 65535");
+}
+
+// There must be at least two servers provided.
+TEST_F(HAConfigTest, singlePeer) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "secondary server required in the load balancing configuration");
+}
+
+// Server name must not be empty.
+TEST_F(HAConfigTest, emptyPeerName) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "peer name must not be empty");
+}
+
+// Can't have two servers with the same name.
+TEST_F(HAConfigTest, duplicatePeerName) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "peer with name 'server1' already specified");
+}
+
+// URL must be valid.
+TEST_F(HAConfigTest, invalidURL) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "invalid URL: url ://127.0.0.1:8080/ lacks http or"
+        " https scheme for server server2");
+}
+
+// URL hostname must be an address.
+TEST_F(HAConfigTest, badURLName) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://localhost:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "bad url 'http://localhost:8080/': "
+        "Failed to convert string to address 'localhost': "
+        "Invalid argument for server server2");
+}
+
+// URL HTTPS scheme is not supported.
+TEST_F(HAConfigTest, badURLHttps) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"https://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "bad url 'https://127.0.0.1:8080/': "
+        "https scheme is not supported for server server2 "
+        "where TLS is disabled");
+}
+
+// Only certain roles are allowed.
+TEST_F(HAConfigTest, unsupportedRole) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"unsupported\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "unsupported value 'unsupported' for role parameter");
+}
+
+// There must be exactly one primary server.
+TEST_F(HAConfigTest, twoPrimary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "multiple primary servers specified");
+}
+
+// There must be exactly one secondary server.
+TEST_F(HAConfigTest, twoSecondary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "multiple secondary servers specified");
+}
+
+// Only one standby server is allowed.
+TEST_F(HAConfigTest, twoStandby) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "multiple standby servers specified");
+}
+
+// Primary server is required for load balancing.
+TEST_F(HAConfigTest, loadBalancingNoPrimary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "primary server required in the load balancing configuration");
+}
+
+// Secondary server is required for load balancing.
+TEST_F(HAConfigTest, loadBalancingNoSecondary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "secondary server required in the load balancing configuration");
+}
+
+// Primary server is required for hot standby mode.
+TEST_F(HAConfigTest, hotStandbyNoPrimary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "primary server required in the hot standby configuration");
+}
+
+// Standby server is required for hot standby mode.
+TEST_F(HAConfigTest, hotStandbyNoStandby) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "standby server required in the hot standby configuration");
+}
+
+// Standby server must not be specified in the load balancing mode.
+TEST_F(HAConfigTest, loadBalancingStandby) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "standby servers not allowed in the load balancing configuration");
+}
+
+// Secondary server must not be specified in the hot standby mode.
+TEST_F(HAConfigTest, hotStandbySecondary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "secondary servers not allowed in the hot standby configuration");
+}
+
+// state-machine parameter must be a map.
+TEST_F(HAConfigTest, invalidStateMachine) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ],"
+        "        \"state-machine\": ["
+        "            {"
+        "                \"state\": \"foo\","
+        "                \"pause\": \"always\""
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'state-machine' parameter must be a map");
+}
+
+// states within state-machine must be a list.
+TEST_F(HAConfigTest, invalidStatesList) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ],"
+        "        \"state-machine\": {"
+        "            \"states\": {"
+        "            }"
+        "        }"
+        "    }"
+        "]",
+        "'states' parameter must be a list");
+}
+
+// State name must be recognized.
+TEST_F(HAConfigTest, invalidStateName) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ],"
+        "        \"state-machine\": {"
+        "            \"states\": ["
+        "                {"
+        "                    \"state\": \"foo\","
+        "                    \"pause\": \"always\""
+        "                }"
+        "            ]"
+        "        }"
+        "    }"
+        "]",
+        "unknown state foo");
+}
+
+// Pause value must be recognized.
+TEST_F(HAConfigTest, invalidPauseValue) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ],"
+        "        \"state-machine\": {"
+        "            \"states\": ["
+        "                {"
+        "                    \"state\": \"waiting\","
+        "                    \"pause\": \"foo\""
+        "                }"
+        "            ]"
+        "        }"
+        "    }"
+        "]",
+        "unsupported value foo of 'pause' parameter");
+}
+
+// Must not specify configuration for the same state twice.
+TEST_F(HAConfigTest, duplicatedStates) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ],"
+        "        \"state-machine\": {"
+        "            \"states\": ["
+        "                {"
+        "                    \"state\": \"waiting\","
+        "                    \"pause\": \"always\""
+        "                },"
+        "                {"
+        "                    \"state\": \"ready\","
+        "                    \"pause\": \"always\""
+        "                },"
+        "                {"
+        "                    \"state\": \"waiting\","
+        "                    \"pause\": \"always\""
+        "                }"
+        "            ]"
+        "        }"
+        "    }"
+        "]",
+        "duplicated configuration for the 'waiting' state");
+}
+
+// Test that wait-backup-ack must not be enabled in the load-balancing
+// configuration.
+TEST_F(HAConfigTest, waitBackupAckLoadBalancing) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"wait-backup-ack\": true,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'wait-backup-ack' must be set to false in the load balancing configuration");
+}
+
+// Test that wait-backup-ack must not be enabled in the hot-standby
+// configuration.
+TEST_F(HAConfigTest, waitBackupAckHotStandby) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"wait-backup-ack\": true,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'wait-backup-ack' must be set to false in the hot standby configuration");
+}
+
+// Test that secondary server is not allowed in the passive-backup mode.
+TEST_F(HAConfigTest, passiveBackupSecondaryServer) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"secondary\""
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "secondary servers not allowed in the passive backup configuration");
+}
+
+// Test that standby server is not allowed in the passive-backup mode.
+TEST_F(HAConfigTest, passiveBackupStandbyServer) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"standby\""
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "standby servers not allowed in the passive backup configuration");
+}
+
+// Test that primary server is required in the passive-backup mode.
+TEST_F(HAConfigTest, passiveBackupNoPrimary) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"backup\""
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "primary server required in the passive backup configuration");
+}
+
+// Test that empty name id is forbidden for basic HTTP authentication.
+TEST_F(HAConfigTest, invalidUser) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"basic-auth-user\": \"foo:bar\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "user 'foo:bar' must not contain a ':' in peer 'server2'");
+}
+
+// Test that setting delayed-updates-limit is not allowed in hot-standby mode.
+TEST_F(HAConfigTest, hotStandbyDelayedUpdatesLimit) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"hot-standby\","
+        "        \"delayed-updates-limit\": 1,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"standby\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'delayed-updates-limit' must be set to 0 in the hot standby configuration");
+}
+
+// Test that setting delayed-updates-limit is not allowed in passive-backup mode.
+TEST_F(HAConfigTest, passiveBackupDelayedUpdatesLimit) {
+    testInvalidConfig(
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"delayed-updates-limit\": 1,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\""
+        "            }"
+        "        ]"
+        "    }"
+        "]",
+        "'delayed-updates-limit' must be set to 0 in the passive backup configuration");
+}
+
+#if (defined(WITH_OPENSSL) || defined(WITH_BOTAN_BOOST))
+/// Test that TLS parameters are correctly inherited.
+TEST_F(HAConfigTest, tlsParameterInheritance) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"my-server\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "        \"cert-file\": \"!CA!/kea-client.crt\","
+        "        \"key-file\": \"!CA!/kea-client.key\","
+        "        \"require-client-certs\": false,"
+        "        \"restrict-commands\": true,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"my-server\","
+        "                \"url\": \"https://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"overwrite\","
+        "                \"trust-anchor\": \"!CA!\","
+        "                \"cert-file\": \"!CA!/kea-server.crt\","
+        "                \"key-file\": \"!CA!/kea-server.key\","
+        "                \"url\": \"https://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            },"
+        "            {"
+        "                \"name\": \"disable\","
+        "                \"trust-anchor\": \"\","
+        "                \"cert-file\": \"\","
+        "                \"key-file\": \"\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"backup\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    HAImplPtr impl(new HAImpl());
+    ASSERT_NO_THROW(impl->configure(Element::fromJSON(patched)));
+
+    // Check the global parameters.
+    std::string expected;
+    EXPECT_FALSE(impl->getConfig()->getTrustAnchor().unspecified());
+    expected = TEST_CA_DIR;
+    expected += "/kea-ca.crt";
+    EXPECT_EQ(expected, impl->getConfig()->getTrustAnchor().get());
+    EXPECT_FALSE(impl->getConfig()->getCertFile().unspecified());
+    expected = TEST_CA_DIR;
+    expected += "/kea-client.crt";
+    EXPECT_EQ(expected, impl->getConfig()->getCertFile().get());
+    EXPECT_FALSE(impl->getConfig()->getKeyFile().unspecified());
+    expected = TEST_CA_DIR;
+    expected += "/kea-client.key";
+    EXPECT_EQ(expected, impl->getConfig()->getKeyFile().get());
+    EXPECT_FALSE(impl->getConfig()->getRequireClientCerts());
+    EXPECT_TRUE(impl->getConfig()->getRestrictCommands());
+
+    // Check the first peer parameters: it inherits them from the global level.
+    HAConfig::PeerConfigPtr cfg = impl->getConfig()->getThisServerConfig();
+    ASSERT_TRUE(cfg);
+    EXPECT_TRUE(cfg->getTlsContext());
+
+    // Check the second peer parameters: it overwrites them.
+    cfg = impl->getConfig()->getPeerConfig("overwrite");
+    ASSERT_TRUE(cfg);
+    EXPECT_FALSE(cfg->getTrustAnchor().unspecified());
+    expected = TEST_CA_DIR;
+    EXPECT_EQ(expected, cfg->getTrustAnchor().get());
+    EXPECT_FALSE(cfg->getCertFile().unspecified());
+    expected = TEST_CA_DIR;
+    expected += "/kea-server.crt";
+    EXPECT_EQ(expected, cfg->getCertFile().get());
+    EXPECT_FALSE(cfg->getKeyFile().unspecified());
+    expected = TEST_CA_DIR;
+    expected += "/kea-server.key";
+    EXPECT_EQ(expected, cfg->getKeyFile().get());
+    EXPECT_TRUE(cfg->getTlsContext());
+
+    // Check the last peer parameters: it disables TLS by setting them to "".
+    cfg = impl->getConfig()->getPeerConfig("disable");
+    ASSERT_TRUE(cfg);
+    EXPECT_FALSE(cfg->getTrustAnchor().unspecified());
+    EXPECT_EQ("", cfg->getTrustAnchor().get());
+    EXPECT_FALSE(cfg->getCertFile().unspecified());
+    EXPECT_EQ("", cfg->getCertFile().get());
+    EXPECT_FALSE(cfg->getKeyFile().unspecified());
+    EXPECT_EQ("", cfg->getKeyFile().get());
+    // The TLS context should be null.
+    EXPECT_FALSE(cfg->getTlsContext());
+}
+
+// Test that a missing trust-anchor in the HTTPS parameter set raise an error.
+TEST_F(HAConfigTest, missingTrustAnchor) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "        \"cert-file\": \"!CA!/kea-client.crt\","
+        "        \"key-file\": \"!CA!/kea-client.key\","
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"trust-anchor\": \"\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    std::string expected = "bad TLS config for server server2: ";
+    expected += "trust-anchor parameter is missing or empty: ";
+    expected += "all or none of TLS parameters must be set";
+    testInvalidConfig(patched, expected);
+}
+
+// Test that a missing cert-file in the HTTPS parameter set raise an error.
+TEST_F(HAConfigTest, missingCertFile) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "        \"cert-file\": \"!CA!/kea-client.crt\","
+        "        \"key-file\": \"!CA!/kea-client.key\","
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"cert-file\": \"\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    std::string expected = "bad TLS config for server server2: ";
+    expected += "cert-file parameter is missing or empty: ";
+    expected += "all or none of TLS parameters must be set";
+    testInvalidConfig(patched, expected);
+}
+
+// Test that a missing key-file in the HTTPS parameter set raise an error.
+TEST_F(HAConfigTest, missingKeyFile) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "        \"cert-file\": \"!CA!/kea-client.crt\","
+        "        \"key-file\": \"!CA!/kea-client.key\","
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"key-file\": \"\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    std::string expected = "bad TLS config for server server2: ";
+    expected += "key-file parameter is missing or empty: ";
+    expected += "all or none of TLS parameters must be set";
+    testInvalidConfig(patched, expected);
+}
+
+// Test that a bad trust-anchor in the HTTPS parameter set raise an error.
+TEST_F(HAConfigTest, badTrustAnchor) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"/this-file-does-not-exist\","
+        "        \"cert-file\": \"!CA!/kea-client.crt\","
+        "        \"key-file\": \"!CA!/kea-client.key\","
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    std::string expected = "bad TLS config for server server1: ";
+    expected += "load of CA file '/this-file-does-not-exist' failed: ";
+    // Backend dependent.
+#ifdef WITH_OPENSSL
+    expected += "No such file or directory";
+#else
+    expected += "I/O error: DataSource: Failure opening file /this-file-does-not-exist";
+#endif
+    testInvalidConfig(patched, expected);
+}
+
+// Test that a bad cert-file in the HTTPS parameter set raise an error.
+TEST_F(HAConfigTest, badCertFile) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "        \"cert-file\": \"/this-file-does-not-exist\","
+        "        \"key-file\": \"!CA!/kea-client.key\","
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    std::string expected = "bad TLS config for server server1: ";
+    expected += "load of cert file '/this-file-does-not-exist' failed: ";
+    // Backend dependent.
+#ifdef WITH_OPENSSL
+    expected += "No such file or directory";
+#else
+    expected += "I/O error: DataSource: Failure opening file /this-file-does-not-exist";
+#endif
+    testInvalidConfig(patched, expected);
+}
+
+// Test that a bad key-file in the HTTPS parameter set raise an error.
+TEST_F(HAConfigTest, badKeyFile) {
+    const std::string ha_config =
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"load-balancing\","
+        "        \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "        \"cert-file\": \"!CA!/kea-client.crt\","
+        "        \"key-file\": \"/this-file-does-not-exist\","
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"auto-failover\": false"
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"secondary\","
+        "                \"auto-failover\": true"
+        "            }"
+        "        ]"
+        "    }"
+        "]";
+    const std::string& patched = replaceInConfig(ha_config, "!CA!",
+                                                 TEST_CA_DIR);
+    std::string expected = "bad TLS config for server server1: ";
+    expected += "load of private key file '/this-file-does-not-exist' failed: ";
+    // Backend dependent.
+#ifdef WITH_OPENSSL
+    expected += "No such file or directory";
+#else
+    expected += "I/O error: DataSource: Failure opening file /this-file-does-not-exist";
+#endif
+    testInvalidConfig(patched, expected);
+}
+#endif // WITH_OPENSSL || WITH_BOTAN_BOOST
+
+// Test that conversion of the role names works correctly.
+TEST_F(HAConfigTest, stringToRole) {
+    EXPECT_EQ(HAConfig::PeerConfig::PRIMARY,
+              HAConfig::PeerConfig::stringToRole("primary"));
+    EXPECT_EQ(HAConfig::PeerConfig::SECONDARY,
+              HAConfig::PeerConfig::stringToRole("secondary"));
+    EXPECT_EQ(HAConfig::PeerConfig::STANDBY,
+              HAConfig::PeerConfig::stringToRole("standby"));
+    EXPECT_EQ(HAConfig::PeerConfig::BACKUP,
+              HAConfig::PeerConfig::stringToRole("backup"));
+    EXPECT_THROW(HAConfig::PeerConfig::stringToRole("unsupported"),
+                 BadValue);
+}
+
+// Test that role name is generated correctly.
+TEST_F(HAConfigTest, roleToString) {
+    EXPECT_EQ("primary",
+              HAConfig::PeerConfig::roleToString(HAConfig::PeerConfig::PRIMARY));
+    EXPECT_EQ("secondary",
+              HAConfig::PeerConfig::roleToString(HAConfig::PeerConfig::SECONDARY));
+    EXPECT_EQ("standby",
+              HAConfig::PeerConfig::roleToString(HAConfig::PeerConfig::STANDBY));
+    EXPECT_EQ("backup",
+              HAConfig::PeerConfig::roleToString(HAConfig::PeerConfig::BACKUP));
+}
+
+// Test that conversion of the HA mode names works correctly.
+TEST_F(HAConfigTest, stringToHAMode) {
+    EXPECT_EQ(HAConfig::LOAD_BALANCING, HAConfig::stringToHAMode("load-balancing"));
+    EXPECT_EQ(HAConfig::HOT_STANDBY, HAConfig::stringToHAMode("hot-standby"));
+    EXPECT_EQ(HAConfig::PASSIVE_BACKUP, HAConfig::stringToHAMode("passive-backup"));
+}
+
+// Test that HA mode name is generated correctly.
+TEST_F(HAConfigTest, HAModeToString) {
+    EXPECT_EQ("load-balancing", HAConfig::HAModeToString(HAConfig::LOAD_BALANCING));
+    EXPECT_EQ("hot-standby", HAConfig::HAModeToString(HAConfig::HOT_STANDBY));
+    EXPECT_EQ("passive-backup", HAConfig::HAModeToString(HAConfig::PASSIVE_BACKUP));
+}
+
+// Test that conversion of the 'pause' value works correctly.
+TEST_F(HAConfigTest, stringToPausing) {
+    EXPECT_EQ(STATE_PAUSE_ALWAYS,
+              HAConfig::StateConfig::stringToPausing("always"));
+    EXPECT_EQ(STATE_PAUSE_NEVER,
+              HAConfig::StateConfig::stringToPausing("never"));
+    EXPECT_EQ(STATE_PAUSE_ONCE,
+              HAConfig::StateConfig::stringToPausing("once"));
+}
+
+// Test that pause parameter value is generated correctly.
+TEST_F(HAConfigTest, pausingToString) {
+    EXPECT_EQ("always",
+              HAConfig::StateConfig::pausingToString(STATE_PAUSE_ALWAYS));
+    EXPECT_EQ("never",
+              HAConfig::StateConfig::pausingToString(STATE_PAUSE_NEVER));
+    EXPECT_EQ("once",
+              HAConfig::StateConfig::pausingToString(STATE_PAUSE_ONCE));
+}
+
+// Verifies permutations of HA+MT configuration.
+TEST_F(HAConfigTest, multiThreadingPermutations) {
+
+    // Structure describing a test scenario.
+    struct Scenario {
+        std::string desc_;              // Description of the scenario.
+        std::string mt_json_;           // multi-threading config to use.
+        bool dhcp_mt_enabled_;          // True if DHCP multi-threading is enabled.
+        uint32_t dhcp_threads_;         // Value of DHCP thread-pool-size.
+        bool exp_ha_mt_enabled_;        // If HA+MT should be enabled
+        bool exp_listener_;             // If HA+MT should use dedicated listener.
+        uint32_t exp_listener_threads_; // Expected number of listener threads.
+        uint32_t exp_client_threads_;   // Expected number of client threads.
+    };
+
+    // Mnemonic constants.
+    bool dhcp_mt = true;
+    bool ha_mt = true;
+    bool listener = true;
+
+    std::vector<Scenario> scenarios {
+        {
+            "1 ha+mt by default",
+            "",
+            dhcp_mt, 4,
+            ha_mt, listener, 4, 4
+        },
+        {
+            "2 dhcp mt enabled, ha mt disabled",
+            makeHAMtJson(!ha_mt, !listener, 0, 0),
+            dhcp_mt, 4,
+            !ha_mt, !listener, 0, 0
+        },
+        {
+            "3 dhcp mt disabled, ha mt enabled",
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            !dhcp_mt, 4,
+            !ha_mt, !listener, 0, 0
+        },
+        {
+            "4 dhcp mt enabled, ha mt enabled, listener disabled",
+            makeHAMtJson(ha_mt, !listener, 0, 0),
+            dhcp_mt, 4,
+            ha_mt, !listener, 4, 4
+        },
+        {
+            "5 dhcp mt enabled, ha mt enabled, listener enabled",
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            dhcp_mt, 4,
+            ha_mt, listener, 4, 4
+        },
+        {
+            "6 explicit DHCP threads, explicit thread values",
+            makeHAMtJson(ha_mt, listener, 5, 6),
+            dhcp_mt, 4,
+            ha_mt, listener, 5, 6
+        },
+        {
+            "7 explicit DHCP threads, zero thread values",
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            dhcp_mt, 8,
+            ha_mt, listener, 8, 8
+        },
+        {
+            "8 DHCP auto detect threads, zero thread values",
+            // Special case: if system reports supported threads as 0
+            // then HA+MT should be disabled.  Otherwise it should
+            // be enabled with listener and client threads set to the
+            // reported value.
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            dhcp_mt, 0,
+            (hardware_threads_ > 0), listener, hardware_threads_, hardware_threads_
+        }
+    };
+
+    // Iterate over the scenarios.
+    for (auto const& scenario : scenarios) {
+        SCOPED_TRACE(scenario.desc_);
+
+        // Build the HA JSON configuration.
+        std::stringstream ss;
+        ss <<
+            "["
+            "    {"
+            "        \"this-server-name\": \"server1\","
+            "        \"mode\": \"passive-backup\","
+            "        \"wait-backup-ack\": true,"
+            "        \"peers\": ["
+            "            {"
+            "                \"name\": \"server1\","
+            "                \"url\": \"http://127.0.0.1:8080/\","
+            "                \"role\": \"primary\""
+            "            },"
+            "            {"
+            "                \"name\": \"server2\","
+            "                \"url\": \"http://127.0.0.1:8081/\","
+            "                \"role\": \"backup\""
+            "            }"
+            "        ]";
+
+        if (!scenario.mt_json_.empty()) {
+            ss << "," << scenario.mt_json_;
+        }
+
+        ss << "}]";
+        ConstElementPtr config_json;
+        ASSERT_NO_THROW_LOG(config_json = Element::fromJSON(ss.str()));
+
+        // Set DHCP multi-threading configuration in CfgMgr.
+        setDHCPMultiThreadingConfig(scenario.dhcp_mt_enabled_, scenario.dhcp_threads_);
+
+        // Create and configure the implementation.
+        HAImplPtr impl(new HAImpl());
+        ASSERT_NO_THROW_LOG(impl->configure(config_json));
+
+        // Fetch the updated config.
+        HAConfigPtr ha_config = impl->getConfig();
+
+        // Verify the configuration is as expected.
+        if (!scenario.exp_ha_mt_enabled_) {
+            // When HA+MT is disabled, the other values are moot.
+            ASSERT_FALSE(ha_config->getEnableMultiThreading());
+        } else {
+            ASSERT_TRUE(ha_config->getEnableMultiThreading());
+            EXPECT_EQ(ha_config->getHttpDedicatedListener(), scenario.exp_listener_);
+            EXPECT_EQ(ha_config->getHttpListenerThreads(), scenario.exp_listener_threads_);
+            EXPECT_EQ(ha_config->getHttpClientThreads(), scenario.exp_client_threads_);
+       }
+    }
+}
+
+// Check that an IPv6 address can be used as part of a value for "url".
+TEST_F(HAConfigTest, ipv6Url) {
+    std::string const ha_config(R"(
+        [
+            {
+                "mode": "load-balancing",
+                "peers": [
+                    {
+                        "name": "server1",
+                        "role": "primary",
+                        "url": "http://[2001:db8::1]:8080/"
+                    },
+                    {
+                        "name": "server2",
+                        "role": "secondary",
+                        "url": "http://[2001:db8::2]:8080/"
+                    }
+                ],
+                "this-server-name": "server1"
+            }
+        ]
+    )");
+
+    // Configure HA.
+    HAImplPtr impl(new HAImpl());
+    ASSERT_NO_THROW_LOG(impl->configure(Element::fromJSON(ha_config)));
+
+    // Check the URL.
+    EXPECT_EQ(impl->getConfig()->getThisServerConfig()->getUrl().toText(), "http://[2001:db8::1]:8080/");
+}
+
+}  // namespace
diff --git a/src/hooks/dhcp/high_availability/tests/ha_impl_unittest.cc b/src/hooks/dhcp/high_availability/tests/ha_impl_unittest.cc
new file mode 100644
index 0000000..9cfe46c
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/ha_impl_unittest.cc
@@ -0,0 +1,814 @@
+// Copyright (C) 2018-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <ha_test.h>
+#include <ha_impl.h>
+#include <asiolink/io_address.h>
+#include <cc/command_interpreter.h>
+#include <cc/data.h>
+#include <dhcp/dhcp4.h>
+#include <dhcp/dhcp6.h>
+#include <dhcp/hwaddr.h>
+#include <dhcpsrv/lease.h>
+#include <dhcpsrv/network_state.h>
+#include <hooks/hooks_manager.h>
+#include <testutils/gtest_utils.h>
+#include <boost/pointer_cast.hpp>
+#include <gtest/gtest.h>
+#include <string>
+
+using namespace isc::asiolink;
+using namespace isc::config;
+using namespace isc::data;
+using namespace isc::dhcp;
+using namespace isc::ha;
+using namespace isc::ha::test;
+using namespace isc::hooks;
+
+namespace {
+
+/// @brief Structure that holds registered hook indexes.
+///
+/// This allows us to park packets.
+struct TestHooks {
+    /// @brief Index of leases4_committed callout.
+    int hook_index_leases4_committed_;
+
+    /// @brief Index of leases6_committed callout.
+    int hook_index_leases6_committed_;
+
+    /// @brief Constructor
+    ///
+    /// The constructor registers hook points for callout tests.
+    TestHooks() {
+        hook_index_leases4_committed_ =
+            HooksManager::registerHook("leases4_committed");
+        hook_index_leases6_committed_ =
+            HooksManager::registerHook("leases6_committed");
+    }
+};
+
+TestHooks test_hooks;
+
+/// @brief Derivation of the @c HAImpl which provides access to protected
+/// methods and members.
+class TestHAImpl : public HAImpl {
+public:
+
+    using HAImpl::config_;
+    using HAImpl::service_;
+};
+
+/// @brief Test fixture class for @c HAImpl.
+class HAImplTest : public HATest {
+public:
+
+    /// @brief Tests handler of a ha-sync command.
+    ///
+    /// It always expects that the error result is returned. The expected
+    /// error text should be provided as function argument.
+    ///
+    /// @param ha_sync_command command provided as text.
+    /// @param expected_response expected text response.
+    void testSynchronizeHandler(const std::string& ha_sync_command,
+                                const std::string& expected_response) {
+        HAImpl ha_impl;
+        ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+        // Starting the service is required prior to running any callouts.
+        NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+        ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                             HAServerType::DHCPv4));
+
+        ConstElementPtr command = Element::fromJSON(ha_sync_command);
+
+        CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+        callout_handle->setArgument("command", command);
+
+        ASSERT_NO_THROW(ha_impl.synchronizeHandler(*callout_handle));
+
+        ConstElementPtr response;
+        callout_handle->getArgument("response", response);
+        ASSERT_TRUE(response);
+
+        checkAnswer(response, CONTROL_RESULT_ERROR, expected_response);
+    }
+};
+
+// Tests that HAService object is created for DHCPv4 service.
+TEST_F(HAImplTest, startService) {
+    // Valid configuration must be provided prior to starting the service.
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Network state is also required.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+
+    // Start the service for DHCPv4 server.
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    // Make sure that the HA service has been created for the requested
+    // server type.
+    ASSERT_TRUE(ha_impl.service_);
+    EXPECT_EQ(HAServerType::DHCPv4, ha_impl.service_->getServerType());
+}
+
+// Tests that HAService object is created for DHCPv6 service.
+TEST_F(HAImplTest, startService6) {
+    // Valid configuration must be provided prior to starting the service.
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Network state is also required.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv6));
+
+    // Start the service for DHCPv4 server.
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv6));
+
+    // Make sure that the HA service has been created for the requested
+    // server type.
+    ASSERT_TRUE(ha_impl.service_);
+    EXPECT_EQ(HAServerType::DHCPv6, ha_impl.service_->getServerType());
+}
+
+// Tests for buffer4_receive callout implementation.
+TEST_F(HAImplTest, buffer4Receive) {
+    // Use hot-standby mode to make sure that this server instance is selected
+    // to process each received query. This is going to give predictable results.
+    ConstElementPtr ha_config = createValidJsonConfiguration(HAConfig::HOT_STANDBY);
+
+    // Create implementation object and configure it.
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(ha_config));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    // Initially the HA service is in the waiting state and serves no scopes.
+    // We need to explicitly enable the scope to be served.
+    ha_impl.service_->serveDefaultScopes();
+
+    // Create callout handle to be used for passing arguments to the
+    // callout.
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    ASSERT_TRUE(callout_handle);
+
+    // Create the BOOTP message. We can use it for testing message parsing
+    // failure case because BOOTP is not supported. We will later turn it
+    // into the DHCP message to test successful parsing.
+    std::vector<uint8_t> msg = {
+        1, // BOOTREQUEST
+        1, // ethernet
+        6, // HW address length = 6
+        0, // hops = 0
+        1, 2, 3, 4, // xid
+        0, 0, // secs = 0
+        0, 0, // flags
+        0, 0, 0, 0, // ciaddr = 0
+        0, 0, 0, 0, // yiaddr = 0
+        0, 0, 0, 0, // siaddr = 0
+        0, 0, 0, 0, // giaddr = 0
+        1, 2, 3, 4, 5, 6, // chaddr
+    };
+
+    // fill chaddr reminder, sname and file with zeros
+    msg.insert(msg.end(), 10 + 128 + 64, 0);
+
+    // Create DHCPv4 message object from the BOOTP message. This should be
+    // successful because the message is not parsed yet.
+    Pkt4Ptr query4(new Pkt4(&msg[0], msg.size()));
+
+    // Set buffer4_receive callout arguments.
+    callout_handle->setArgument("query4", query4);
+
+    // Invoke the buffer4_receive callout.
+    ASSERT_NO_THROW(ha_impl.buffer4Receive(*callout_handle));
+
+    // The BOOTP messages are not supported so trying to unpack the message
+    // should trigger an error. The callout should set the next step to
+    // DROP treating the message as malformed.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_DROP, callout_handle->getStatus());
+    // Malformed message should not be classified.
+    EXPECT_TRUE(query4->getClasses().empty());
+
+    // Turn this into the DHCP message by appending a magic cookie and the
+    // options.
+    std::vector<uint8_t> magic_cookie = {
+        99, 130, 83, 99
+    };
+
+    // Provide DHCP message type option, truncated vendor option and domain name.
+    // Parsing this message should be successful but vendor option should be
+    // skipped.
+    std::vector<uint8_t> options = {
+        53, 1, 1, // Message type = DHCPDISCOVER
+        125, 6, // vendor options
+            1, 2, 3, 4, // enterprise id
+            8, 1, // data len 8 but the actual length is 1 (truncated options)
+        15, 3, 'a', 'b', 'c' // Domain name = abc
+    };
+
+    // Append the magic cookie and the options to our BOOTP message.
+    msg.insert(msg.end(), magic_cookie.begin(), magic_cookie.end());
+    msg.insert(msg.end(), options.begin(), options.end());
+
+    // Create new query and pass it to the callout.
+    query4.reset(new Pkt4(&msg[0], msg.size()));
+    callout_handle->setArgument("query4", query4);
+
+    // Invoke the callout again.
+    ASSERT_NO_THROW(ha_impl.buffer4Receive(*callout_handle));
+
+    // This time the callout should set the next step to SKIP to indicate to
+    // the DHCP server that the message has been already parsed.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_SKIP, callout_handle->getStatus());
+
+    // The client class should be assigned to the message to indicate that the
+    // server1 should process this message.
+    ASSERT_EQ(1, query4->getClasses().size());
+    EXPECT_TRUE(query4->inClass("HA_server1"));
+
+    // Check that the message has been parsed. The DHCP message type should
+    // be set in this case.
+    EXPECT_EQ(DHCPDISCOVER, static_cast<int>(query4->getType()));
+    // Vendor option should be skipped because it was truncated.
+    EXPECT_FALSE(query4->getOption(DHO_VIVSO_SUBOPTIONS));
+    // Domain name should not be skipped because the vendor option was truncated.
+    EXPECT_TRUE(query4->getOption(DHO_DOMAIN_NAME));
+}
+
+// Tests for buffer6_receive callout implementation.
+TEST_F(HAImplTest, buffer6Receive) {
+    // Use hot-standby mode to make sure that this server instance is selected
+    // to process each received query. This is going to give predictable results.
+    ConstElementPtr ha_config = createValidJsonConfiguration(HAConfig::HOT_STANDBY);
+
+    // Create implementation object and configure it.
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(ha_config));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv6));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv6));
+
+    // Initially the HA service is in the waiting state and serves no scopes.
+    // We need to explicitly enable the scope to be served.
+    ha_impl.service_->serveDefaultScopes();
+
+    // Create callout handle to be used for passing arguments to the
+    // callout.
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    ASSERT_TRUE(callout_handle);
+
+    // Create DHCPv6 message. It initially has no transaction id so should be
+    // considered malformed.
+    std::vector<uint8_t> msg = {
+        1, // Solicit
+    };
+
+    // Create DHCPv4 message object from the BOOTP message. This should be
+    // successful because the message is not parsed yet.
+    Pkt6Ptr query6(new Pkt6(&msg[0], msg.size()));
+
+    // Set buffer6_receive callout arguments.
+    callout_handle->setArgument("query6", query6);
+
+    // Invoke the buffer6_receive callout.
+    ASSERT_NO_THROW(ha_impl.buffer6Receive(*callout_handle));
+
+    // Our DHCP messages contains no transaction id so it should cause
+    // parsing error. The next step is set to DROP for malformed messages.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_DROP, callout_handle->getStatus());
+    // Malformed message should not be classified.
+    EXPECT_TRUE(query6->getClasses().empty());
+
+    // Append transaction id (3 bytes, each set to 1).
+    msg.insert(msg.end(), 3, 1);
+
+    // Include 3 options in the DHCPv6 message: ORO, truncated vendor option
+    // and the NIS Domain Name option. This should be parsed correctly but the
+    // last option should be skipped because of the preceding option being
+    // truncated.
+    std::vector<uint8_t> options = {
+        0, 6, 0, 2, 0, 29, // option ORO requesting option 29
+        0, 17, // vendor options
+        0, 9, // option length = 10
+            1, 2, 3, 4, // enterprise id
+            0, 1, 0, 10, // code 1, (invalid) length = 10
+            1, // ONLY 1 byte of data (truncated)
+        0, 29, 0, 3, 'a', 'b', 'c' // NIS Domain Name = abc
+    };
+
+    msg.insert(msg.end(), options.begin(), options.end());
+
+    // Create new query and pass it to the callout.
+    query6.reset(new Pkt6(&msg[0], msg.size()));
+    callout_handle->setArgument("query6", query6);
+
+    // Invoke the callout again.
+    ASSERT_NO_THROW(ha_impl.buffer6Receive(*callout_handle));
+
+    // This time the callout should set the next step to SKIP to indicate to
+    // the DHCP server that the message has been already parsed.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_SKIP, callout_handle->getStatus());
+
+    // The client class should be assigned to the message to indicate that the
+    // server1 should process this message.
+    ASSERT_EQ(1, query6->getClasses().size());
+    EXPECT_TRUE(query6->inClass("HA_server1"));
+
+    // Check that the message has been parsed. The DHCP message type should
+    // be set in this case.
+    EXPECT_EQ(DHCPV6_SOLICIT, static_cast<int>(query6->getType()));
+    // Domain name should be skipped because the vendor option was truncated.
+    EXPECT_FALSE(query6->getOption(D6O_NIS_DOMAIN_NAME));
+}
+
+// Tests leases4_committed callout implementation.
+TEST_F(HAImplTest, leases4Committed) {
+    // Create implementation object and configure it.
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    // Make sure we wait for the acks from the backup server to be able to
+    // test the case of sending lease updates even though the service is
+    // in the state in which the lease updates are normally not sent.
+    ha_impl.config_->setWaitBackupAck(true);
+
+    // Create callout handle to be used for passing arguments to the
+    // callout.
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    ASSERT_TRUE(callout_handle);
+
+    // Set the hook index so we can park packets.
+    callout_handle->setCurrentHook(test_hooks.hook_index_leases4_committed_);
+
+    // query4
+    Pkt4Ptr query4 = createMessage4(DHCPREQUEST, 1, 0, 0);
+    callout_handle->setArgument("query4", query4);
+
+    // leases4
+    Lease4CollectionPtr leases4(new Lease4Collection());
+    callout_handle->setArgument("leases4", leases4);
+
+    // deleted_leases4
+    Lease4CollectionPtr deleted_leases4(new Lease4Collection());
+    callout_handle->setArgument("deleted_leases4", deleted_leases4);
+
+    // Set initial status.
+    callout_handle->setStatus(CalloutHandle::NEXT_STEP_CONTINUE);
+
+    // Park the packet.
+    HooksManager::park("leases4_committed", query4, []{});
+
+    // There are no leases so the callout should return.
+    ASSERT_NO_THROW(ha_impl.leases4Committed(*callout_handle));
+
+    // No updates are generated so the default status should not be modified.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_CONTINUE, callout_handle->getStatus());
+    EXPECT_TRUE(callout_handle->getParkingLotHandlePtr()->drop(query4));
+
+    // Create a lease and pass it to the callout, but temporarily disable lease
+    // updates.
+    HWAddrPtr hwaddr(new HWAddr(std::vector<uint8_t>(6, 1), HTYPE_ETHER));
+    Lease4Ptr lease4(new Lease4(IOAddress("192.1.2.3"), hwaddr,
+                                static_cast<const uint8_t*>(0), 0,
+                                60, 0, 1));
+    leases4->push_back(lease4);
+    callout_handle->setArgument("leases4", leases4);
+
+    ha_impl.config_->setSendLeaseUpdates(false);
+
+    // Park the packet.
+    HooksManager::park("leases4_committed", query4, []{});
+
+    // Run the callout again.
+    ASSERT_NO_THROW(ha_impl.leases4Committed(*callout_handle));
+
+    // No updates are generated so the default status should not be modified.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_CONTINUE, callout_handle->getStatus());
+    EXPECT_TRUE(callout_handle->getParkingLotHandlePtr()->drop(query4));
+
+    // Enable updates and retry.
+    ha_impl.config_->setSendLeaseUpdates(true);
+    callout_handle->setArgument("leases4", leases4);
+
+    // Park the packet.
+    HooksManager::park("leases4_committed", query4, []{});
+
+    // Run the callout again.
+    ASSERT_NO_THROW(ha_impl.leases4Committed(*callout_handle));
+
+    // This time the lease update should be generated and the status should
+    // be set to "park".
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_PARK, callout_handle->getStatus());
+    EXPECT_TRUE(callout_handle->getParkingLotHandlePtr()->drop(query4));
+}
+
+// Tests leases6_committed callout implementation.
+TEST_F(HAImplTest, leases6Committed) {
+    // Create implementation object and configure it.
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv6));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv6));
+
+    // Make sure we wait for the acks from the backup server to be able to
+    // test the case of sending lease updates even though the service is
+    // in the state in which the lease updates are normally not sent.
+    ha_impl.config_->setWaitBackupAck(true);
+
+    // Create callout handle to be used for passing arguments to the
+    // callout.
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    ASSERT_TRUE(callout_handle);
+
+    // Set the hook index so we can park packets.
+    callout_handle->setCurrentHook(test_hooks.hook_index_leases6_committed_);
+
+    // query6
+    Pkt6Ptr query6 = createMessage6(DHCPV6_REQUEST, 1, 0);
+    callout_handle->setArgument("query6", query6);
+
+    // leases6
+    Lease6CollectionPtr leases6(new Lease6Collection());
+    callout_handle->setArgument("leases6", leases6);
+
+    // deleted_leases6
+    Lease6CollectionPtr deleted_leases6(new Lease6Collection());
+    callout_handle->setArgument("deleted_leases6", deleted_leases6);
+
+    // Set initial status.
+    callout_handle->setStatus(CalloutHandle::NEXT_STEP_CONTINUE);
+
+    // Park the packet.
+    HooksManager::park("leases6_committed", query6, []{});
+
+    // There are no leases so the callout should return.
+    ASSERT_NO_THROW(ha_impl.leases6Committed(*callout_handle));
+
+    // No updates are generated so the default status should not be modified.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_CONTINUE, callout_handle->getStatus());
+    EXPECT_TRUE(callout_handle->getParkingLotHandlePtr()->drop(query6));
+
+    // Create a lease and pass it to the callout, but temporarily disable lease
+    // updates.
+    DuidPtr duid(new DUID(std::vector<uint8_t>(8, 2)));
+    Lease6Ptr lease6(new Lease6(Lease::TYPE_NA, IOAddress("2001:db8:1::cafe"), duid,
+                                1234, 50, 60, 1));
+    leases6->push_back(lease6);
+    callout_handle->setArgument("leases6", leases6);
+
+    ha_impl.config_->setSendLeaseUpdates(false);
+
+    // Park the packet.
+    HooksManager::park("leases6_committed", query6, []{});
+
+    // Run the callout again.
+    ASSERT_NO_THROW(ha_impl.leases6Committed(*callout_handle));
+
+    // No updates are generated so the default status should not be modified.
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_CONTINUE, callout_handle->getStatus());
+    EXPECT_TRUE(callout_handle->getParkingLotHandlePtr()->drop(query6));
+
+    // Enable updates and retry.
+    ha_impl.config_->setSendLeaseUpdates(true);
+    callout_handle->setArgument("leases6", leases6);
+
+    // Park the packet.
+    HooksManager::park("leases6_committed", query6, []{});
+
+    // Run the callout again.
+    ASSERT_NO_THROW(ha_impl.leases6Committed(*callout_handle));
+
+    // This time the lease update should be generated and the status should
+    // be set to "park".
+    EXPECT_EQ(CalloutHandle::NEXT_STEP_PARK, callout_handle->getStatus());
+    EXPECT_TRUE(callout_handle->getParkingLotHandlePtr()->drop(query6));
+}
+
+// Tests ha-sync command handler with correct and incorrect arguments.
+TEST_F(HAImplTest, synchronizeHandler) {
+    {
+        // This syntax is correct. The error returned is simply a result of
+        // trying to connect to the server which is offline, which should
+        // result in connection refused error.
+        SCOPED_TRACE("Correct syntax");
+        testSynchronizeHandler("{"
+                               "    \"command\": \"ha-sync\","
+                               "    \"arguments\": {"
+                               "        \"server-name\": \"server2\""
+                               "    }"
+                               "}", "Connection refused");
+    }
+
+    {
+        SCOPED_TRACE("No arguments");
+        testSynchronizeHandler("{"
+                               "    \"command\": \"ha-sync\""
+                               "}", "arguments not found in the 'ha-sync' command");
+    }
+
+    {
+        SCOPED_TRACE("No server name");
+        testSynchronizeHandler("{"
+                               "    \"command\": \"ha-sync\","
+                               "    \"arguments\": {"
+                               "        \"max-period\": 20"
+                               "    }"
+                               "}", "'server-name' is mandatory for the 'ha-sync' command");
+    }
+
+    {
+        SCOPED_TRACE("Server name is not a string");
+        testSynchronizeHandler("{"
+                               "    \"command\": \"ha-sync\","
+                               "    \"arguments\": {"
+                               "        \"server-name\": 20"
+                               "    }"
+                               "}", "'server-name' must be a string in the 'ha-sync' command");
+    }
+
+    {
+        SCOPED_TRACE("Max period is not a number");
+        testSynchronizeHandler("{"
+                               "    \"command\": \"ha-sync\","
+                               "    \"arguments\": {"
+                               "        \"server-name\": \"server2\","
+                               "        \"max-period\": \"20\""
+                               "    }"
+                               "}", "'max-period' must be a positive integer in the 'ha-sync'"
+                               " command");
+    }
+
+    {
+        SCOPED_TRACE("Max period must be positive");
+        testSynchronizeHandler("{"
+                               "    \"command\": \"ha-sync\","
+                               "    \"arguments\": {"
+                               "        \"server-name\": \"server2\","
+                               "        \"max-period\": \"20\""
+                               "    }"
+                               "}", "'max-period' must be a positive integer in the 'ha-sync'"
+                               " command");
+    }
+
+}
+
+// Tests ha-continue command handler.
+TEST_F(HAImplTest, continueHandler) {
+    HAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    ConstElementPtr command = Element::fromJSON("{ \"command\": \"ha-continue\" }");
+
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    callout_handle->setArgument("command", command);
+
+    ASSERT_NO_THROW(ha_impl.continueHandler(*callout_handle));
+
+    ConstElementPtr response;
+    callout_handle->getArgument("response", response);
+    ASSERT_TRUE(response);
+
+    checkAnswer(response, CONTROL_RESULT_SUCCESS, "HA state machine is not paused.");
+}
+
+// Tests status-get command processed handler.
+TEST_F(HAImplTest, statusGet) {
+    HAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    std::string name = "status-get";
+    ConstElementPtr response =
+        Element::fromJSON("{ \"arguments\": { \"pid\": 1 }, \"result\": 0 }");
+
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+
+    callout_handle->setArgument("name", name);
+    callout_handle->setArgument("response", response);
+
+    ASSERT_NO_THROW(ha_impl.commandProcessed(*callout_handle));
+
+    ConstElementPtr got;
+    callout_handle->getArgument("response", got);
+    ASSERT_TRUE(got);
+
+    std::string expected =
+        "{"
+        "    \"arguments\": {"
+        "        \"high-availability\": ["
+        "            {"
+        "                \"ha-mode\": \"load-balancing\","
+        "                \"ha-servers\": {"
+        "                    \"local\": {"
+        "                        \"role\": \"primary\","
+        "                        \"scopes\": [  ],"
+        "                        \"state\": \"waiting\""
+        "                    },"
+        "                    \"remote\": {"
+        "                        \"age\": 0,"
+        "                        \"in-touch\": false,"
+        "                        \"last-scopes\": [ ],"
+        "                        \"last-state\": \"\","
+        "                        \"role\": \"secondary\","
+        "                        \"communication-interrupted\": false,"
+        "                        \"connecting-clients\": 0,"
+        "                        \"unacked-clients\": 0,"
+        "                        \"unacked-clients-left\": 0,"
+        "                        \"analyzed-packets\": 0"
+        "                    }"
+        "                }"
+        "            }"
+        "        ],"
+        "        \"pid\": 1"
+        "    },"
+        "    \"result\": 0"
+        "}";
+    EXPECT_TRUE(isEquivalent(got, Element::fromJSON(expected)));
+}
+
+// Tests status-get command processed handler for backup server.
+TEST_F(HAImplTest, statusGetBackupServer) {
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+    ha_impl.config_->setThisServerName("server3");
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    std::string name = "status-get";
+    ConstElementPtr response =
+        Element::fromJSON("{ \"arguments\": { \"pid\": 1 }, \"result\": 0 }");
+
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+
+    callout_handle->setArgument("name", name);
+    callout_handle->setArgument("response", response);
+
+    ASSERT_NO_THROW(ha_impl.commandProcessed(*callout_handle));
+
+    ConstElementPtr got;
+    callout_handle->getArgument("response", got);
+    ASSERT_TRUE(got);
+
+    std::string expected =
+        "{"
+        "    \"arguments\": {"
+        "        \"high-availability\": ["
+        "            {"
+        "                \"ha-mode\": \"load-balancing\","
+        "                \"ha-servers\": {"
+        "                    \"local\": {"
+        "                        \"role\": \"backup\","
+        "                        \"scopes\": [  ],"
+        "                        \"state\": \"backup\""
+        "                    }"
+        "                }"
+        "            }"
+        "        ],"
+        "        \"pid\": 1"
+        "    },"
+        "    \"result\": 0"
+        "}";
+    EXPECT_TRUE(isEquivalent(got, Element::fromJSON(expected)));
+}
+
+// Tests status-get command processed handler for primary server being in the
+// passive-backup state.
+TEST_F(HAImplTest, statusGetPassiveBackup) {
+    TestHAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidPassiveBackupJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    std::string name = "status-get";
+    ConstElementPtr response =
+        Element::fromJSON("{ \"arguments\": { \"pid\": 1 }, \"result\": 0 }");
+
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+
+    callout_handle->setArgument("name", name);
+    callout_handle->setArgument("response", response);
+
+    ASSERT_NO_THROW(ha_impl.commandProcessed(*callout_handle));
+
+    ConstElementPtr got;
+    callout_handle->getArgument("response", got);
+    ASSERT_TRUE(got);
+
+    std::string expected =
+        "{"
+        "    \"arguments\": {"
+        "        \"high-availability\": ["
+        "            {"
+        "                \"ha-mode\": \"passive-backup\","
+        "                \"ha-servers\": {"
+        "                    \"local\": {"
+        "                        \"role\": \"primary\","
+        "                        \"scopes\": [ \"server1\" ],"
+        "                        \"state\": \"passive-backup\""
+        "                    }"
+        "                }"
+        "            }"
+        "        ],"
+        "        \"pid\": 1"
+        "    },"
+        "    \"result\": 0"
+        "}";
+    EXPECT_TRUE(isEquivalent(got, Element::fromJSON(expected)));
+}
+
+// Test ha-maintenance-notify command handler.
+TEST_F(HAImplTest, maintenanceNotify) {
+    HAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    ConstElementPtr command = Element::fromJSON(
+        "{"
+        "    \"command\": \"ha-maintenance-notify\","
+        "    \"arguments\": {"
+        "        \"cancel\": false"
+        "    }"
+         "}"
+    );
+
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    callout_handle->setArgument("command", command);
+
+    ASSERT_NO_THROW(ha_impl.maintenanceNotifyHandler(*callout_handle));
+
+    ConstElementPtr response;
+    callout_handle->getArgument("response", response);
+    ASSERT_TRUE(response);
+
+    checkAnswer(response, CONTROL_RESULT_SUCCESS, "Server is in-maintenance state.");
+}
+
+// Test ha-reset command handler.
+TEST_F(HAImplTest, haReset) {
+    HAImpl ha_impl;
+    ASSERT_NO_THROW(ha_impl.configure(createValidJsonConfiguration()));
+
+    // Starting the service is required prior to running any callouts.
+    NetworkStatePtr network_state(new NetworkState(NetworkState::DHCPv4));
+    ASSERT_NO_THROW(ha_impl.startService(io_service_, network_state,
+                                         HAServerType::DHCPv4));
+
+    ConstElementPtr command = Element::fromJSON(
+        "{"
+        "    \"command\": \"ha-reset\""
+        "}"
+    );
+
+    CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+    callout_handle->setArgument("command", command);
+
+    ASSERT_NO_THROW(ha_impl.haResetHandler(*callout_handle));
+
+    ConstElementPtr response;
+    callout_handle->getArgument("response", response);
+    ASSERT_TRUE(response);
+
+    checkAnswer(response, CONTROL_RESULT_SUCCESS, "HA state machine already in WAITING state.");
+}
+
+}
diff --git a/src/hooks/dhcp/high_availability/tests/ha_mt_unittest.cc b/src/hooks/dhcp/high_availability/tests/ha_mt_unittest.cc
new file mode 100644
index 0000000..8aac8b0
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/ha_mt_unittest.cc
@@ -0,0 +1,561 @@
+// Copyright (C) 2021-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <asiolink/asio_wrapper.h>
+#include <config/cmd_response_creator.h>
+#include <ha_test.h>
+#include <ha_config.h>
+#include <ha_service.h>
+
+#include <util/multi_threading_mgr.h>
+#include <testutils/gtest_utils.h>
+
+#include <gtest/gtest.h>
+
+using namespace isc::asiolink;
+using namespace isc::config;
+using namespace isc::data;
+using namespace isc::dhcp;
+
+using namespace isc::ha;
+using namespace isc::ha::test;
+using namespace isc::http;
+using namespace isc::util;
+
+namespace {
+
+/// @brief Derivation of the @c HAService which provides access to
+/// protected methods and members.
+class TestHAService : public HAService {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// @param io_service Pointer to the IO service used by the DHCP server.
+    /// @param network_state Object holding state of the DHCP service
+    /// (enabled/disabled).
+    /// @param config Parsed HA hook library configuration.
+    /// @param server_type Server type, i.e. DHCPv4 or DHCPv6 server.
+    TestHAService(const IOServicePtr& io_service,
+                  const NetworkStatePtr& network_state,
+                  const HAConfigPtr& config,
+                  const HAServerType& server_type = HAServerType::DHCPv4)
+        : HAService(io_service, network_state, config, server_type) {
+    }
+
+    /// @brief Test version of the @c HAService::runModel.
+    ///
+    /// The original implementation of this method returns control when
+    /// @c NOP_EVT is found. This implementation runs a
+    /// single handler to allow the tests to verify if the state machine
+    /// transitions to an expected state before it is run again.
+    virtual void runModel(unsigned int event) {
+        try {
+            postNextEvent(event);
+            getState(getCurrState())->run();
+
+        } catch (const std::exception& ex) {
+            abortModel(ex.what());
+        }
+    }
+
+    /// @brief Schedules asynchronous "dhcp-disable" command to the specified
+    /// server.
+    ///
+    /// This variant of the method uses default HTTP client for communication.
+    ///
+    /// @param server_name name of the server to which the command should be
+    /// sent.
+    /// @param max_period maximum number of seconds for which the DHCP service
+    /// should be disabled.
+    /// @param post_request_action pointer to the function to be executed when
+    /// the request is completed.
+    void asyncDisableDHCPService(const std::string& server_name,
+                                 const unsigned int max_period,
+                                 const PostRequestCallback& post_request_action) {
+        HAService::asyncDisableDHCPService(*client_, server_name, max_period,
+                                           post_request_action);
+    }
+
+    /// @brief Schedules asynchronous "dhcp-enable" command to the specified
+    /// server.
+    ///
+    /// This variant of the method uses default HTTP client for communication.
+    ///
+    /// @param server_name name of the server to which the command should be
+    /// sent.
+    /// @param post_request_action pointer to the function to be executed when
+    /// the request is completed.
+    void asyncEnableDHCPService(const std::string& server_name,
+                                const PostRequestCallback& post_request_action) {
+        HAService::asyncEnableDHCPService(*client_, server_name, post_request_action);
+    }
+
+    using HAService::asyncSendHeartbeat;
+    using HAService::asyncSyncLeases;
+    using HAService::postNextEvent;
+    using HAService::transition;
+    using HAService::verboseTransition;
+    using HAService::shouldSendLeaseUpdates;
+    using HAService::shouldQueueLeaseUpdates;
+    using HAService::pendingRequestSize;
+    using HAService::getPendingRequest;
+    using HAService::network_state_;
+    using HAService::config_;
+    using HAService::communication_state_;
+    using HAService::query_filter_;
+    using HAService::lease_update_backlog_;
+    using HAService::client_;
+    using HAService::listener_;
+};
+
+/// @brief Pointer to the @c TestHAService.
+typedef boost::shared_ptr<TestHAService> TestHAServicePtr;
+
+/// @brief Test fixture class for @c HAService multi-threading.
+class HAMtServiceTest : public HATest {
+public:
+
+    /// @brief Constructor.
+    HAMtServiceTest()
+        : HATest() {
+        MultiThreadingMgr::instance().setMode(true);
+        CmdResponseCreator::command_accept_list_.clear();
+    }
+
+    /// @brief Destructor.
+    ///
+    /// Stops all test servers.
+    ~HAMtServiceTest() {
+        io_service_->get_io_service().reset();
+        io_service_->poll();
+        MultiThreadingMgr::instance().setMode(false);
+        CmdResponseCreator::command_accept_list_.clear();
+    }
+
+    /// @brief Callback function invoke upon test timeout.
+    ///
+    /// It stops the IO service and reports test timeout.
+    ///
+    /// @param fail_on_timeout Specifies if test failure should be reported.
+    void timeoutHandler(const bool fail_on_timeout) {
+        if (fail_on_timeout) {
+            ADD_FAILURE() << "Timeout occurred while running the test!";
+        }
+        io_service_->stop();
+    }
+};
+
+// Verifies HA+MT start, pause, resume, and stop. Note
+// that pause and resume are tested indirectly through
+// entry and exit of a critical section.
+TEST_F(HAMtServiceTest, multiThreadingBasics) {
+
+    // Build the HA JSON configuration.
+    std::stringstream ss;
+    ss <<
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"wait-backup-ack\": true,"
+        "        \"restrict-commands\": true,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"http://127.0.0.1:8080/\","
+        "                \"role\": \"primary\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"http://127.0.0.1:8081/\","
+        "                \"role\": \"backup\""
+        "            }"
+        "        ]";
+
+    // Enable MT, listener, and 3 threads for both client and listener.
+    ss << "," << makeHAMtJson(true, true, 3, 3) << "}]";
+    ConstElementPtr config_json;
+    ASSERT_NO_THROW_LOG(config_json = Element::fromJSON(ss.str()));
+
+    // Enable DHCP multi-threading configuration in CfgMgr with 3 threads.
+    setDHCPMultiThreadingConfig(true, 3);
+
+    // Create the HA configuration
+    HAConfigPtr ha_config(new HAConfig());
+    HAConfigParser parser;
+    ASSERT_NO_THROW_LOG(parser.parse(ha_config, config_json));
+
+    // Instantiate the service.
+    TestHAServicePtr service;
+    ASSERT_NO_THROW_LOG(service.reset(new TestHAService(io_service_, network_state_,
+                                                        ha_config)));
+    // Multi-threading should be enabled.
+    ASSERT_TRUE(ha_config->getEnableMultiThreading());
+
+    // Command filtering is enabled.
+    EXPECT_FALSE(CmdResponseCreator::command_accept_list_.empty());
+
+    // Now we'll start, pause, resume and stop a few times.
+    for (int i = 0; i < 3; ++i) {
+        // Verify we're stopped.
+        // Client should exist but be stopped.
+        ASSERT_TRUE(service->client_);
+        ASSERT_TRUE(service->client_->isStopped());
+        if (i == 0) {
+            EXPECT_FALSE(service->client_->getThreadIOService()->stopped());
+        } else {
+            EXPECT_TRUE(service->client_->getThreadIOService()->stopped());
+        }
+
+        // Listener should exist but be stopped.
+        ASSERT_TRUE(service->listener_);
+        ASSERT_TRUE(service->listener_->isStopped());
+        EXPECT_FALSE(service->listener_->getThreadIOService());
+
+        // Start client and listener.
+        ASSERT_NO_THROW_LOG(service->startClientAndListener());
+
+        // Verify we've started.
+        // Client should be running.
+        ASSERT_TRUE(service->client_->isRunning());
+        ASSERT_TRUE(service->client_->getThreadIOService());
+        EXPECT_FALSE(service->client_->getThreadIOService()->stopped());
+        EXPECT_EQ(service->client_->getThreadPoolSize(), 3);
+        EXPECT_EQ(service->client_->getThreadCount(), 3);
+
+        // Listener should be running.
+        ASSERT_TRUE(service->listener_->isRunning());
+        ASSERT_TRUE(service->listener_->getThreadIOService());
+        EXPECT_FALSE(service->listener_->getThreadIOService()->stopped());
+        EXPECT_EQ(service->listener_->getThreadPoolSize(), 3);
+        EXPECT_EQ(service->listener_->getThreadCount(), 3);
+
+        {
+            // Entering a critical section should pause both client
+            // and listener.
+            MultiThreadingCriticalSection cs;
+
+            // Client should be paused.
+            ASSERT_TRUE(service->client_->isPaused());
+            EXPECT_TRUE(service->client_->getThreadIOService()->stopped());
+
+            // Listener should be paused.
+            ASSERT_TRUE(service->listener_->isPaused());
+            EXPECT_TRUE(service->listener_->getThreadIOService()->stopped());
+        }
+
+        // Exiting critical section should resume both client
+        // and listener.
+
+        // Client should be running.
+        ASSERT_TRUE(service->client_->isRunning());
+        EXPECT_FALSE(service->client_->getThreadIOService()->stopped());
+
+        // Listener should be running.
+        ASSERT_TRUE(service->listener_->isRunning());
+        EXPECT_FALSE(service->listener_->getThreadIOService()->stopped());
+
+        // Stop should succeed.
+        ASSERT_NO_THROW_LOG(service->stopClientAndListener());
+    }
+}
+
+// Verifies multiThreadingBasics can be extended to use HTTPS/TLS>
+TEST_F(HAMtServiceTest, multiThreadingTls) {
+
+    // Build the HA JSON configuration.
+    std::stringstream ss;
+    ss <<
+        "["
+        "    {"
+        "        \"this-server-name\": \"server1\","
+        "        \"mode\": \"passive-backup\","
+        "        \"wait-backup-ack\": true,"
+        "        \"require-client-certs\": false,"
+        "        \"peers\": ["
+        "            {"
+        "                \"name\": \"server1\","
+        "                \"url\": \"https://127.0.0.1:8080/\","
+        "                \"role\": \"primary\","
+        "                \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "                \"cert-file\": \"!CA!/kea-server.crt\","
+        "                \"key-file\": \"!CA!/kea-server.key\""
+        "            },"
+        "            {"
+        "                \"name\": \"server2\","
+        "                \"url\": \"https://127.0.0.1:8081/\","
+        "                \"role\": \"backup\","
+        "                \"trust-anchor\": \"!CA!/kea-ca.crt\","
+        "                \"cert-file\": \"!CA!/kea-client.crt\","
+        "                \"key-file\": \"!CA!/kea-client.key\""
+        "            }"
+        "        ]";
+
+    // Enable MT, listener, and 3 threads for both client and listener.
+    ss << "," << makeHAMtJson(true, true, 3, 3) << "}]";
+    ConstElementPtr config_json;
+    const std::string& patched = replaceInConfig(ss.str(), "!CA!",
+                                                 TEST_CA_DIR);
+    ASSERT_NO_THROW_LOG(config_json = Element::fromJSON(patched));
+
+    // Enable DHCP multi-threading configuration in CfgMgr with 3 threads.
+    setDHCPMultiThreadingConfig(true, 3);
+
+    // Create the HA configuration
+    HAConfigPtr ha_config(new HAConfig());
+    HAConfigParser parser;
+    ASSERT_NO_THROW_LOG(parser.parse(ha_config, config_json));
+
+    // Instantiate the service.
+    TestHAServicePtr service;
+    ASSERT_NO_THROW_LOG(service.reset(new TestHAService(io_service_, network_state_,
+                                                        ha_config)));
+    // Multi-threading should be enabled.
+    ASSERT_TRUE(ha_config->getEnableMultiThreading());
+
+    // Now we'll start, pause, resume and stop a few times.
+    for (int i = 0; i < 3; ++i) {
+        // Verify we're stopped.
+        // Client should exist but be stopped.
+        ASSERT_TRUE(service->client_);
+        ASSERT_TRUE(service->client_->isStopped());
+        if (i == 0) {
+            EXPECT_FALSE(service->client_->getThreadIOService()->stopped());
+        } else {
+            EXPECT_TRUE(service->client_->getThreadIOService()->stopped());
+        }
+
+        // Listener should exist but be stopped.
+        ASSERT_TRUE(service->listener_);
+        ASSERT_TRUE(service->listener_->isStopped());
+        EXPECT_FALSE(service->listener_->getThreadIOService());
+
+        // Start client and listener.
+        ASSERT_NO_THROW_LOG(service->startClientAndListener());
+
+        // Verify we've started.
+        // Client should be running.
+        ASSERT_TRUE(service->client_->isRunning());
+        ASSERT_TRUE(service->client_->getThreadIOService());
+        EXPECT_FALSE(service->client_->getThreadIOService()->stopped());
+        EXPECT_EQ(service->client_->getThreadPoolSize(), 3);
+        EXPECT_EQ(service->client_->getThreadCount(), 3);
+
+        // Listener should be running.
+        ASSERT_TRUE(service->listener_->isRunning());
+        ASSERT_TRUE(service->listener_->getThreadIOService());
+        EXPECT_FALSE(service->listener_->getThreadIOService()->stopped());
+        EXPECT_EQ(service->listener_->getThreadPoolSize(), 3);
+        EXPECT_EQ(service->listener_->getThreadCount(), 3);
+
+        {
+            // Entering a critical section should pause both client
+            // and listener.
+            MultiThreadingCriticalSection cs;
+
+            // Client should be paused.
+            ASSERT_TRUE(service->client_->isPaused());
+            EXPECT_TRUE(service->client_->getThreadIOService()->stopped());
+
+            // Listener should be paused.
+            ASSERT_TRUE(service->listener_->isPaused());
+            EXPECT_TRUE(service->listener_->getThreadIOService()->stopped());
+        }
+
+        // Exiting critical section should resume both client
+        // and listener.
+
+        // Client should be running.
+        ASSERT_TRUE(service->client_->isRunning());
+        EXPECT_FALSE(service->client_->getThreadIOService()->stopped());
+
+        // Listener should be running.
+        ASSERT_TRUE(service->listener_->isRunning());
+        EXPECT_FALSE(service->listener_->getThreadIOService()->stopped());
+
+        // Stop should succeed.
+        ASSERT_NO_THROW_LOG(service->stopClientAndListener());
+    }
+}
+
+// Verifies permutations of HA+MT configuration and start-up.
+TEST_F(HAMtServiceTest, multiThreadingConfigStartup) {
+
+    // Structure describing a test scenario.
+    struct Scenario {
+        std::string desc_;              // Description of the scenario.
+        std::string mt_json_;           // multi-threading config to use.
+        bool dhcp_mt_enabled_;          // True if DHCP multi-threading is enabled.
+        uint32_t dhcp_threads_;         // Value of DHCP thread-pool-size.
+        bool exp_ha_mt_enabled_;        // If HA+MT should be enabled
+        bool exp_listener_;             // If HA+MT should use dedicated listener.
+        uint32_t exp_listener_threads_; // Expected number of listener threads.
+        uint32_t exp_client_threads_;   // Expected number of client threads.
+    };
+
+    // Mnemonic constants.
+    bool dhcp_mt = true;
+    bool ha_mt = true;
+    bool listener = true;
+
+    // Number of threads the system reports as supported.
+    uint32_t sys_threads = MultiThreadingMgr::detectThreadCount();
+
+    std::vector<Scenario> scenarios {
+        {
+            "1 ha+mt by default",
+            "",
+            dhcp_mt, 4,
+            ha_mt, listener, 4, 4
+        },
+        {
+            "2 dhcp mt enabled, ha mt disabled",
+            makeHAMtJson(!ha_mt, !listener, 0, 0),
+            dhcp_mt, 4,
+            !ha_mt, !listener, 0, 0
+        },
+        {
+            "3 dhcp mt disabled, ha mt enabled",
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            !dhcp_mt, 4,
+            !ha_mt, !listener, 0, 0
+        },
+        {
+            "4 dhcp mt enabled, ha mt enabled, listener disabled",
+            makeHAMtJson(ha_mt, !listener, 0, 0),
+            dhcp_mt, 4,
+            ha_mt, !listener, 4, 4
+        },
+        {
+            "5 dhcp mt enabled, ha mt enabled, listener enabled",
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            dhcp_mt, 4,
+            ha_mt, listener, 4, 4
+        },
+        {
+            "6 explicit DHCP threads, explicit thread values",
+            makeHAMtJson(ha_mt, listener, 5, 6),
+            dhcp_mt, 4,
+            ha_mt, listener, 5, 6
+        },
+        {
+            "7 explicit DHCP threads, zero thread values",
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            dhcp_mt, 8,
+            ha_mt, listener, 8, 8
+        },
+        {
+            "8 DHCP auto detect threads, zero thread values",
+            // Special case: if system reports supported threads as 0
+            // then HA+MT should be disabled.  Otherwise it should
+            // be enabled with listener and client threads set to the
+            // reported value.
+            makeHAMtJson(ha_mt, listener, 0, 0),
+            dhcp_mt, 0,
+            (sys_threads > 0), listener, sys_threads, sys_threads
+        }
+    };
+
+    // Iterate over the scenarios.
+    for (auto const& scenario : scenarios) {
+        SCOPED_TRACE(scenario.desc_);
+
+        // Build the HA JSON configuration.
+        std::stringstream ss;
+        ss <<
+            "["
+            "    {"
+            "        \"this-server-name\": \"server1\","
+            "        \"mode\": \"passive-backup\","
+            "        \"wait-backup-ack\": true,"
+            "        \"peers\": ["
+            "            {"
+            "                \"name\": \"server1\","
+            "                \"url\": \"http://127.0.0.1:8080/\","
+            "                \"role\": \"primary\""
+            "            },"
+            "            {"
+            "                \"name\": \"server2\","
+            "                \"url\": \"http://127.0.0.1:8081/\","
+            "                \"role\": \"backup\""
+            "            }"
+            "        ]";
+
+        if (!scenario.mt_json_.empty()) {
+            ss << "," << scenario.mt_json_;
+        }
+
+        ss << "}]";
+        ConstElementPtr config_json;
+        ASSERT_NO_THROW_LOG(config_json = Element::fromJSON(ss.str()));
+
+        // Set DHCP multi-threading configuration in CfgMgr.
+        setDHCPMultiThreadingConfig(scenario.dhcp_mt_enabled_, scenario.dhcp_threads_);
+        /// @todo this is a hack... we have chicken-egg...  CmdHttpListener won't
+        /// start if MT is not enabled BUT that happens after config hook point
+        MultiThreadingMgr::instance().setMode(scenario.dhcp_mt_enabled_);
+
+        // Create the HA configuration
+        HAConfigPtr ha_config(new HAConfig());
+        HAConfigParser parser;
+        ASSERT_NO_THROW_LOG(parser.parse(ha_config, config_json));
+
+        // Instantiate the service.
+        TestHAServicePtr service;
+        ASSERT_NO_THROW_LOG(service.reset(new TestHAService(io_service_, network_state_,
+                                                            ha_config)));
+        ASSERT_NO_THROW_LOG(service->startClientAndListener());
+
+        // Verify the configuration is as expected.
+        if (!scenario.exp_ha_mt_enabled_) {
+            // When HA+MT is disabled, client should be single-threaded.
+            ASSERT_TRUE(service->client_);
+            EXPECT_FALSE(service->client_->getThreadIOService());
+            EXPECT_EQ(service->client_->getThreadPoolSize(), 0);
+            EXPECT_EQ(service->client_->getThreadCount(), 0);
+
+            // Listener should not exist.
+            ASSERT_FALSE(service->listener_);
+            continue;
+        }
+
+        // Multi-threading should be enabled.
+        ASSERT_TRUE(ha_config->getEnableMultiThreading());
+
+        // When HA+MT is enabled, client should be multi-threaded.
+        ASSERT_TRUE(service->client_);
+        EXPECT_TRUE(service->client_->isRunning());
+        EXPECT_TRUE(service->client_->getThreadIOService());
+        EXPECT_EQ(service->client_->getThreadPoolSize(), scenario.exp_client_threads_);
+
+        // Currently thread count should be the same as thread pool size.  This might
+        // change if we go to so some sort of dynamic thread instance management.
+        EXPECT_EQ(service->client_->getThreadCount(), scenario.exp_client_threads_);
+
+        if (!scenario.exp_listener_) {
+            // We should not have a listener.
+            ASSERT_FALSE(service->listener_);
+            continue;
+        }
+
+        // We should have a running listener with the expected number of threads.
+        ASSERT_TRUE(service->listener_);
+        EXPECT_TRUE(service->listener_->isRunning());
+        ASSERT_TRUE(service->listener_->getThreadIOService());
+        EXPECT_EQ(service->listener_->getThreadPoolSize(), scenario.exp_listener_threads_);
+
+        // Currently thread count should be the same as thread pool size.  This might
+        // change if we go to so some sort of dynamic thread instance management.
+        EXPECT_EQ(service->listener_->getThreadCount(), scenario.exp_listener_threads_);
+
+        ASSERT_NO_THROW_LOG(service->stopClientAndListener());
+    }
+}
+
+}  // namespace
diff --git a/src/hooks/dhcp/high_availability/tests/ha_service_unittest.cc b/src/hooks/dhcp/high_availability/tests/ha_service_unittest.cc
new file mode 100644
index 0000000..f0c286e
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/ha_service_unittest.cc
@@ -0,0 +1,8037 @@
+// Copyright (C) 2018-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <asiolink/asio_wrapper.h>
+#include <ha_test.h>
+#include <ha_config.h>
+#include <ha_service.h>
+#include <ha_service_states.h>
+#include <asiolink/interval_timer.h>
+#include <asiolink/io_address.h>
+#include <asiolink/io_service.h>
+#include <cc/command_interpreter.h>
+#include <cc/data.h>
+#include <dhcp/classify.h>
+#include <dhcp/dhcp4.h>
+#include <dhcp/dhcp6.h>
+#include <dhcp/duid.h>
+#include <dhcp/hwaddr.h>
+#include <dhcp/pkt4.h>
+#include <dhcpsrv/cfgmgr.h>
+#include <dhcpsrv/lease.h>
+#include <dhcpsrv/lease_mgr.h>
+#include <dhcpsrv/lease_mgr_factory.h>
+#include <dhcpsrv/network_state.h>
+#include <dhcpsrv/subnet_id.h>
+#include <hooks/parking_lots.h>
+#include <http/basic_auth_config.h>
+#include <http/date_time.h>
+#include <http/http_types.h>
+#include <http/listener.h>
+#include <http/post_request_json.h>
+#include <http/response_creator.h>
+#include <http/response_creator_factory.h>
+#include <http/response_json.h>
+#include <util/multi_threading_mgr.h>
+#include <testutils/gtest_utils.h>
+
+#include <boost/date_time/posix_time/posix_time.hpp>
+#include <boost/pointer_cast.hpp>
+#include <boost/shared_ptr.hpp>
+#include <gtest/gtest.h>
+
+#include <functional>
+#include <sstream>
+#include <set>
+#include <string>
+#include <vector>
+
+using namespace isc::asiolink;
+using namespace isc::config;
+using namespace isc::data;
+using namespace isc::dhcp;
+using namespace isc::ha;
+using namespace isc::ha::test;
+using namespace isc::hooks;
+using namespace isc::http;
+using namespace isc::util;
+
+/// @file The tests herein were created prior to HA+MT but are very valuable
+/// for testing HA single-threaded operation and overall HA behavior.
+/// HA+MT testing is done elsewhere.
+
+namespace {
+
+/// @brief IP address to which HTTP service is bound.
+const std::string SERVER_ADDRESS = "127.0.0.1";
+
+/// @brief Port number to which HTTP service is bound.
+const unsigned short SERVER_PORT = 18123;
+
+/// @brief Request Timeout used in most of the tests (ms).
+const long REQUEST_TIMEOUT = 10000;
+
+/// @brief Persistent connection idle timeout used in most of the tests (ms).
+const long IDLE_TIMEOUT = 10000;
+
+/// @brief Test timeout (ms).
+const long TEST_TIMEOUT = 10000;
+
+/// @brief Generates IPv4 leases to be used by the tests.
+///
+/// @param [out] leases reference to the container where leases are stored.
+void generateTestLeases(std::vector<Lease4Ptr>& leases) {
+    for (uint8_t i = 1; i <= 10; ++i) {
+        uint32_t lease_address = 0xC0000201 + 256 * i;
+        std::vector<uint8_t> hwaddr(6, i);
+        Lease4Ptr lease(new Lease4(IOAddress(lease_address),
+                                   HWAddrPtr(new HWAddr(hwaddr, HTYPE_ETHER)),
+                                   ClientIdPtr(),
+                                   60,
+                                   static_cast<time_t>(1000 + i),
+                                   SubnetID(i)));
+        leases.push_back(lease);
+    }
+}
+
+/// @brief Generates IPv6 leases to be used by the tests.
+///
+/// @param [out] leases reference to the container where leases are stored.
+void generateTestLeases(std::vector<Lease6Ptr>& leases) {
+    std::vector<uint8_t> address_bytes = IOAddress("2001:db8:1::1").toBytes();
+    for (uint8_t i = 1; i <= 10; ++i) {
+        DuidPtr duid(new DUID(std::vector<uint8_t>(10, i)));
+        address_bytes[6] += i;
+        Lease6Ptr lease(new Lease6(Lease::TYPE_NA,
+                                   IOAddress::fromBytes(AF_INET6, &address_bytes[0]),
+                                   duid, 1, 50, 60, SubnetID(i)));
+        leases.push_back(lease);
+    }
+}
+
+/// @brief Returns generated leases in JSON format.
+///
+/// @tparam LeasesVec vector of IPv4 or IPv6 lease pointers.
+/// @param leases reference to the container holding leases to be
+/// converted to JSON format.
+template<typename LeasesVec>
+ConstElementPtr getLeasesAsJson(const LeasesVec& leases) {
+    ElementPtr leases_json = Element::createList();
+    for (auto l = leases.begin(); l != leases.end(); ++l) {
+        leases_json->add((*l)->toElement());
+    }
+    return (leases_json);
+}
+
+/// @brief Derivation of the @c HAService which provides access to
+/// protected methods and members.
+class TestHAService : public HAService {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// @param io_service Pointer to the IO service used by the DHCP server.
+    /// @param network_state Object holding state of the DHCP service
+    /// (enabled/disabled).
+    /// @param config Parsed HA hook library configuration.
+    /// @param server_type Server type, i.e. DHCPv4 or DHCPv6 server.
+    TestHAService(const IOServicePtr& io_service,
+                  const NetworkStatePtr& network_state,
+                  const HAConfigPtr& config,
+                  const HAServerType& server_type = HAServerType::DHCPv4)
+        : HAService(io_service, network_state, config, server_type) {
+    }
+
+    /// @brief Test version of the @c HAService::runModel.
+    ///
+    /// The original implementation of this method returns control when
+    /// @c NOP_EVT is found. This implementation runs a
+    /// single handler to allow the tests to verify if the state machine
+    /// transitions to an expected state before it is run again.
+    virtual void runModel(unsigned int event) {
+        try {
+            postNextEvent(event);
+            getState(getCurrState())->run();
+
+        } catch (const std::exception& ex) {
+            abortModel(ex.what());
+        }
+    }
+
+    /// @brief Schedules asynchronous "dhcp-disable" command to the specified
+    /// server.
+    ///
+    /// This variant of the method uses default HTTP client for communication.
+    ///
+    /// @param server_name name of the server to which the command should be
+    /// sent.
+    /// @param max_period maximum number of seconds for which the DHCP service
+    /// should be disabled.
+    /// @param post_request_action pointer to the function to be executed when
+    /// the request is completed.
+    void asyncDisableDHCPService(const std::string& server_name,
+                                 const unsigned int max_period,
+                                 const PostRequestCallback& post_request_action) {
+        HAService::asyncDisableDHCPService(*client_, server_name, max_period,
+                                           post_request_action);
+    }
+
+    /// @brief Schedules asynchronous "dhcp-enable" command to the specified
+    /// server.
+    ///
+    /// This variant of the method uses default HTTP client for communication.
+    ///
+    /// @param server_name name of the server to which the command should be
+    /// sent.
+    /// @param post_request_action pointer to the function to be executed when
+    /// the request is completed.
+    void asyncEnableDHCPService(const std::string& server_name,
+                                const PostRequestCallback& post_request_action) {
+        HAService::asyncEnableDHCPService(*client_, server_name, post_request_action);
+    }
+
+    using HAService::asyncSendHeartbeat;
+    using HAService::asyncSyncLeases;
+    using HAService::postNextEvent;
+    using HAService::transition;
+    using HAService::verboseTransition;
+    using HAService::shouldSendLeaseUpdates;
+    using HAService::shouldQueueLeaseUpdates;
+    using HAService::pendingRequestSize;
+    using HAService::getPendingRequest;
+    using HAService::network_state_;
+    using HAService::config_;
+    using HAService::communication_state_;
+    using HAService::query_filter_;
+    using HAService::lease_update_backlog_;
+    using HAService::client_;
+    using HAService::listener_;
+};
+
+/// @brief Pointer to the @c TestHAService.
+typedef boost::shared_ptr<TestHAService> TestHAServicePtr;
+
+/// @brief Test HTTP response creator.
+///
+/// It records received requests and allows the tests to retrieve them
+/// to verify that they include expected values.
+class TestHttpResponseCreator : public HttpResponseCreator {
+public:
+
+    /// @brief Constructor.
+    TestHttpResponseCreator() :
+        requests_(), control_result_(CONTROL_RESULT_SUCCESS),
+        arguments_(), per_request_control_result_(),
+        per_request_arguments_(), request_index_(), basic_auth_() {
+    }
+
+    /// @brief Removes all received requests.
+    void clearReceivedRequests() {
+        std::lock_guard<std::mutex> lk(mutex_);
+        requests_.clear();
+    }
+
+    /// @brief Returns a vector of received requests.
+    std::vector<PostHttpRequestJsonPtr> getReceivedRequests() {
+        std::lock_guard<std::mutex> lk(mutex_);
+        return (requests_);
+    }
+
+    /// @brief Finds a received request which includes two strings.
+    ///
+    /// @param str1 First string which must be included in the request.
+    /// @param str2 Second string which must be included in the request.
+    /// @param str3 Third string which must be included in the request.
+    /// It is optional and defaults to empty string which means "do not
+    /// match".
+    ///
+    /// @return Pointer to the request found, or null pointer if there is
+    /// no such request.
+    PostHttpRequestJsonPtr
+    findRequest(const std::string& str1, const std::string& str2,
+                const std::string& str3 = "") {
+        std::lock_guard<std::mutex> lk(mutex_);
+        for (auto r = requests_.begin(); r < requests_.end(); ++r) {
+            std::string request_as_string = (*r)->toString();
+            if (request_as_string.find(str1) != std::string::npos) {
+                if (request_as_string.find(str2) != std::string::npos) {
+                    if (str3.empty() ||
+                        (request_as_string.find(str3) != std::string::npos))
+                    return (*r);
+                }
+            }
+        }
+
+        // Request not found.
+        return (PostHttpRequestJsonPtr());
+    }
+
+    /// @brief Sets control result  to be included in the responses.
+    ///
+    /// @param control_result new control result value.
+    void setControlResult(const int control_result) {
+        control_result_ = control_result;
+    }
+
+    /// @brief Sets control result to be returned for the particular command.
+    ///
+    /// @param command_name command name.
+    /// @param control_result new control result value.
+    void setControlResult(const std::string& command_name,
+                          const int control_result) {
+        std::lock_guard<std::mutex> lk(mutex_);
+        per_request_control_result_[command_name] = control_result;
+    }
+
+    /// @brief Sets arguments to be included in the responses.
+    ///
+    /// @param arguments pointer to the arguments.
+    void setArguments(const ElementPtr& arguments) {
+        std::lock_guard<std::mutex> lk(mutex_);
+        arguments_ = arguments;
+    }
+
+    /// @brief Sets arguments to be included in the response to a particular
+    /// command.
+    ///
+    /// Some tests require that the returned arguments vary for the same
+    /// command sent multiple times. One example of such command is the
+    /// @c lease4-get-page command sent multiple times to fetch pages of
+    /// leases from the partner. This method facilitates such test scenario.
+    /// In order to set different arguments for consecutive requests this
+    /// method must be called multiple times. Each call results in adding
+    /// arguments to be returned when the command is issued multiple times.
+    ///
+    /// @param command_name command name.
+    /// @param arguments pointer to the arguments.
+    void setArguments(const std::string& command_name,
+                      const ElementPtr& arguments) {
+        std::lock_guard<std::mutex> lk(mutex_);
+        per_request_arguments_[command_name].push_back(isc::data::copy(arguments));
+        // Create request index for this command if it doesn't exist.
+        if (request_index_.count(command_name) == 0) {
+            request_index_[command_name] = 0;
+        }
+    }
+
+    /// @brief Create a new request.
+    ///
+    /// @return Pointer to the new instance of the @ref HttpRequest.
+    virtual HttpRequestPtr
+    createNewHttpRequest() const {
+        return (HttpRequestPtr(new PostHttpRequestJson()));
+    }
+
+    /// @brief Get basic HTTP authentication credentials.
+    ///
+    /// @return Basic HTTP authentication credentials and user id map.
+    const BasicHttpAuthMap& getCredentials() const {
+        return (basic_auth_.getCredentialMap());
+    }
+
+    /// @brief Add basic HTTP authentication client.
+    ///
+    /// @param user The user id to authorize.
+    /// @param password The password.
+    void addBasicAuth(const std::string& user, const std::string& password) {
+        std::lock_guard<std::mutex> lk(mutex_);
+        basic_auth_.add(user, "", password, "");
+    }
+
+private:
+
+    /// @brief Creates HTTP response.
+    ///
+    /// @param request Pointer to the HTTP request.
+    /// @return Pointer to the generated HTTP response.
+    virtual HttpResponsePtr
+    createStockHttpResponse(const HttpRequestPtr& request,
+                            const HttpStatusCode& status_code) const {
+        // The request hasn't been finalized so the request object
+        // doesn't contain any information about the HTTP version number
+        // used. But, the context should have this data (assuming the
+        // HTTP version is parsed ok).
+        HttpVersion http_version(request->context()->http_version_major_,
+                                 request->context()->http_version_minor_);
+        // This will generate the response holding JSON content.
+        HttpResponseJsonPtr response(new HttpResponseJson(http_version, status_code));
+        response->finalize();
+        return (response);
+    }
+
+    /// @brief Creates HTTP response.
+    ///
+    /// It records received request so it may be later validated by the tests.
+    /// The returned status code and arguments are set using @c setControlResult
+    /// and @c setArguments methods. The per-request control result and arguments
+    /// take precedence over global values.
+    ///
+    /// @param request Pointer to the HTTP request.
+    /// @return Pointer to the generated HTTP OK response.
+    virtual HttpResponsePtr
+    createDynamicHttpResponse(HttpRequestPtr request) {
+        std::unique_lock<std::mutex> lk(mutex_);
+        // Check authentication.
+        const BasicHttpAuthMap& credentials = getCredentials();
+        if (!credentials.empty()) {
+            bool authorized = false;
+            try {
+                std::string value = request->getHeaderValue("Authorization");
+                if (value.size() < 8) {
+                    isc_throw(isc::BadValue, "header content is too short");
+                }
+                if (value.substr(0, 6) != "Basic ") {
+                    isc_throw(isc::BadValue, "no basic authentication");
+                }
+                value = value.substr(6);
+                if (credentials.count(value) != 0) {
+                    authorized = true;
+                }
+            } catch (const std::exception&) {
+                authorized = false;
+            }
+            if (!authorized) {
+                return (createStockHttpResponse(request,
+                                                HttpStatusCode::UNAUTHORIZED));
+            }
+        }
+
+        // Request must always be JSON.
+        PostHttpRequestJsonPtr request_json =
+            boost::dynamic_pointer_cast<PostHttpRequestJson>(request);
+
+        // Remember the request received.
+        requests_.push_back(request_json);
+
+        // The request must always contain non-empty Host header.
+        bool invalid_host = false;
+        try {
+            auto host_hdr = request_json->getHeader("Host");
+            if (host_hdr->getValue().empty()) {
+                invalid_host = true;
+            }
+
+        } catch (...) {
+            // Host header does not exist.
+            invalid_host = true;
+        }
+
+        // If invalid host then return Bad Request.
+        if (invalid_host) {
+            return (createStockHttpResponse(request, HttpStatusCode::BAD_REQUEST));
+        }
+
+        int control_result = -1;
+        ElementPtr arguments;
+
+        // First, check if the request contains a body with a command.
+        // If so, we may include a specific error code and arguments in the
+        // response based on the command name.
+        ConstElementPtr body = request_json->getBodyAsJson();
+        if (body && (body->getType() == Element::map)) {
+            ConstElementPtr command = body->get("command");
+            if (command && (command->getType() == Element::string)) {
+                std::string command_name = command->stringValue();
+
+                // Check if there is specific error code to be returned for this
+                // command.
+                if (per_request_control_result_.count(command_name) > 0) {
+                    control_result = per_request_control_result_[command_name];
+                }
+
+                // Check if there are specific arguments to be returned for this
+                // command.
+                if (per_request_arguments_.count(command_name) > 0) {
+                    // For certain requests we may return different arguments for consecutive
+                    // instances of the same command. The request_index_ tracks the current
+                    // index of the arguments to be returned.
+                    arguments = per_request_arguments_[command_name][request_index_[command_name]];
+
+                    // If we have reached the last arguments do not increase the index. Simply
+                    // continue returning the same arguments.
+                    if (request_index_[command_name] + 1 < per_request_arguments_[command_name].size()) {
+                        // Not the last arguments, increase the index.
+                        ++request_index_[command_name];
+                    }
+                }
+            }
+        }
+
+        HttpResponseJsonPtr response(new HttpResponseJson(request->getHttpVersion(),
+                                                          HttpStatusCode::OK));
+        // Body is a list of responses from multiple servers listed in "service"
+        // argument of the request.
+        ElementPtr response_body = Element::createList();
+
+        // No per-command control result specified, so include the global result.
+        if (control_result < 0) {
+            control_result = control_result_;
+        }
+
+        // No per-command arguments specified, so include the global arguments.
+        if (!arguments) {
+            arguments = arguments_;
+        }
+
+        // Insert current date-time if not statically provided.
+        if (arguments && !arguments->contains("date-time")) {
+            arguments->set("date-time", Element::create(HttpDateTime().rfc1123Format()));
+        }
+
+        // Insert unsent-update-count if not present.
+        if (arguments && !arguments->contains("unsent-update-count")) {
+            arguments->set("unsent-update-count", Element::create(int64_t(0)));
+        }
+
+        response_body->add(boost::const_pointer_cast<Element>
+                           (createAnswer(control_result, "response returned",
+                                         arguments)));
+        response->setBodyAsJson(response_body);
+        response->finalize();
+        return (response);
+    }
+
+    /// @brief Holds received HTTP requests.
+    std::vector<PostHttpRequestJsonPtr> requests_;
+
+    /// @brief Control result to be returned in the server responses.
+    int control_result_;
+
+    /// @brief Arguments to be included in the responses.
+    ElementPtr arguments_;
+
+    /// @brief Command specific control results.
+    std::map<std::string, int> per_request_control_result_;
+
+    /// @brief Command specific response arguments.
+    std::map<std::string, std::vector<ElementPtr> > per_request_arguments_;
+
+    /// @brief Index of the next request of the given type.
+    std::map<std::string, size_t> request_index_;
+
+    /// @brief Basic HTTP authentication configuration.
+    BasicHttpAuthConfig basic_auth_;
+
+    /// @brief Mutex to protect member access.
+    std::mutex mutex_;
+};
+
+/// @brief Shared pointer to the @c TestHttpResponseCreator.
+typedef boost::shared_ptr<TestHttpResponseCreator> TestHttpResponseCreatorPtr;
+
+/// @brief Implementation of the test @ref HttpResponseCreatorFactory.
+///
+/// This factory class creates @ref TestHttpResponseCreator instances.
+class TestHttpResponseCreatorFactory : public HttpResponseCreatorFactory {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// Initializes common HTTP response creator instance.
+    TestHttpResponseCreatorFactory()
+        : creator_(new TestHttpResponseCreator()) {
+    }
+
+    /// @brief Creates @ref TestHttpResponseCreator instance.
+    virtual HttpResponseCreatorPtr create() const {
+        return (creator_);
+    }
+
+    /// @brief Returns instance of the response creator constructed by this
+    /// factory.
+    TestHttpResponseCreatorPtr getResponseCreator() const {
+        return (boost::dynamic_pointer_cast<TestHttpResponseCreator>(creator_));
+    }
+
+private:
+
+    /// @brief Pointer to the common HTTP response creator.
+    HttpResponseCreatorPtr creator_;
+};
+
+/// @brief Pointer to the @c TestHttpResponseCreatorFactory.
+typedef boost::shared_ptr<TestHttpResponseCreatorFactory>
+TestHttpResponseCreatorFactoryPtr;
+
+/// @brief Test fixture class for @c HAService.
+///
+/// It creates 3 HTTP listeners (servers) which are used in the unit tests.
+class HAServiceTest : public HATest {
+public:
+
+    struct MyState {
+        explicit MyState(const int state)
+            : state_(state) {
+        }
+        int state_;
+    };
+
+    struct PartnerState {
+        explicit PartnerState(const int state)
+            : state_(state) {
+        }
+        int state_;
+    };
+
+    struct FinalState {
+        explicit FinalState(const int state)
+            : state_(state) {
+        }
+        int state_;
+    };
+
+    /// @brief Constructor.
+    HAServiceTest()
+        : HATest(),
+          service_(),
+          factory_(new TestHttpResponseCreatorFactory()),
+          factory2_(new TestHttpResponseCreatorFactory()),
+          factory3_(new TestHttpResponseCreatorFactory()),
+          listener_(new HttpListener(*io_service_, IOAddress(SERVER_ADDRESS),
+                                     SERVER_PORT, TlsContextPtr(), factory_,
+                                     HttpListener::RequestTimeout(REQUEST_TIMEOUT),
+                                     HttpListener::IdleTimeout(IDLE_TIMEOUT))),
+          listener2_(new HttpListener(*io_service_, IOAddress(SERVER_ADDRESS),
+                                      SERVER_PORT + 1, TlsContextPtr(), factory2_,
+                                      HttpListener::RequestTimeout(REQUEST_TIMEOUT),
+                                      HttpListener::IdleTimeout(IDLE_TIMEOUT))),
+          listener3_(new HttpListener(*io_service_, IOAddress(SERVER_ADDRESS),
+                                      SERVER_PORT + 2, TlsContextPtr(), factory3_,
+                                      HttpListener::RequestTimeout(REQUEST_TIMEOUT),
+                                      HttpListener::IdleTimeout(IDLE_TIMEOUT))),
+          leases4_(),
+          leases6_(),
+          user1_(""),
+          password1_(""),
+          user2_(""),
+          password2_(""),
+          user3_(""),
+          password3_("") {
+        MultiThreadingMgr::instance().setMode(false);
+    }
+
+    /// @brief Destructor.
+    ///
+    /// Stops all test servers.
+    ~HAServiceTest() {
+        listener_->stop();
+        listener2_->stop();
+        listener3_->stop();
+        io_service_->get_io_service().reset();
+        io_service_->poll();
+        MultiThreadingMgr::instance().setMode(false);
+    }
+
+    /// @brief Callback function invoke upon test timeout.
+    ///
+    /// It stops the IO service and reports test timeout.
+    ///
+    /// @param fail_on_timeout Specifies if test failure should be reported.
+    void timeoutHandler(const bool fail_on_timeout) {
+        if (fail_on_timeout) {
+            ADD_FAILURE() << "Timeout occurred while running the test!";
+        }
+        io_service_->stop();
+    }
+
+    /// @brief Creates a TestHAService instance with HA+MT disabled.
+    ///
+    /// @param network_state Object holding state of the DHCP service
+    /// (enabled/disabled).
+    /// @param config Parsed HA hook library configuration.
+    /// @param server_type server type, i.e. DHCPv4 or DHCPv6.
+    void createSTService(const NetworkStatePtr& state,
+                         const HAConfigPtr config,
+                         const HAServerType& server_type = HAServerType::DHCPv4) {
+
+        ASSERT_FALSE(config->getEnableMultiThreading());
+        ASSERT_NO_THROW_LOG(service_.reset(new TestHAService(io_service_, state,
+                                           config, server_type)));
+        ASSERT_TRUE(service_->client_);
+        ASSERT_FALSE(service_->client_->getThreadIOService());
+        ASSERT_FALSE(service_->listener_);
+    }
+
+    /// @brief Generates IPv4 leases to be used by the tests.
+    void generateTestLeases4() {
+        generateTestLeases(leases4_);
+    }
+
+    /// @brief Returns range of generated IPv4 leases in JSON format.
+    ///
+    /// @param first_index Index of the first lease to be returned.
+    /// @param last_index Index of the last lease to be returned.
+    ConstElementPtr getTestLeases4AsJson(const size_t first_index,
+                                         const size_t last_index) const {
+        return (getLeasesAsJson(std::vector<Lease4Ptr>(leases4_.begin() + first_index,
+                                                       leases4_.begin() + last_index)));
+    }
+
+    /// @brief Generates IPv6 leases to be used by the tests.
+    void generateTestLeases6() {
+        generateTestLeases(leases6_);
+    }
+
+    /// @brief Returns range of generated IPv6 leases in JSON format.
+    ///
+    /// @param first_index Index of the first lease to be returned.
+    /// @param last_index Index of the last lease to be returned.
+    ConstElementPtr getTestLeases6AsJson(const size_t first_index,
+                                         const size_t last_index) const {
+        return (getLeasesAsJson(std::vector<Lease6Ptr>(leases6_.begin() + first_index,
+                                                       leases6_.begin() + last_index)));
+    }
+
+    /// @brief Configure server side to return  IPv4 leases in 3-element
+    /// chunks.
+    ///
+    /// Leases are fetched in pages, so the lease4-get-page should be
+    /// sent multiple times. The server is configured to return leases
+    /// in 3-element chunks. Note that the HA configis set to ask for 3
+    /// leases.
+    void createPagedSyncResponses4() {
+        ElementPtr response_arguments = Element::createMap();
+
+        // First, return leases with indexes from 0 to 2.
+        response_arguments->set("leases", getTestLeases4AsJson(0, 3));
+        factory2_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+
+        // Next, return leases with indexes from 3 to 5.
+        response_arguments->set("leases", getTestLeases4AsJson(3, 6));
+        factory2_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+
+        // Then, return leases with indexes from 6 to 8.
+        response_arguments->set("leases", getTestLeases4AsJson(6, 9));
+        factory2_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+
+        // Finally, there is one lease with index 9 to be returned.
+        // When the server requests a page of 3 leases and gets 1 it
+        // means that the last page was returned. At this point, the
+        // server ends synchronization.
+        response_arguments->set("leases", getTestLeases4AsJson(9, 10));
+        factory2_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+    }
+
+    /// @brief Configure server side to return  IPv6 leases in 3-element
+    /// chunks.
+    ///
+    /// Leases are fetched in pages, so the lease6-get-page should be
+    /// sent multiple times. The server is configured to return leases
+    /// in 3-element chunks. Note that the HA configis set to ask for 3
+    /// leases.
+    void createPagedSyncResponses6() {
+        ElementPtr response_arguments = Element::createMap();
+
+        // First, return leases with indexes from 0 to 2.
+        response_arguments->set("leases", getTestLeases6AsJson(0, 3));
+        factory2_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+
+        // Next, return leases with indexes from 3 to 5.
+        response_arguments->set("leases", getTestLeases6AsJson(3, 6));
+        factory2_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+
+        // Then, return leases with indexes from 6 to 8.
+        response_arguments->set("leases", getTestLeases6AsJson(6, 9));
+        factory2_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+
+        // Finally, there is one lease with index 9 to be returned.
+        // When the server requests a page of 3 leases and gets 1 it
+        // means that the last page was returned. At this point, the
+        // server ends synchronization.
+        response_arguments->set("leases", getTestLeases6AsJson(9, 10));
+        factory2_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+        factory3_->getResponseCreator()->setArguments("lease6-get-page", response_arguments);
+    }
+
+    /// @brief Set basic HTTP authentication in a config.
+    ///
+    /// @param config Configuration to update.
+    void setBasicAuth(HAConfigPtr config) {
+        if (!config) {
+            ADD_FAILURE() << "null config";
+            return;
+        }
+        if (!user1_.empty()) {
+            HAConfig::PeerConfigPtr peer = config->getPeerConfig("server1");
+            if (!peer) {
+                ADD_FAILURE() << "null server1 config";
+                return;
+            }
+            BasicHttpAuthPtr& auth = peer->getBasicAuth();
+            auth.reset(new BasicHttpAuth(user1_, password1_));
+        }
+        if (!user2_.empty()) {
+            HAConfig::PeerConfigPtr peer = config->getPeerConfig("server2");
+            if (!peer) {
+                ADD_FAILURE() << "null server2 config";
+                return;
+            }
+            BasicHttpAuthPtr& auth = peer->getBasicAuth();
+            auth.reset(new BasicHttpAuth(user2_, password2_));
+        }
+        if (!user3_.empty()) {
+            HAConfig::PeerConfigPtr peer = config->getPeerConfig("server3");
+            if (!peer) {
+                ADD_FAILURE() << "null server3 config";
+                return;
+            }
+            BasicHttpAuthPtr& auth = peer->getBasicAuth();
+            auth.reset(new BasicHttpAuth(user3_, password3_));
+        }
+    }
+
+    /// @brief Tests scenarios when lease updates are sent to a partner while
+    /// the partner is online or offline.
+    ///
+    /// @param unpark_handler a function called when packet is unparked.
+    /// @param should_fail indicates if the update is expected to be unsuccessful.
+    /// @param num_updates expected number of servers to which lease updates are
+    /// sent.
+    /// @param my_state state of the server while lease updates are sent.
+    /// @param wait_backup_ack indicates if the server should wait for the acknowledgment
+    /// from the backup servers.
+    /// @param create_service a boolean flag indicating whether the test should
+    /// re-create HA service and communication state.
+    void testSendLeaseUpdates(std::function<void()> unpark_handler,
+                              const bool should_fail,
+                              const size_t num_updates,
+                              const MyState& my_state = MyState(HA_LOAD_BALANCING_ST),
+                              const bool wait_backup_ack = false,
+                              const bool create_service = true) {
+        // Create parking lot where query is going to be parked and unparked.
+        ParkingLotPtr parking_lot(new ParkingLot());
+        ParkingLotHandlePtr parking_lot_handle(new ParkingLotHandle(parking_lot));
+
+        // Create query.
+        Pkt4Ptr query(new Pkt4(DHCPREQUEST, 1234));
+
+        // Create leases collection and put the lease there.
+        Lease4CollectionPtr leases4(new Lease4Collection());
+        HWAddrPtr hwaddr(new HWAddr(std::vector<uint8_t>(6, 1), HTYPE_ETHER));
+        Lease4Ptr lease4(new Lease4(IOAddress("192.1.2.3"), hwaddr,
+                                    static_cast<const uint8_t*>(0), 0,
+                                    60, 0, 1));
+        leases4->push_back(lease4);
+
+        // Create deleted leases collection and put the lease there too.
+        Lease4CollectionPtr deleted_leases4(new Lease4Collection());
+        Lease4Ptr deleted_lease4(new Lease4(IOAddress("192.2.3.4"), hwaddr,
+                                            static_cast<const uint8_t*>(0), 0,
+                                            60, 0, 1));
+        deleted_leases4->push_back(deleted_lease4);
+
+        if (create_service) {
+            // Create HA configuration for 3 servers. This server is
+            // server 1.
+            HAConfigPtr config_storage = createValidConfiguration();
+            config_storage->setWaitBackupAck(wait_backup_ack);
+            // Let's override the default value. The lower value makes it easier
+            // for some unit tests to simulate the server's overflow. Simulating it
+            // requires appending leases to the backlog. It is easier to add 10
+            // than 100.
+            config_storage->setDelayedUpdatesLimit(10);
+            setBasicAuth(config_storage);
+
+            // The communication state is the member of the HAServce object. We have to
+            // replace this object with our own implementation to have an ability to
+            // modify its poke time.
+            NakedCommunicationState4Ptr state(new NakedCommunicationState4(io_service_,
+                                                                       config_storage));
+            // Set poke time 30s in the past. If the state is poked it will be reset
+            // to the current time. This allows for testing whether the object has been
+            // poked by the HA service.
+            state->modifyPokeTime(-30);
+
+            // Create HA service.
+            createSTService(network_state_, config_storage);
+            service_->communication_state_ = state;
+        }
+
+        service_->transition(my_state.state_, HAService::NOP_EVT);
+
+        // Schedule lease updates.
+        EXPECT_EQ(num_updates,
+                  service_->asyncSendLeaseUpdates(query, leases4, deleted_leases4,
+                                                  parking_lot_handle));
+
+        // The number of pending requests should be 2 times the number of
+        // contacted servers because we send one lease update and one
+        // lease deletion to each contacted server from which we expect
+        // an acknowledgment.
+        EXPECT_EQ(2 * num_updates, service_->getPendingRequest(query));
+
+        // Let's park the packet and associate it with the callback function which
+        // simply records the fact that it has been called. We expect that it wasn't
+        // because the parked packet should be dropped as a result of lease updates
+        // failures.
+        ASSERT_NO_THROW(parking_lot->park(query, unpark_handler));
+
+        ASSERT_NO_THROW(parking_lot->reference(query));
+
+        // Actually perform the lease updates.
+        ASSERT_NO_THROW(runIOService(TEST_TIMEOUT, [this]() {
+            // Finish running IO service when there are no more pending requests.
+            return (service_->pendingRequestSize() == 0);
+        }));
+
+        // Only if we wait for lease updates to complete it makes sense to test
+        // that the packet was either dropped or unparked.
+        if (num_updates > 0) {
+            // Try to drop the packet. We expect that the packet has been already
+            // dropped so this should return false.
+            EXPECT_FALSE(parking_lot_handle->drop(query));
+        }
+
+        // The updates should not be sent to this server.
+        EXPECT_TRUE(factory_->getResponseCreator()->getReceivedRequests().empty());
+
+        if (should_fail) {
+            EXPECT_EQ(HA_UNAVAILABLE_ST, service_->communication_state_->getPartnerState());
+        } else {
+            EXPECT_NE(service_->communication_state_->getPartnerState(), HA_UNAVAILABLE_ST);
+        }
+    }
+
+    /// @brief Tests scenarios when IPv6 lease updates are sent to a partner while
+    /// the partner is online or offline.
+    ///
+    /// @param unpark_handler a function called when packet is unparked.
+    /// @param should_fail indicates if the update is expected to be unsuccessful.
+    /// @param num_updates expected number of servers to which lease updates are
+    /// sent.
+    /// @param my_state state of the server while lease updates are sent.
+    /// @param wait_backup_ack indicates if the server should wait for the acknowledgment
+    /// from the backup servers.
+    /// @param create_service a boolean flag indicating whether the test should
+    /// re-create HA service and communication state.
+    void testSendLeaseUpdates6(std::function<void()> unpark_handler,
+                               const bool should_fail,
+                               const size_t num_updates,
+                               const MyState& my_state = MyState(HA_LOAD_BALANCING_ST),
+                               const bool wait_backup_ack = false,
+                               const bool create_service = true) {
+
+        // Create parking lot where query is going to be parked and unparked.
+        ParkingLotPtr parking_lot(new ParkingLot());
+        ParkingLotHandlePtr parking_lot_handle(new ParkingLotHandle(parking_lot));
+
+        DuidPtr duid(new DUID(std::vector<uint8_t>(8, 2)));
+
+        // Create query.
+        Pkt6Ptr query(new Pkt6(DHCPV6_SOLICIT, 1234));
+        OptionPtr opt_client_id(new Option(Option::V6,
+                                           D6O_CLIENTID,
+                                           duid->getDuid().begin(),
+                                           duid->getDuid().end()));
+        query->addOption(opt_client_id);
+
+        // Create leases collection and put the lease there.
+        Lease6CollectionPtr leases6(new Lease6Collection());
+        Lease6Ptr lease6(new Lease6(Lease::TYPE_NA, IOAddress("2001:db8:1::cafe"), duid,
+                                    1234, 50, 60, 1));
+        leases6->push_back(lease6);
+
+        // Create deleted leases collection and put the lease there too.
+        Lease6CollectionPtr deleted_leases6(new Lease6Collection());
+        Lease6Ptr deleted_lease6(new Lease6(Lease::TYPE_NA, IOAddress("2001:db8:1::efac"),
+                                            duid, 1234, 50, 60, 1));
+        deleted_leases6->push_back(deleted_lease6);
+
+        if (create_service) {
+            // Create HA configuration for 3 servers. This server is
+            // server 1.
+            HAConfigPtr config_storage = createValidConfiguration();
+            config_storage->setDelayedUpdatesLimit(10);
+            config_storage->setWaitBackupAck(wait_backup_ack);
+            setBasicAuth(config_storage);
+
+            // The communication state is the member of the HAServce object. We have to
+            // replace this object with our own implementation to have an ability to
+            // modify its poke time.
+            NakedCommunicationState6Ptr state(new NakedCommunicationState6(io_service_,
+                                                                       config_storage));
+            // Set poke time 30s in the past. If the state is poked it will be reset
+            // to the current time. This allows for testing whether the object has been
+            // poked by the HA service.
+            state->modifyPokeTime(-30);
+
+            // Create HA service.
+            createSTService(network_state_, config_storage, HAServerType::DHCPv6);
+            service_->communication_state_ = state;
+        }
+
+        service_->transition(my_state.state_, HAService::NOP_EVT);
+
+        // Schedule lease updates.
+        EXPECT_EQ(num_updates,
+                  service_->asyncSendLeaseUpdates(query, leases6, deleted_leases6,
+                                                  parking_lot_handle));
+
+        // The number of requests we send is equal to the number of servers
+        // from which we expect an acknowledgement. We send both lease updates
+        // and the deletions in a single bulk update command.
+        EXPECT_EQ(num_updates, service_->getPendingRequest(query));
+
+        EXPECT_FALSE(boost::dynamic_pointer_cast<NakedCommunicationState6>
+                     (service_->communication_state_)->isPoked());
+
+        // Let's park the packet and associate it with the callback function which
+        // simply records the fact that it has been called. We expect that it wasn't
+        // because the parked packet should be dropped as a result of lease updates
+        // failures.
+        ASSERT_NO_THROW(parking_lot->park(query, unpark_handler));
+
+        ASSERT_NO_THROW(parking_lot->reference(query));
+
+        // Actually perform the lease updates.
+        ASSERT_NO_THROW(runIOService(TEST_TIMEOUT, [this]() {
+            // Finish running IO service when there are no more pending requests.
+            return (service_->pendingRequestSize() == 0);
+        }));
+
+        // Only if we wait for lease updates to complete it makes sense to test
+        // that the packet was either dropped or unparked.
+        if (num_updates > 0) {
+            // Try to drop the packet. We expect that the packet has been already
+            // dropped so this should return false.
+            EXPECT_FALSE(parking_lot_handle->drop(query));
+        }
+
+        // The updates should not be sent to this server.
+        EXPECT_TRUE(factory_->getResponseCreator()->getReceivedRequests().empty());
+
+        if (should_fail) {
+            EXPECT_EQ(HA_UNAVAILABLE_ST, service_->communication_state_->getPartnerState());
+        }
+    }
+
+    /// @brief Tests scenarios when recurring heartbeat has been enabled
+    /// and the partner is online or offline.
+    ///
+    /// @param control_result control result that the servers should return.
+    /// @param should_pass boolean value indicating if the heartbeat should
+    /// be successful or not.
+    void testRecurringHeartbeat(const int control_result,
+                                const bool should_pass) {
+        // Create HA configuration for 3 servers. This server is
+        // server 1.
+        HAConfigPtr config_storage = createValidConfiguration();
+        config_storage->setHeartbeatDelay(1000);
+        setBasicAuth(config_storage);
+
+        // Create a valid static response to the heartbeat command.
+        ElementPtr response_arguments = Element::createMap();
+        response_arguments->set("state", Element::create(std::string("load-balancing")));
+
+        // Both server 2 and server 3 are configured to send this response.
+        factory2_->getResponseCreator()->setArguments(response_arguments);
+        factory3_->getResponseCreator()->setArguments(response_arguments);
+
+        // Configure server 2 and server 3 to send a specified control result.
+        factory2_->getResponseCreator()->setControlResult(control_result);
+        factory3_->getResponseCreator()->setControlResult(control_result);
+
+        // The communication state is the member of the HAServce object. We have to
+        // replace this object with our own implementation to have an ability to
+        // modify its poke time.
+        NakedCommunicationState4Ptr state(new NakedCommunicationState4(io_service_,
+                                                                       config_storage));
+        // Set poke time 30s in the past. If the state is poked it will be reset
+        // to the current time. This allows for testing whether the object has been
+        // poked by the HA service.
+        state->modifyPokeTime(-30);
+
+        // Create the service and replace the default communication state object.
+        TestHAService service(io_service_, network_state_, config_storage);
+        service.communication_state_ = state;
+
+        EXPECT_FALSE(state->isPoked());
+
+        // Let's explicitly transition the state machine to the load balancing state
+        // in which the periodic heartbeats should be generated.
+        ASSERT_NO_THROW(service.verboseTransition(HA_LOAD_BALANCING_ST));
+        ASSERT_NO_THROW(service.runModel(HAService::NOP_EVT));
+
+        // Run the IO service to allow the heartbeat interval timers to execute.
+        ASSERT_NO_THROW(runIOService(2000));
+
+        // Server 1 and server 3 must never receive heartbeats because the former
+        // is the one that generates them and the latter is a backup server.
+        EXPECT_TRUE(factory_->getResponseCreator()->getReceivedRequests().empty());
+        EXPECT_TRUE(factory3_->getResponseCreator()->getReceivedRequests().empty());
+
+        // If should pass, the communication state should be poked.
+        if (should_pass) {
+            EXPECT_TRUE(state->isPoked());
+        } else {
+            EXPECT_FALSE(state->isPoked());
+        }
+    }
+
+    /// @brief Tests scenarios when all lease updates are sent successfully.
+    void testSendSuccessfulUpdates() {
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates([&unpark_called] { unpark_called = true; },
+                             false, 1);
+
+        // Expecting that the packet was unparked because lease
+        // updates are expected to be successful.
+        EXPECT_TRUE(unpark_called);
+
+        // Updates have been sent so this counter should remain 0.
+        EXPECT_EQ(0, service_->communication_state_->getUnsentUpdateCount());
+
+        // The server 2 should have received two commands.
+        EXPECT_EQ(2, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 2 has received lease4-update command.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_TRUE(update_request2);
+
+        // Check that the server 2 has received lease4-del command.
+        auto delete_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_TRUE(delete_request2);
+
+        // Lease updates should be successfully sent to server3.
+        EXPECT_EQ(2, factory3_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 3 has received lease4-update command.
+        auto update_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_TRUE(update_request3);
+
+        // Check that the server 3 has received lease4-del command.
+        auto delete_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_TRUE(delete_request3);
+    }
+
+    /// @brief Tests that DHCPv4 lease updates are queued when the server is in the
+    /// communication-recovery state and later sent before transitioning back to
+    /// the load-balancing state.
+    void testSendUpdatesCommunicationRecovery() {
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates([&unpark_called] { unpark_called = true; },
+                             false, 0, MyState(HA_COMMUNICATION_RECOVERY_ST));
+
+        // Packet shouldn't be unparked because no updates went out. We merely
+        // queued the updates.
+        EXPECT_FALSE(unpark_called);
+
+        // Let's make sure they have been queued.
+        EXPECT_EQ(2, service_->lease_update_backlog_.size());
+
+        // Make partner available.
+        service_->communication_state_->poke();
+        service_->communication_state_->setPartnerState("load-balancing");
+
+        // This should cause the server to send outstanding lease updates and
+        // because they are all successful the server should transition to the
+        // load-balancing state and continue normal operation.
+        testSynchronousCommands([this]() {
+            service_->runModel(HAService::NOP_EVT);
+            EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+        });
+
+        // Lease updates should have been sent.
+        EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                                 "192.1.2.3"));
+        EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                                 "192.2.3.4"));
+
+        // Backlog should be empty.
+        EXPECT_EQ(0, service_->lease_update_backlog_.size());
+    }
+
+    /// @brief Test the cases when the trying to recover from the communication
+    /// interruption and sending lease updates or/and ha-reset fails.
+    ///
+    /// @param partner_state partner state when communication is re-established.
+    /// @param lease_update_result control result returned to lease updates.
+    /// @param ha_reset_result control result returned to ha-reset command.
+    /// @param overflow boolean value indicating if this test should verify the
+    /// case when the leases backlog is overflown (when true), or not (when
+    /// false).
+    void testSendUpdatesCommunicationRecoveryFailed(const std::string& partner_state,
+                                                    const int lease_update_result,
+                                                    const int ha_reset_result,
+                                                    const bool overflow = false) {
+        // Partner responds with a specified control result to lease updates.
+        factory2_->getResponseCreator()->setControlResult("lease4-update",
+                                                          lease_update_result);
+        factory2_->getResponseCreator()->setControlResult("lease4-del",
+                                                          lease_update_result);
+        // Partner returns specified control result to ha-reset.
+        factory2_->getResponseCreator()->setControlResult("ha-reset", ha_reset_result);
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates([&unpark_called] { unpark_called = true; },
+                             false, 0, MyState(HA_COMMUNICATION_RECOVERY_ST));
+
+        // Packet shouldn't be unparked because no updates went out. We merely
+        // queued the updates.
+        EXPECT_FALSE(unpark_called);
+
+        // Let's make sure they have been queued.
+        EXPECT_EQ(2, service_->lease_update_backlog_.size());
+
+        // When testing the case when the backlog should be overflown, we need
+        // to add several more leases to the backlog to exceed the limit.
+        if (overflow) {
+            ASSERT_NO_THROW(generateTestLeases4());
+            for (auto lease : leases4_) {
+                service_->lease_update_backlog_.push(LeaseUpdateBacklog::ADD, lease);
+            }
+        }
+
+        // Make partner available.
+        service_->communication_state_->poke();
+        service_->communication_state_->setPartnerState(partner_state);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This should cause the server to attempt to send outstanding lease
+        // updates to the partner.
+        testSynchronousCommands([this, ha_reset_result]() {
+            service_->runModel(HAService::NOP_EVT);
+            // If the ha-reset returns success the server should transition to the
+            // waiting state and begin synchronization. Otherwise, if the ha-reset
+            // fails the server should wait in the communication-recovery state
+            // until it succeeds.
+            if (ha_reset_result == CONTROL_RESULT_SUCCESS) {
+                EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+            } else {
+                EXPECT_EQ(HA_COMMUNICATION_RECOVERY_ST, service_->getCurrState());
+            }
+        });
+
+        // The server will only send lease updates if it is not overflown. If
+        // it is overflown, it will rather transition to the waiting state to
+        // initiate full synchronization.
+        if (!overflow) {
+            // Deletions are scheduled first and this should cause the failure.
+            EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                                     "192.2.3.4"));
+            // This lease update should not be sent because the first update
+            // triggered an error.
+            EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                                      "192.1.2.3"));
+        }
+
+        if ((partner_state == "load-balancing") || (partner_state == "communication-recovery")) {
+            // The lease updates failed and the partner remains in the load-balancing or
+            // communication-recovery state, so the server should send ha-reset to the
+            // partner to cause it to transition to the waiting state and synchronize
+            // the lease database.
+            EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-reset", ""));
+        } else {
+            // The lease updates failed but the partner is already synchronizing lease
+            // database. In this case, don't send the ha-reset.
+            EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("ha-reset", ""));
+        }
+
+        // The backlog should be empty.
+        EXPECT_EQ(0, service_->lease_update_backlog_.size());
+    }
+
+    /// @brief Tests scenarios when lease updates are not sent to the failover peer.
+    void testSendUpdatesPartnerDown() {
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates([&unpark_called] { unpark_called = true; },
+                             false, 0, MyState(HA_PARTNER_DOWN_ST));
+
+        // There were no lease updates for which we have been waiting
+        // to complete so the packet was never unparked. Note that in
+        // such situation the packet is not parked either.
+        EXPECT_FALSE(unpark_called);
+
+        // In the partner-down state we don't send lease updates. We
+        // should count transactions for which lease updates were not sent.
+        // This is later returned in the heartbeat so the partner can
+        // determine whether it should synchronize its lease database or
+        // not.
+        EXPECT_EQ(1, service_->communication_state_->getUnsentUpdateCount());
+
+        // Server 2 should not receive lease4-update.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_FALSE(update_request2);
+
+        // Server 2 should not receive lease4-del.
+        auto delete_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_FALSE(delete_request2);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which
+    /// updates are sent is offline.
+    void testSendUpdatesActiveServerOffline() {
+        // Start only two servers out of three. The server 3 is not running.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, true, 1);
+
+        // Server 2 should not receive lease4-update.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_FALSE(update_request2);
+
+        // Server 2 should not receive lease4-del.
+        auto delete_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_FALSE(delete_request2);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which a
+    /// lease update is sent returns an error.
+    void testSendUpdatesControlResultError() {
+        // Instruct the server 2 to return an error as a result of receiving a command.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, true, 1);
+
+        // The updates should be sent to server 2 and this server should
+        // return error code.
+        EXPECT_EQ(2, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Server 2 should receive lease4-update.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_TRUE(update_request2);
+
+        // Server 2 should receive lease4-del.
+        auto delete_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_TRUE(delete_request2);
+
+        // Lease updates should be successfully sent to server3.
+        EXPECT_EQ(2, factory3_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 3 has received lease4-update command.
+        auto update_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_TRUE(update_request3);
+
+        // Check that the server 3 has received lease4-del command.
+        auto delete_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_TRUE(delete_request3);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which a
+    /// lease update is sent does not authorize the local server.
+    void testSendUpdatesControlResultUnauthorized() {
+        // Instruct the server 2 to require authentication.
+        factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, true, 1);
+
+        // The updates should be sent to server 2 and this server should
+        // return error code.
+        EXPECT_TRUE(factory2_->getResponseCreator()->getReceivedRequests().empty());
+
+        // Lease updates should be successfully sent to server3.
+        EXPECT_EQ(2, factory3_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 3 has received lease4-update command.
+        auto update_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_TRUE(update_request3);
+
+        // Check that the server 3 has received lease4-del command.
+        auto delete_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_TRUE(delete_request3);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which
+    /// updates are sent is offline.
+    void testSendUpdatesBackupServerOffline() {
+        // Start only two servers out of three. The server 2 is not running.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+        });
+
+        bool unpark_called = false;
+        testSendLeaseUpdates([&unpark_called] { unpark_called = true; },
+                             false, 1);
+
+        EXPECT_TRUE(unpark_called);
+
+        // The server 2 should have received two commands.
+        EXPECT_EQ(2, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 2 has received lease4-update command.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_TRUE(update_request2);
+
+        // Check that the server 2 has received lease4-del command.
+        auto delete_request2 =
+            factory2_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_TRUE(delete_request2);
+
+        // Server 3 should not receive lease4-update.
+        auto update_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-update",
+                                                         "192.1.2.3");
+        EXPECT_FALSE(update_request3);
+
+        // Server 3 should not receive lease4-del.
+        auto delete_request3 =
+            factory3_->getResponseCreator()->findRequest("lease4-del",
+                                                         "192.2.3.4");
+        EXPECT_FALSE(delete_request3);
+    }
+
+    /// @brief Test the scenario when the servers receiving a lease update
+    /// return the conflict status code.
+    void testSendUpdatesControlResultConflict() {
+        // Instruct the server 2 to return an error as a result of receiving a command.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_CONFLICT);
+        factory3_->getResponseCreator()->setControlResult(CONTROL_RESULT_CONFLICT);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, false, 1);
+
+        // Ensure that the server has recorded a lease update conflict. The conflict
+        // reported by the backup server should not count.
+        EXPECT_EQ(1, service_->communication_state_->getRejectedLeaseUpdatesCount());
+
+        // Change the partner's response to success.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_SUCCESS);
+
+        // Try sending the lease updates again. The previously rejected lease should
+        // now be accepted and the counter should be 0.
+        bool unpark_called = false;
+        testSendLeaseUpdates([&unpark_called] {
+            unpark_called = true;
+        }, false, 1, MyState(HA_LOAD_BALANCING_ST), true, false);
+        EXPECT_TRUE(unpark_called);
+        EXPECT_EQ(0, service_->communication_state_->getRejectedLeaseUpdatesCount());
+    }
+
+    /// @brief Tests scenarios when all lease updates are sent successfully.
+    void testSendSuccessfulUpdates6() {
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] { unpark_called = true; },
+                              false, 1);
+
+        // Expecting that the packet was unparked because lease
+        // updates are expected to be successful.
+        EXPECT_TRUE(unpark_called);
+
+        // Updates have been sent so this counter should remain 0.
+        EXPECT_EQ(0, service_->communication_state_->getUnsentUpdateCount());
+
+        // The server 2 should have received one command.
+        EXPECT_EQ(1, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 2 has received lease6-bulk-apply command.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_TRUE(update_request2);
+
+        // Lease updates should be successfully sent to server3.
+        EXPECT_EQ(1, factory3_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 3 has received lease6-bulk-apply command.
+        auto update_request3 =
+            factory3_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_TRUE(update_request3);
+    }
+
+    /// @brief Tests that DHCPv6 lease updates are queued when the server is in the
+    /// communication-recovery state and later sent before transitioning back to
+    /// the load-balancing state.
+    void testSendUpdatesCommunicationRecovery6() {
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] { unpark_called = true; },
+                             false, 0, MyState(HA_COMMUNICATION_RECOVERY_ST));
+
+        // Packet shouldn't be unparked because no updates went out. We merely
+        // queued the updates.
+        EXPECT_FALSE(unpark_called);
+
+        // Let's make sure they have been queued.
+        EXPECT_EQ(2, service_->lease_update_backlog_.size());
+
+        // Make partner available.
+        service_->communication_state_->poke();
+        service_->communication_state_->setPartnerState("load-balancing");
+
+        // This should cause the server to send outstanding lease updates and
+        // because they are all successful the server should transition to the
+        // load-balancing state and continue normal operation.
+        testSynchronousCommands([this]() {
+            service_->runModel(HAService::NOP_EVT);
+            EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+        });
+
+        // Bulk lease update should have been sent.
+        EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                                 "2001:db8:1::cafe",
+                                                                 "2001:db8:1::efac"));
+
+        // Backlog should be empty.
+        EXPECT_EQ(0, service_->lease_update_backlog_.size());
+    }
+
+    /// @brief Test the cases when the trying to recover from the communication
+    /// interruption and sending lease updates or/and ha-reset fails.
+    ///
+    /// @param partner_state partner state when communication is re-established.
+    /// @param lease_update_result control result returned to lease updates.
+    /// @param ha_reset_result control result returned to ha-reset command.
+    /// @param overflow boolean value indicating if this test should verify the
+    /// case when the leases backlog is overflown (when true), or not (when
+    /// false).
+    void testSendUpdatesCommunicationRecovery6Failed(const std::string& partner_state,
+                                                     const int lease_update_result,
+                                                     const int ha_reset_result,
+                                                     const bool overflow = false) {
+        // Partner responds with a specified control result to lease updates.
+        factory2_->getResponseCreator()->setControlResult("lease6-bulk-apply",
+                                                          lease_update_result);
+        // Partner returns specified control result to ha-reset.
+        factory2_->getResponseCreator()->setControlResult("ha-reset",
+                                                          ha_reset_result);
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] { unpark_called = true; },
+                             false, 0, MyState(HA_COMMUNICATION_RECOVERY_ST));
+
+        // Packet shouldn't be unparked because no updates went out. We merely
+        // queued the updates.
+        EXPECT_FALSE(unpark_called);
+
+        // Let's make sure they have been queued.
+        EXPECT_EQ(2, service_->lease_update_backlog_.size());
+
+        // When testing the case when the backlog should be overflown, we need
+        // to add several more leases to the backlog to exceed the limit.
+        if (overflow) {
+            ASSERT_NO_THROW(generateTestLeases6());
+            for (auto lease : leases6_) {
+                service_->lease_update_backlog_.push(LeaseUpdateBacklog::ADD, lease);
+            }
+        }
+
+        // Make partner available.
+        service_->communication_state_->poke();
+        service_->communication_state_->setPartnerState(partner_state);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This should cause the server to attempt to send outstanding lease
+        // updates to the partner.
+        testSynchronousCommands([this, ha_reset_result]() {
+            service_->runModel(HAService::NOP_EVT);
+            // If the ha-reset returns success the server should transition to the
+            // waiting state and begin synchronization. Otherwise, if the ha-reset
+            // fails the server should wait in the communication-recovery state
+            // until it succeeds.
+            if (ha_reset_result == CONTROL_RESULT_SUCCESS) {
+                EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+            } else {
+                EXPECT_EQ(HA_COMMUNICATION_RECOVERY_ST, service_->getCurrState());
+            }
+        });
+
+        // The server will only send lease updates if it is not overflown. If
+        // it is overflown, it will rather transition to the waiting state to
+        // initiate full synchronization.
+        if (!overflow) {
+            // The server should have sent lease updates in a single command.
+            EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                                     "2001:db8:1::cafe",
+                                                                     "2001:db8:1::efac"));
+        }
+
+        if ((partner_state == "load-balancing") || (partner_state == "communication-recovery")) {
+            // The lease updates failed and the partner remains in the load-balancing or
+            // communication-recovery state, so the server should send ha-reset to the
+            // partner to cause it to transition to the waiting state and synchronize
+            // the lease database.
+            EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-reset", ""));
+        } else {
+            // The lease updates failed but the partner is already synchronizing lease
+            // database. In this case, don't send the ha-reset.
+            EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("ha-reset", ""));
+        }
+
+        // Backlog should be empty.
+        EXPECT_EQ(0, service_->lease_update_backlog_.size());
+    }
+
+    /// @brief Tests scenarios when lease updates are not sent to the failover peer.
+    void testSendUpdatesPartnerDown6() {
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] { unpark_called = true; },
+                              false, 0, MyState(HA_PARTNER_DOWN_ST));
+
+        // There were no lease updates for which we have been waiting to
+        // complete so the packet was never unparked. Note that in such
+        // situation the packet is not parked either.
+        EXPECT_FALSE(unpark_called);
+
+        // In the partner-down state we don't send lease updates. We
+        // should count transactions for which lease updates were not sent.
+        // This is later returned in the heartbeat so the partner can
+        // determine whether it should synchronize its lease database or
+        // not.
+        EXPECT_EQ(1, service_->communication_state_->getUnsentUpdateCount());
+
+        // Server 2 should not receive lease6-bulk-apply.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_FALSE(update_request2);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which
+    /// updates are sent is offline.
+    void testSendUpdatesActiveServerOffline6() {
+        // Start only two servers out of three. The server 3 is not running.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates6([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, true, 1);
+
+        // Server 2 should not receive lease6-bulk-apply.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_FALSE(update_request2);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which
+    /// updates are sent is offline.
+    void testSendUpdatesBackupServerOffline6() {
+        // Start only two servers out of three. The server 2 is not running.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+        });
+
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] { unpark_called = true; },
+                              false, 1);
+
+        EXPECT_TRUE(unpark_called);
+
+        // The server 2 should have received one command.
+        EXPECT_EQ(1, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 2 has received lease6-bulk-apply command.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_TRUE(update_request2);
+
+        // Server 3 should not receive lease6-bulk-apply.
+        auto update_request3 =
+            factory3_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_FALSE(update_request3);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which a
+    /// lease update is sent returns an error.
+    void testSendUpdatesControlResultError6() {
+        // Instruct the server 2 to return an error as a result of receiving a command.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates6([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, true, 1);
+
+        // The updates should be sent to server 2 and this server should return error code.
+        EXPECT_EQ(1, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Server 2 should receive lease6-bulk-apply.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_TRUE(update_request2);
+    }
+
+    /// @brief Tests scenarios when one of the servers to which a
+    /// lease update is sent does not authorize the local server.
+    void testSendUpdatesControlResultUnauthorized6() {
+        // Instruct the server 2 to require authentication.
+        factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates6([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, true, 1);
+
+        // The updates should be sent to server 2 and this server should return error code.
+        EXPECT_TRUE(factory2_->getResponseCreator()->getReceivedRequests().empty());
+    }
+
+    /// @brief These tests verify that the server accepts the response
+    /// to the lease6-bulk-apply command including
+    /// failed-deleted-leases and failed-leases parameters.
+    void testSendUpdatesFailedLeases6() {
+        // Create a dummy lease which failed to be deleted.
+        auto failed_deleted_lease = Element::createMap();
+        failed_deleted_lease->set("type",
+                                  Element::create("IA_NA"));
+        failed_deleted_lease->set("ip-address",
+                                  Element::create("2001:db8:1::1"));
+        failed_deleted_lease->set("subnet-id",
+                                  Element::create(1));
+        failed_deleted_lease->set("result",
+                                  Element::create(CONTROL_RESULT_EMPTY));
+        failed_deleted_lease->set("error-message",
+                                  Element::create("no lease found"));
+
+        // Crate a dummy lease which failed to be created.
+        auto failed_lease = Element::createMap();
+        failed_lease->set("type",
+                          Element::create("IA_PD"));
+        failed_lease->set("ip-address",
+                          Element::create("2001:db8:1::"));
+        failed_lease->set("subnet-id",
+                          Element::create(2));
+        failed_lease->set("result",
+                          Element::create(CONTROL_RESULT_ERROR));
+        failed_lease->set("error-message",
+                          Element::create("failed to create lease"));
+
+        // Create the "failed-deleted-leases" list.
+        auto failed_deleted_leases = Element::createList();
+        failed_deleted_leases->add(failed_deleted_lease);
+
+        // Create the "failed-leases" list.
+        auto failed_leases = Element::createList();
+        failed_leases->add(failed_lease);
+
+        // Add both lists to the arguments.
+        ElementPtr arguments = Element::createMap();
+        arguments->set("failed-deleted-leases", failed_deleted_leases);
+        arguments->set("failed-leases", failed_leases);
+
+        // Configure the server to return this response.
+        factory2_->getResponseCreator()->setArguments("lease6-bulk-apply",
+                                                      arguments);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        // This flag will be set to true if unpark is called.
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] { unpark_called = true; },
+                              false, 1);
+
+        // Expecting that the packet was unparked because lease
+        // updates are expected to be successful.
+        EXPECT_TRUE(unpark_called);
+
+        // The server 2 should have received one command.
+        EXPECT_EQ(1, factory2_->getResponseCreator()->getReceivedRequests().size());
+
+        // Check that the server 2 has received lease6-bulk-apply command.
+        auto update_request2 =
+            factory2_->getResponseCreator()->findRequest("lease6-bulk-apply",
+                                                         "2001:db8:1::cafe",
+                                                         "2001:db8:1::efac");
+        EXPECT_TRUE(update_request2);
+    }
+
+    /// @brief Test the scenario when the servers receiving a lease update
+    /// return the conflict status code.
+    void testSendUpdatesControlResultConflict6() {
+        // Crate a dummy lease for which a conflict status code is returned.
+        auto failed_lease = Element::createMap();
+        failed_lease->set("type",
+                          Element::create("IA_PD"));
+        failed_lease->set("ip-address",
+                          Element::create("2001:db8:1::"));
+        failed_lease->set("subnet-id",
+                          Element::create(2));
+        failed_lease->set("result",
+                          Element::create(CONTROL_RESULT_CONFLICT));
+        failed_lease->set("error-message",
+                          Element::create("failed to create lease"));
+
+        // Create the "failed-leases" list.
+        auto failed_leases = Element::createList();
+        failed_leases->add(failed_lease);
+
+        // Add the list to the arguments.
+        ElementPtr arguments = Element::createMap();
+        arguments->set("failed-leases", failed_leases);
+
+        // Active server returns an empty status code to indicate that no lease
+        // has been created. It should cause the HA service to look into the
+        // status codes for individual leases.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_EMPTY);
+        factory3_->getResponseCreator()->setControlResult(CONTROL_RESULT_CONFLICT);
+
+        // Configure the server to return our response.
+        factory2_->getResponseCreator()->setArguments("lease6-bulk-apply",
+                                                      arguments);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+                listener3_->start();
+        });
+
+        testSendLeaseUpdates6([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, false, 1);
+
+        // Ensure that the server has recorded the lease update conflict. The conflict
+        // reported by the backup server should not count.
+        EXPECT_EQ(1, service_->communication_state_->getRejectedLeaseUpdatesCount());
+
+        // Change the active server's response to success. The initially rejected
+        // lease update should no longer be tracked.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_SUCCESS);
+
+        bool unpark_called = false;
+        testSendLeaseUpdates6([&unpark_called] {
+            unpark_called = true;
+        }, false, 1, MyState(HA_LOAD_BALANCING_ST), true, false);
+        EXPECT_TRUE(unpark_called);
+        EXPECT_EQ(0, service_->communication_state_->getRejectedLeaseUpdatesCount());
+    }
+
+    /// @brief Test the scenario when the server receiving a lease update returns
+    /// both the conflict and error status code. The latter should take precedence.
+    void testSendUpdatesControlResultConflict6ErrorPrecedence() {
+        // Create the "failed-leases" list.
+        auto failed_leases = Element::createList();
+
+        // Crate a dummy lease for which a conflict status code is returned.
+        auto failed_lease = Element::createMap();
+        failed_lease->set("type",
+                          Element::create("IA_NA"));
+        failed_lease->set("ip-address",
+                          Element::create("2001:db8:1::"));
+        failed_lease->set("subnet-id",
+                          Element::create(2));
+        failed_lease->set("result",
+                          Element::create(CONTROL_RESULT_CONFLICT));
+        failed_lease->set("error-message",
+                          Element::create("failed to create lease"));
+        failed_leases->add(failed_lease);
+
+        // Create another lease with an error status code.
+        failed_lease = Element::createMap();
+        failed_lease->set("type",
+                          Element::create("IA_NA"));
+        failed_lease->set("ip-address",
+                          Element::create("2001:db8:1::2"));
+        failed_lease->set("subnet-id",
+                          Element::create(2));
+        failed_lease->set("result",
+                          Element::create(CONTROL_RESULT_ERROR));
+        failed_lease->set("error-message",
+                          Element::create("failed to create lease"));
+        failed_leases->add(failed_lease);
+
+        // Add the list to the arguments.
+        ElementPtr arguments = Element::createMap();
+        arguments->set("failed-leases", failed_leases);
+
+        // Active server returns an empty status code to indicate that no lease
+        // has been created. It should cause the HA service to look into the
+        // status codes for individual leases.
+        factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_EMPTY);
+
+        // Configure the server to return our response.
+        factory2_->getResponseCreator()->setArguments("lease6-bulk-apply",
+                                                      arguments);
+
+        // Start HTTP servers.
+        ASSERT_NO_THROW({
+                listener_->start();
+                listener2_->start();
+        });
+
+        testSendLeaseUpdates6([] {
+            ADD_FAILURE() << "unpark function called but expected that "
+                "the packet is dropped";
+        }, false, 1);
+
+        // The conflict should not be recorded because the error status code
+        // takes precedence.
+        EXPECT_EQ(0, service_->communication_state_->getRejectedLeaseUpdatesCount());
+    }
+
+    /// @brief Runs HAService::processSynchronize for the DHCPv4 server and
+    /// returns a response.
+    ///
+    /// The HAService::processSynchronize is synchronous. Therefore, the IO service
+    /// for HTTP servers is run in a thread. The unit test is responsible for setting
+    /// up the status codes to be returned by the servers, verifying a response and
+    /// leases in the lease database.
+    ///
+    /// @param [out] rsp pointer to the object where response will be stored.
+    void runProcessSynchronize4(ConstElementPtr& rsp) {
+        // Create lease manager.
+        ASSERT_NO_THROW(LeaseMgrFactory::create("universe=4 type=memfile persist=false"));
+
+        // Create IPv4 leases which will be fetched from the other server.
+        ASSERT_NO_THROW(generateTestLeases4());
+
+        // Create HA configuration for 3 servers. This server is
+        // server 1.
+        HAConfigPtr config_storage = createValidConfiguration();
+        setBasicAuth(config_storage);
+
+        // Leases are fetched in pages, so the lease4-get-page should be
+        // sent multiple times. The server is configured to return leases
+        // in 3-element chunks.
+        createPagedSyncResponses4();
+
+        // Start the servers.
+        ASSERT_NO_THROW({
+            listener_->start();
+            listener2_->start();
+            listener3_->start();
+        });
+
+        HAService service(io_service_, network_state_, config_storage);
+
+        // The tested function is synchronous, so we need to run server side IO service
+        // in bakckground to not block the main thread.
+        auto thread = runIOServiceInThread();
+
+        // Process ha-sync command.
+        ASSERT_NO_THROW(rsp = service.processSynchronize("server2", 20));
+
+        // Stop the IO service. This should cause the thread to terminate.
+        io_service_->stop();
+        thread->join();
+        io_service_->get_io_service().reset();
+        io_service_->poll();
+    }
+
+    /// @brief Runs HAService::processSynchronize for the DHCPv6 server
+    /// and returns a response.
+    ///
+    /// The HAService::processSynchronize is synchronous. Therefore, the IO service
+    /// for HTTP servers is run in a thread. The unit test is responsible for setting
+    /// up the status codes to be returned by the servers, verifying a response and
+    /// leases in the lease database.
+    ///
+    /// @param [out] rsp pointer to the object where response will be stored.
+    void runProcessSynchronize6(ConstElementPtr& rsp) {
+        // Create lease manager.
+        ASSERT_NO_THROW(LeaseMgrFactory::create("universe=6 type=memfile persist=false"));
+
+        // Create IPv4 leases which will be fetched from the other server.
+        ASSERT_NO_THROW(generateTestLeases6());
+
+        // Create HA configuration for 3 servers. This server is
+        // server 1.
+        HAConfigPtr config_storage = createValidConfiguration();
+        setBasicAuth(config_storage);
+
+        // Leases are fetched in pages, so the lease6-get-page should be
+        // sent multiple times. The server is configured to return leases
+        // in 3-element chunks.
+        createPagedSyncResponses6();
+
+        // Start the servers.
+        ASSERT_NO_THROW({
+            listener_->start();
+            listener2_->start();
+            listener3_->start();
+        });
+
+        HAService service(io_service_, network_state_, config_storage,
+                          HAServerType::DHCPv6);
+
+        // The tested function is synchronous, so we need to run server side IO service
+        // in bakckground to not block the main thread.
+        auto thread = runIOServiceInThread();
+
+        // Process ha-sync command.
+        ASSERT_NO_THROW(rsp = service.processSynchronize("server2", 20));
+
+        // Stop the IO service. This should cause the thread to terminate.
+        io_service_->stop();
+        thread->join();
+        io_service_->get_io_service().reset();
+        io_service_->poll();
+    }
+
+    /// @brief Pointer to the HA service under test.
+    TestHAServicePtr service_;
+
+    /// @brief HTTP response factory for server 1.
+    TestHttpResponseCreatorFactoryPtr factory_;
+
+    /// @brief HTTP response factory for server 2.
+    TestHttpResponseCreatorFactoryPtr factory2_;
+
+    /// @brief HTTP response factory for server 3.
+    TestHttpResponseCreatorFactoryPtr factory3_;
+
+    /// @brief Test HTTP server 1.
+    HttpListenerPtr listener_;
+
+    /// @brief Test HTTP server 2.
+    HttpListenerPtr listener2_;
+
+    /// @brief Test HTTP server 3.
+    HttpListenerPtr listener3_;
+
+    /// @brief IPv4 leases to be used in the tests.
+    std::vector<Lease4Ptr> leases4_;
+
+    /// @brief IPv6 leases to be used in the tests.
+    std::vector<Lease6Ptr> leases6_;
+
+    /// @brief Basic HTTP authentication user id for server1.
+    std::string user1_;
+
+    /// @brief Basic HTTP authentication password for server1.
+    std::string password1_;
+
+    /// @brief Basic HTTP authentication user id for server2.
+    std::string user2_;
+
+    /// @brief Basic HTTP authentication password for server2.
+    std::string password2_;
+
+    /// @brief Basic HTTP authentication user id for server3.
+    std::string user3_;
+
+    /// @brief Basic HTTP authentication password for server3.
+    std::string password3_;
+};
+
+// Test that server performs load balancing and assigns appropriate classes
+// to the queries.
+TEST_F(HAServiceTest, loadBalancingScopeSelection) {
+    // Create HA configuration for load balancing.
+    HAConfigPtr config_storage = createValidConfiguration();
+    // ... and HA service using this configuration.
+    TestHAService service(io_service_, network_state_, config_storage);
+    service.verboseTransition(HA_LOAD_BALANCING_ST);
+    service.runModel(HAService::NOP_EVT);
+
+    // Count number of in scope packets.
+    unsigned in_scope = 0;
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+
+        // Some queries should be processed by this server, some not.
+        if (service.inScope(query4)) {
+            // If the query is to be processed by this server the query
+            // should be assigned to the "HA_server1" class but not to
+            // the "HA_server2" class.
+            ASSERT_TRUE(query4->inClass(ClientClass("HA_server1")));
+            ASSERT_FALSE(query4->inClass(ClientClass("HA_server2")));
+            ++in_scope;
+
+        } else {
+            // If the query is to be processed by another server, the
+            // "HA_server2" class should be assigned instead.
+            ASSERT_FALSE(query4->inClass(ClientClass("HA_server1")));
+            ASSERT_TRUE(query4->inClass(ClientClass("HA_server2")));
+        }
+    }
+
+    // We should have roughly 50/50 split of in scope and out of scope queries.
+    // However, we don't know exactly how many. To be safe we simply assume that
+    // we got more than 25% of in scope and more than 25% out of scope queries.
+    EXPECT_GT(in_scope, static_cast<unsigned>(queries_num / 4));
+    EXPECT_GT(queries_num - in_scope, static_cast<unsigned>(queries_num / 4));
+}
+
+// Test that primary server in hot standby configuration processes all queries.
+TEST_F(HAServiceTest, hotStandbyScopeSelectionThisPrimary) {
+    HAConfigPtr config_storage = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config_storage->getPeerConfig("server2")->setRole("standby");
+
+    // ... and HA service using this configuration.
+    TestHAService service(io_service_, network_state_, config_storage);
+    service.verboseTransition(HA_HOT_STANDBY_ST);
+    service.runModel(HAService::NOP_EVT);
+
+    // Check the reported info about servers.
+    ConstElementPtr ha_servers = service.processStatusGet();
+    ASSERT_TRUE(ha_servers);
+    std::string expected = "{"
+        "    \"local\": {"
+        "        \"role\": \"primary\","
+        "        \"scopes\": [ \"server1\" ],"
+        "        \"state\": \"hot-standby\""
+        "    }, "
+        "    \"remote\": {"
+        "        \"age\": 0,"
+        "        \"in-touch\": false,"
+        "        \"role\": \"standby\","
+        "        \"last-scopes\": [ ],"
+        "        \"last-state\": \"\","
+        "        \"communication-interrupted\": false,"
+        "        \"connecting-clients\": 0,"
+        "        \"unacked-clients\": 0,"
+        "        \"unacked-clients-left\": 0,"
+        "        \"analyzed-packets\": 0"
+        "    }"
+        "}";
+    EXPECT_TRUE(isEquivalent(Element::fromJSON(expected), ha_servers));
+
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+
+        // All queries should be processed by this server.
+        ASSERT_TRUE(service.inScope(query4));
+        // The query should be assigned to the "HA_server1" class but not to
+        // the "HA_server2" class.
+        ASSERT_TRUE(query4->inClass(ClientClass("HA_server1")));
+        ASSERT_FALSE(query4->inClass(ClientClass("HA_server2")));
+    }
+}
+
+// Test that secondary server in hot standby configuration processes no queries.
+TEST_F(HAServiceTest, hotStandbyScopeSelectionThisStandby) {
+    HAConfigPtr config_storage = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config_storage->getPeerConfig("server2")->setRole("standby");
+    config_storage->setThisServerName("server2");
+
+    // ... and HA service using this configuration.
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Check the reported info about servers.
+    ConstElementPtr ha_servers = service.processStatusGet();
+    ASSERT_TRUE(ha_servers);
+
+    std::string expected = "{"
+        "    \"local\": {"
+        "        \"role\": \"standby\","
+        "        \"scopes\": [ ],"
+        "        \"state\": \"waiting\""
+        "    }, "
+        "    \"remote\": {"
+        "        \"age\": 0,"
+        "        \"in-touch\": false,"
+        "        \"role\": \"primary\","
+        "        \"last-scopes\": [ ],"
+        "        \"last-state\": \"\","
+        "        \"communication-interrupted\": false,"
+        "        \"connecting-clients\": 0,"
+        "        \"unacked-clients\": 0,"
+        "        \"unacked-clients-left\": 0,"
+        "        \"analyzed-packets\": 0"
+        "    }"
+        "}";
+    EXPECT_TRUE(isEquivalent(Element::fromJSON(expected), ha_servers));
+
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+
+        // No queries should be processed by this server.
+        ASSERT_FALSE(service.inScope(query4));
+        // The query should be assigned to the "HA_server1" class but not to
+        // the "HA_server2" class.
+        ASSERT_TRUE(query4->inClass(ClientClass("HA_server1")));
+        ASSERT_FALSE(query4->inClass(ClientClass("HA_server2")));
+    }
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdates) {
+    testSendSuccessfulUpdates();
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdatesMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendSuccessfulUpdates();
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state for later send.
+TEST_F(HAServiceTest, sendUpdatesCommunicationRecovery) {
+    testSendUpdatesCommunicationRecovery();
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state for later send. Multi threading case.
+TEST_F(HAServiceTest, sendUpdatesCommunicationRecoveryMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecovery();
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in load-balancing
+// state when the communication is re-established, so the test expects that the
+// ha-reset command is sent to the partner.
+TEST_F(HAServiceTest, communicationRecoveryFailedPartnerLoadBalancing) {
+    testSendUpdatesCommunicationRecoveryFailed("load-balancing", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_SUCCESS);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in load-balancing
+// state when the communication is re-established, so the test expects that the
+// ha-reset command is sent to the partner.
+TEST_F(HAServiceTest, communicationRecoveryFailedPartnerLoadBalancingMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecoveryFailed("load-balancing", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_SUCCESS);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later fails. The partner server in the load-balancing
+// state but sending ha-reset fails. The server should remain in the
+// communication-recovery state.
+TEST_F(HAServiceTest, communicationRecoveryFailedResetFailed) {
+    testSendUpdatesCommunicationRecoveryFailed("load-balancing", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_ERROR);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later fails. The partner server in the load-balancing
+// state but sending ha-reset fails. The server should remain in the
+// communication-recovery state.
+TEST_F(HAServiceTest, communicationRecoveryFailedResetFailedMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecoveryFailed("load-balancing", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_ERROR);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in ready state
+// when the communication is re-established, so the test expects that the
+// ha-reset command is NOT sent to the partner. The lease backlog is overflown,
+// so the server should transition to the waiting state.
+TEST_F(HAServiceTest, communicationRecoveryFailedPartnerReady) {
+    testSendUpdatesCommunicationRecoveryFailed("ready", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_SUCCESS, true);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in ready state
+// when the communication is re-established, so the test expects that the
+// ha-reset command is NOT sent to the partner. The lease backlog is overflown,
+// so the server should transition to the waiting state.
+TEST_F(HAServiceTest, communicationRecoveryFailedPartnerReadyMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecoveryFailed("ready", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_SUCCESS, true);
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdatesAuthorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+    testSendSuccessfulUpdates();
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdatesAuthorizedMultiThreading) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+    MultiThreadingMgr::instance().setMode(true);
+    testSendSuccessfulUpdates();
+}
+
+// Test scenario when lease updates are not sent to the failover peer.
+TEST_F(HAServiceTest, sendUpdatesPartnerDown) {
+    testSendUpdatesPartnerDown();
+}
+
+// Test scenario when lease updates are not sent to the failover peer.
+TEST_F(HAServiceTest, sendUpdatesPartnerDownMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesPartnerDown();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesActiveServerOffline) {
+    testSendUpdatesActiveServerOffline();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesActiveServerOfflineMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesActiveServerOffline();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesBackupServerOffline) {
+    testSendUpdatesBackupServerOffline();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesBackupServerOfflineMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesBackupServerOffline();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns an error.
+TEST_F(HAServiceTest, sendUpdatesControlResultError) {
+    testSendUpdatesControlResultError();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns an error.
+TEST_F(HAServiceTest, sendUpdatesControlResultErrorMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultError();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// requires not provided authentication.
+TEST_F(HAServiceTest, sendUpdatesControlResultUnauthorized) {
+    testSendUpdatesControlResultUnauthorized();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// requires not provided authentication.
+TEST_F(HAServiceTest, sendUpdatesControlResultUnauthorizedMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultUnauthorized();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns a conflict status code.
+TEST_F(HAServiceTest, sendUpdatesControlResultConflict) {
+    testSendUpdatesControlResultConflict();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns a conflict status code.
+TEST_F(HAServiceTest, sendUpdatesControlResultConflictMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultConflict();
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdates6) {
+    testSendSuccessfulUpdates6();
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdates6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendSuccessfulUpdates6();
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state for later send. Then, the partner refuses lease updates causing the
+// server to transition to the waiting state.
+TEST_F(HAServiceTest, sendUpdatesCommunicationRecovery6) {
+    testSendUpdatesCommunicationRecovery6();
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state for later send. Then, the partner refuses lease updates causing the
+// server to transition to the waiting state.
+TEST_F(HAServiceTest, sendUpdatesCommunicationRecovery6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecovery6();
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in load-balancing
+// state when the communication is re-established, so the test expects that the
+// ha-reset command is sent to the partner.
+TEST_F(HAServiceTest, communicationRecoveryFailed6PartnerLoadBalancing) {
+    testSendUpdatesCommunicationRecovery6Failed("load-balancing", CONTROL_RESULT_ERROR,
+                                                CONTROL_RESULT_SUCCESS);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in load-balancing
+// state when the communication is re-established, so the test expects that the
+// ha-reset command is sent to the partner.
+TEST_F(HAServiceTest, communicationRecovery6FailedPartnerLoadBalancingMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecovery6Failed("load-balancing", CONTROL_RESULT_ERROR,
+                                                CONTROL_RESULT_SUCCESS);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later fails. The partner server in the load-balancing
+// state but sending ha-reset fails. The server should remain in the
+// communication-recovery state.
+TEST_F(HAServiceTest, communicationRecovery6FailedResetFailed) {
+    testSendUpdatesCommunicationRecovery6Failed("load-balancing", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_ERROR);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later fails. The partner server in the load-balancing
+// state but sending ha-reset fails. The server should remain in the
+// communication-recovery state.
+TEST_F(HAServiceTest, communicationRecovery6FailedResetFailedMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecovery6Failed("load-balancing", CONTROL_RESULT_ERROR,
+                                                CONTROL_RESULT_ERROR);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in ready state
+// when the communication is re-established, so the test expects that the
+// ha-reset command is NOT sent to the partner. The lease backlog is overflown,
+// so the server should transition to the waiting state.
+TEST_F(HAServiceTest, communicationRecovery6FailedPartnerReady) {
+    testSendUpdatesCommunicationRecovery6Failed("ready", CONTROL_RESULT_ERROR,
+                                                CONTROL_RESULT_SUCCESS, true);
+}
+
+// Test scenario when lease updates are queued in the communication-recovery
+// state and sending them later is unsuccessful. Partner is in ready state
+// when the communication is re-established, so the test expects that the
+// ha-reset command is NOT sent to the partner. The lease backlog is overflown,
+// so the server should transition to the waiting state.
+TEST_F(HAServiceTest, communicationRecovery6FailedPartnerReadyMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesCommunicationRecovery6Failed("ready", CONTROL_RESULT_ERROR,
+                                               CONTROL_RESULT_SUCCESS, true);
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdates6Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+    testSendSuccessfulUpdates6();
+}
+
+// Test scenario when all lease updates are sent successfully.
+TEST_F(HAServiceTest, sendSuccessfulUpdates6AuthorizedMultiThreading) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+    MultiThreadingMgr::instance().setMode(true);
+    testSendSuccessfulUpdates6();
+}
+
+// Test scenario when lease updates are not sent to the failover peer.
+TEST_F(HAServiceTest, sendUpdatesPartnerDown6) {
+    testSendUpdatesPartnerDown6();
+}
+
+// Test scenario when lease updates are not sent to the failover peer.
+TEST_F(HAServiceTest, sendUpdatesPartnerDown6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesPartnerDown6();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesActiveServerOffline6) {
+    testSendUpdatesActiveServerOffline6();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesActiveServerOffline6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesActiveServerOffline6();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesBackupServerOffline6) {
+    testSendUpdatesBackupServerOffline6();
+}
+
+// Test scenario when one of the servers to which updates are sent is offline.
+TEST_F(HAServiceTest, sendUpdatesBackupServerOffline6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesBackupServerOffline6();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns an error.
+TEST_F(HAServiceTest, sendUpdatesControlResultError6) {
+    testSendUpdatesControlResultError6();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns an error.
+TEST_F(HAServiceTest, sendUpdatesControlResultError6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultError6();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// requires not provided authentication.
+TEST_F(HAServiceTest, sendUpdatesControlResultUnauthorized6) {
+    testSendUpdatesControlResultUnauthorized6();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// requires not provided authentication.
+TEST_F(HAServiceTest, sendUpdatesControlResultUnauthorized6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultUnauthorized6();
+}
+
+// This test verifies that the server accepts the response to the lease6-bulk-apply
+// command including failed-deleted-leases and failed-leases parameters.
+TEST_F(HAServiceTest, sendUpdatesFailedLeases6) {
+    testSendUpdatesFailedLeases6();
+}
+
+// This test verifies that the server accepts the response to the lease6-bulk-apply
+// command including failed-deleted-leases and failed-leases parameters.
+TEST_F(HAServiceTest, sendUpdatesFailedLeases6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesFailedLeases6();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns a conflict status code.
+TEST_F(HAServiceTest, sendUpdatesControlResultConflict6) {
+    testSendUpdatesControlResultConflict6();
+}
+
+// Test scenario when one of the servers to which a lease update is sent
+// returns a conflict status code.
+TEST_F(HAServiceTest, sendUpdatesControlResultConflict6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultConflict6();
+}
+
+// Test the scenario when the server receiving a lease update returns
+// both the conflict and error status code. The latter should take precedence.
+TEST_F(HAServiceTest, sendUpdatesControlResultConflict6ErrorPrecedence) {
+    testSendUpdatesControlResultConflict6ErrorPrecedence();
+}
+
+    // Test the scenario when the server receiving a lease update returns
+// both the conflict and error status code. The latter should take precedence.
+TEST_F(HAServiceTest, sendUpdatesControlResultConflict6ErrorPrecedenceMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    testSendUpdatesControlResultConflict6ErrorPrecedence();
+}
+
+// This test verifies that the heartbeat command is processed successfully.
+TEST_F(HAServiceTest, processHeartbeat) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    std::string config_text =
+        "["
+        "     {"
+        "         \"this-server-name\": \"server1\","
+        "         \"mode\": \"load-balancing\","
+        "         \"peers\": ["
+        "             {"
+        "                 \"name\": \"server1\","
+        "                 \"url\": \"http://127.0.0.1:18123/\","
+        "                 \"role\": \"primary\","
+        "                 \"auto-failover\": true"
+        "             },"
+        "             {"
+        "                 \"name\": \"server2\","
+        "                 \"url\": \"http://127.0.0.1:18124/\","
+        "                 \"role\": \"secondary\","
+        "                 \"auto-failover\": true"
+        "             },"
+        "             {"
+        "                 \"name\": \"server3\","
+        "                 \"url\": \"http://127.0.0.1:18125/\","
+        "                 \"role\": \"backup\","
+        "                 \"auto-failover\": false"
+        "             }"
+        "         ]"
+        "     }"
+        "]";
+
+    // Parse the HA configuration.
+    HAConfigPtr config_storage(new HAConfig());
+    HAConfigParser parser;
+    ASSERT_NO_THROW(parser.parse(config_storage, Element::fromJSON(config_text)));
+
+    TestHAService service(io_service_,  network_state_, config_storage);
+    service.query_filter_.serveDefaultScopes();
+
+    int unsent_updates = 6;
+    for (auto i = 0; i < unsent_updates; ++i) {
+        service.communication_state_->increaseUnsentUpdateCount();
+    }
+
+    // Process heartbeat command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processHeartbeat());
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "HA peer status returned.");
+    // Response must include arguments.
+    ConstElementPtr args = rsp->get("arguments");
+    ASSERT_TRUE(args);
+
+    // Response must include server state as string.
+    ConstElementPtr state = args->get("state");
+    ASSERT_TRUE(state);
+    EXPECT_EQ(Element::string, state->getType());
+    EXPECT_EQ("waiting", state->stringValue());
+
+    // Response must include timestamp when the response was generated.
+    ConstElementPtr date_time = args->get("date-time");
+    ASSERT_TRUE(date_time);
+    EXPECT_EQ(Element::string, date_time->getType());
+
+    auto scopes_list = args->get("scopes");
+    ASSERT_TRUE(scopes_list);
+    EXPECT_EQ(Element::list, scopes_list->getType());
+    ASSERT_EQ(1, scopes_list->size());
+    auto scope = scopes_list->get(0);
+    ASSERT_TRUE(scope);
+    EXPECT_EQ(Element::string, scope->getType());
+    EXPECT_EQ("server1", scope->stringValue());
+
+    // The response should contain the timestamp in the format specified
+    // in RFC1123. We use the HttpDateTime method to parse this timestamp.
+    HttpDateTime t;
+    ASSERT_NO_THROW(t = HttpDateTime::fromRfc1123(date_time->stringValue()));
+
+    // Let's test if the timestamp is accurate. We do it by checking current
+    // time and comparing with the received timestamp.
+    HttpDateTime now;
+    boost::posix_time::time_duration td = now.getPtime() - t.getPtime();
+
+    // Let's allow the response propagation time of 5 seconds to make
+    // sure this test doesn't fail on slow systems.
+    EXPECT_LT(td.seconds(), 5);
+
+    // The response should contain unsent-update-count parameter indicating
+    // how many updates haven't been sent to a partner because the partner
+    // was unavailable.
+    ConstElementPtr unsent_update_count = args->get("unsent-update-count");
+    ASSERT_TRUE(unsent_update_count);
+    EXPECT_EQ(Element::integer, unsent_update_count->getType());
+    EXPECT_EQ(unsent_updates, static_cast<uint64_t>(unsent_update_count->intValue()));
+}
+
+// This test verifies that the correct value of the heartbeat-delay is used.
+TEST_F(HAServiceTest, recurringHeartbeatDelay) {
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Set the heartbeat delay to 6 seconds.
+    config_storage->setHeartbeatDelay(6000);
+
+    // The communication state is the member of the HAServce object. We have to
+    // replace this object with our own implementation to have an ability to
+    // test the setup of the interval timer.
+    NakedCommunicationState4Ptr state(new NakedCommunicationState4(io_service_,
+                                                                   config_storage));
+
+    TestHAService service(io_service_, network_state_, config_storage);
+    service.communication_state_ = state;
+
+    // Let's explicitly transition the state machine to the load balancing state
+    // in which the periodic heartbeats should be generated.
+    ASSERT_NO_THROW(service.verboseTransition(HA_LOAD_BALANCING_ST));
+    ASSERT_NO_THROW(service.runModel(HAService::NOP_EVT));
+
+    ASSERT_TRUE(state->timer_);
+    EXPECT_EQ(6000, state->timer_->getInterval());
+}
+
+// This test verifies that the heartbeat is periodically sent to the
+// other server.
+TEST_F(HAServiceTest, recurringHeartbeat) {
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    // All servers are configured to return success and all servers are online.
+    // The heartbeat should be successful (as indicated by the 'true' argument).
+    ASSERT_NO_FATAL_FAILURE(testRecurringHeartbeat(CONTROL_RESULT_SUCCESS, true));
+
+    // Server 2 should have received the heartbeat
+    EXPECT_GE(factory2_->getResponseCreator()->getReceivedRequests().size(), 0);
+}
+
+// This test verifies that the heartbeat is periodically sent to the
+// other server.
+TEST_F(HAServiceTest, recurringHeartbeatAuthorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    // All servers are configured to return success and all servers are online.
+    // The heartbeat should be successful (as indicated by the 'true' argument).
+    ASSERT_NO_FATAL_FAILURE(testRecurringHeartbeat(CONTROL_RESULT_SUCCESS, true));
+
+    // Server 2 should have received the heartbeat
+    EXPECT_GE(factory2_->getResponseCreator()->getReceivedRequests().size(), 0);
+}
+
+// This test verifies that the heartbeat is considered being unsuccessful if the
+// partner is offline.
+TEST_F(HAServiceTest, recurringHeartbeatServerOffline) {
+    // Start the servers but do not start server 2.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener3_->start();
+    });
+
+    // The servers are configured to return success but the server 2 is offline
+    // so the heartbeat should be unsuccessful.
+    ASSERT_NO_FATAL_FAILURE(testRecurringHeartbeat(CONTROL_RESULT_SUCCESS, false));
+
+    // Server 2 is offline so it would be very weird if it received any command.
+    EXPECT_TRUE(factory2_->getResponseCreator()->getReceivedRequests().empty());
+}
+
+// This test verifies that the heartbeat is considered being unsuccessful if the
+// partner returns error control result.
+TEST_F(HAServiceTest, recurringHeartbeatControlResultError) {
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    // Run the actual test. The servers return a control error and it is expected
+    // that the state is not poked.
+    ASSERT_NO_FATAL_FAILURE(testRecurringHeartbeat(CONTROL_RESULT_ERROR, false));
+
+    // Server 2 should have received the heartbeat.
+    EXPECT_EQ(1, factory2_->getResponseCreator()->getReceivedRequests().size());
+}
+
+// This test verifies that the heartbeat is considered being unsuccessful if
+// the partner requires not provided authentication.
+TEST_F(HAServiceTest, recurringHeartbeatControlResultUnauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    // Run the actual test. The servers return a control error and it is expected
+    // that the state is not poked.
+    ASSERT_NO_FATAL_FAILURE(testRecurringHeartbeat(CONTROL_RESULT_ERROR, false));
+
+    // Server 2 should have received the heartbeat.
+    EXPECT_TRUE(factory2_->getResponseCreator()->getReceivedRequests().empty());
+}
+
+// This test verifies that IPv4 leases can be fetched from the peer and inserted
+// or updated in the local lease database.
+TEST_F(HAServiceTest, asyncSyncLeases4) {
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=4 type=memfile persist=false"));
+
+    // Create IPv4 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases4());
+
+    for (size_t i = 0; i < leases4_.size(); ++i) {
+        // For every even lease index we add this lease to the database to exercise
+        // the scenario when a lease is already in the database and may be updated
+        // by the lease synchronization procedure.
+        if ((i % 2) == 0) {
+            // Add a copy of the lease to make sure that by modifying the lease
+            // contents we don't affect the lease in the database.
+            Lease4Ptr lease_to_add(new Lease4(*leases4_[i]));
+            // Modify valid lifetime of the lease in the database so we can
+            // later use this value to verify if the lease has been updated.
+            --lease_to_add->valid_lft_;
+            LeaseMgrFactory::instance().addLease(lease_to_add);
+        }
+    }
+
+    // Modify cltt of the first lease. This lease should be updated as a result
+    // of synchronization process because cltt is checked and the lease is
+    // updated if the cltt of the fetched lease is later than the cltt of the
+    // existing lease.
+    ++leases4_[0]->cltt_;
+
+    // For the second lease, set the wrong subnet identifier. This should be
+    // rejected and this lease shouldn't be inserted into the database.
+    // Other leases should be inserted/updated just fine.
+    ++leases4_[1]->subnet_id_ = 0;
+
+    // Modify the partner's lease cltt so it is earlier than the local lease.
+    // Therefore, this lease update should be rejected.
+    --leases4_[2]->cltt_;
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Convert leases to the JSON format, the same as used by the lease_cmds
+    // hook library. Configure our test HTTP servers to return those
+    // leases in this format.
+    ElementPtr response_arguments = Element::createMap();
+
+    // Leases are fetched in pages, so the lease4-get-page should be
+    // sent multiple times. The server is configured to return leases
+    // in 3-element chunks.
+    createPagedSyncResponses4();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT, []() {
+        // Stop running the IO service if we see a lease in the lease
+        // database which is expected to be inserted as a result of lease
+        // syncing.
+        return (!LeaseMgrFactory::instance().getLeases4(SubnetID(10)).empty());
+    }));
+
+    // Check if all leases have been stored in the local database.
+    for (size_t i = 0; i < leases4_.size(); ++i) {
+        if (i == 1) {
+            // This lease was purposely malformed and thus shouldn't be
+            // inserted into the database.
+            EXPECT_FALSE(LeaseMgrFactory::instance().getLease4(leases4_[i]->addr_))
+                << "lease " << leases4_[i]->addr_.toText()
+                << " was inserted into the database, but it shouldn't";
+
+        } else {
+            // All other leases should be in the database.
+            Lease4Ptr existing_lease = LeaseMgrFactory::instance().getLease4(leases4_[i]->addr_);
+            ASSERT_TRUE(existing_lease) << "lease " << leases4_[i]->addr_.toText()
+                                        << " not in the lease database";
+            // The lease with #2 returned by the partner is older than its local instance.
+            // The local server should reject this lease.
+            if (i == 2) {
+                // The existing lease should have unmodified timestamp because the
+                // update is expected to be rejected. Same for valid lifetime.
+                EXPECT_LT(leases4_[i]->cltt_, existing_lease->cltt_);
+                EXPECT_NE(leases4_[i]->valid_lft_, existing_lease->valid_lft_);
+
+            } else {
+                // All other leases should have the same cltt.
+                EXPECT_EQ(leases4_[i]->cltt_, existing_lease->cltt_);
+
+                // Leases with even indexes were added to the database with modified
+                // valid lifetime. Thus the local copy of each such lease should have
+                // this modified valid lifetime. The lease #0 should be updated from
+                // the partner because of the partner's cltt was set to later time.
+                if ((i != 0) && (i % 2) == 0) {
+                    EXPECT_EQ(leases4_[i]->valid_lft_ - 1, existing_lease->valid_lft_);
+
+                } else {
+                    // All other leases should have been fetched from the partner and
+                    // inserted with no change.
+                    EXPECT_EQ(leases4_[i]->valid_lft_, existing_lease->valid_lft_);
+                }
+            }
+        }
+    }
+}
+
+// This test verifies that IPv4 leases can be fetched from the peer and inserted
+// or updated in the local lease database.
+TEST_F(HAServiceTest, asyncSyncLeases4Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=4 type=memfile persist=false"));
+
+    // Create IPv4 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases4());
+
+    for (size_t i = 0; i < leases4_.size(); ++i) {
+        // For every even lease index we add this lease to the database to exercise
+        // the scenario when a lease is already in the database and may be updated
+        // by the lease synchronization procedure.
+        if ((i % 2) == 0) {
+            // Add a copy of the lease to make sure that by modifying the lease
+            // contents we don't affect the lease in the database.
+            Lease4Ptr lease_to_add(new Lease4(*leases4_[i]));
+            // Modify valid lifetime of the lease in the database so we can
+            // later use this value to verify if the lease has been updated.
+            --lease_to_add->valid_lft_;
+            LeaseMgrFactory::instance().addLease(lease_to_add);
+        }
+    }
+
+    // Modify cltt of the first lease. This lease should be updated as a result
+    // of synchronization process because cltt is checked and the lease is
+    // updated if the cltt of the fetched lease is later than the cltt of the
+    // existing lease.
+    ++leases4_[0]->cltt_;
+
+    // For the second lease, set the wrong subnet identifier. This should be
+    // rejected and this lease shouldn't be inserted into the database.
+    // Other leases should be inserted/updated just fine.
+    ++leases4_[1]->subnet_id_ = 0;
+
+    // Modify the partner's lease cltt so it is earlier than the local lease.
+    // Therefore, this lease update should be rejected.
+    --leases4_[2]->cltt_;
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Convert leases to the JSON format, the same as used by the lease_cmds
+    // hook library. Configure our test HTTP servers to return those
+    // leases in this format.
+    ElementPtr response_arguments = Element::createMap();
+
+    // Leases are fetched in pages, so the lease4-get-page should be
+    // sent multiple times. The server is configured to return leases
+    // in 3-element chunks.
+    createPagedSyncResponses4();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT, []() {
+        // Stop running the IO service if we see a lease in the lease
+        // database which is expected to be inserted as a result of lease
+        // syncing.
+        return (!LeaseMgrFactory::instance().getLeases4(SubnetID(10)).empty());
+    }));
+
+    // Check if all leases have been stored in the local database.
+    for (size_t i = 0; i < leases4_.size(); ++i) {
+        if (i == 1) {
+            // This lease was purposely malformed and thus shouldn't be
+            // inserted into the database.
+            EXPECT_FALSE(LeaseMgrFactory::instance().getLease4(leases4_[i]->addr_))
+                << "lease " << leases4_[i]->addr_.toText()
+                << " was inserted into the database, but it shouldn't";
+
+        } else {
+            // All other leases should be in the database.
+            Lease4Ptr existing_lease = LeaseMgrFactory::instance().getLease4(leases4_[i]->addr_);
+            ASSERT_TRUE(existing_lease) << "lease " << leases4_[i]->addr_.toText()
+                                        << " not in the lease database";
+            // The lease with #2 returned by the partner is older than its local instance.
+            // The local server should reject this lease.
+            if (i == 2) {
+                // The existing lease should have unmodified timestamp because the
+                // update is expected to be rejected. Same for valid lifetime.
+                EXPECT_LT(leases4_[i]->cltt_, existing_lease->cltt_);
+                EXPECT_NE(leases4_[i]->valid_lft_, existing_lease->valid_lft_);
+
+            } else {
+                // All other leases should have the same cltt.
+                EXPECT_EQ(leases4_[i]->cltt_, existing_lease->cltt_);
+
+                // Leases with even indexes were added to the database with modified
+                // valid lifetime. Thus the local copy of each such lease should have
+                // this modified valid lifetime. The lease #0 should be updated from
+                // the partner because of the partner's cltt was set to later time.
+                if ((i != 0) && (i % 2) == 0) {
+                    EXPECT_EQ(leases4_[i]->valid_lft_ - 1, existing_lease->valid_lft_);
+
+                } else {
+                    // All other leases should have been fetched from the partner and
+                    // inserted with no change.
+                    EXPECT_EQ(leases4_[i]->valid_lft_, existing_lease->valid_lft_);
+                }
+            }
+        }
+    }
+}
+
+// Test that there is no exception thrown during leases synchronization
+// when server returns a wrong answer.
+TEST_F(HAServiceTest, asyncSyncLeases4WrongAnswer) {
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=4 type=memfile persist=false"));
+
+    // Create IPv4 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases4());
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Set empty response. This should cause the HA service to log an
+    // error but not crash.
+    ElementPtr response_arguments = Element::createMap();
+
+    factory2_->getResponseCreator()->setArguments(response_arguments);
+    factory3_->getResponseCreator()->setArguments(response_arguments);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(1000));
+}
+
+// Test that there is no exception thrown during leases synchronization
+// when servers are offline.
+TEST_F(HAServiceTest, asyncSyncLeases4ServerOffline) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service for 1 second.
+    ASSERT_NO_THROW(runIOService(1000));
+}
+
+// Test that there is no exception thrown during leases synchronization
+// when servers require not provided authentication.
+TEST_F(HAServiceTest, asyncSyncLeases4ServerUnauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=4 type=memfile persist=false"));
+
+    // Create IPv4 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases4());
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service for 1 second.
+    ASSERT_NO_THROW(runIOService(1000));
+}
+
+// This test verifies that IPv6 leases can be fetched from the peer and inserted
+// or updated in the local lease database.
+TEST_F(HAServiceTest, asyncSyncLeases6) {
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=6 type=memfile persist=false"));
+
+    // Create IPv6 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases6());
+
+    for (size_t i = 0; i < leases6_.size(); ++i) {
+        // For every even lease index we add this lease to the database to exercise
+        // the scenario when a lease is already in the database and may be updated
+        // by the lease synchronization procedure.
+        if ((i % 2) == 0) {
+            // Add a copy of the lease to make sure that by modifying the lease
+            // contents we don't affect the lease in the database.
+            Lease6Ptr lease_to_add(new Lease6(*leases6_[i]));
+            // Modify valid lifetime of the lease in the database so we can
+            // later use this value to verify if the lease has been updated.
+            --lease_to_add->valid_lft_;
+            LeaseMgrFactory::instance().addLease(lease_to_add);
+        }
+    }
+
+    // Modify cltt of the first lease. This lease should be updated as a result
+    // of synchronization process because cltt is checked and the lease is
+    // updated if the cltt of the fetched lease is later than the cltt of the
+    // existing lease.
+    ++leases6_[0]->cltt_;
+
+    // For the second lease, set the wrong subnet identifier. This should be
+    // rejected and this lease shouldn't be inserted into the database.
+    // Other leases should be inserted/updated just fine.
+    ++leases6_[1]->subnet_id_ = 0;
+
+    // Modify the partner's lease cltt so it is earlier than the local lease.
+    // Therefore, this lease update should be rejected.
+    --leases6_[2]->cltt_;
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Convert leases to the JSON format, the same as used by the lease_cmds
+    // hook library. Configure our test HTTP servers to return those
+    // leases in this format.
+    ElementPtr response_arguments = Element::createMap();
+
+    // Leases are fetched in pages, so the lease4-get-page should be
+    // sent multiple times. We need to configure the server side to
+    // return leases in 3-element chunks.
+    createPagedSyncResponses6();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage,
+                          HAServerType::DHCPv6);
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT, []() {
+        // Stop running the IO service if we see a lease in the lease
+        // database which is expected to be inserted as a result of lease
+        // syncing.
+        return (!LeaseMgrFactory::instance().getLeases6(SubnetID(10)).empty());
+    }));
+
+    // Check if all leases have been stored in the local database.
+    for (size_t i = 0; i < leases6_.size(); ++i) {
+        if (i == 1) {
+            // This lease was purposely malformed and thus shouldn't be
+            // inserted into the database.
+            EXPECT_FALSE(LeaseMgrFactory::instance().getLease6(Lease::TYPE_NA,
+                                                               leases6_[i]->addr_))
+                << "lease " << leases6_[i]->addr_.toText()
+                << " was inserted into the database, but it shouldn't";
+        } else {
+            // Other leases should be inserted/updated.
+            Lease6Ptr existing_lease = LeaseMgrFactory::instance().getLease6(Lease::TYPE_NA,
+                                                                             leases6_[i]->addr_);
+            ASSERT_TRUE(existing_lease) << "lease " << leases6_[i]->addr_.toText()
+                                        << " not in the lease database";
+
+            if (i == 2) {
+                // The existing lease should have unmodified timestamp because the
+                // update is expected to be rejected. Same for valid lifetime.
+                EXPECT_LT(leases6_[i]->cltt_, existing_lease->cltt_);
+                EXPECT_NE(leases6_[i]->valid_lft_, existing_lease->valid_lft_);
+
+            } else {
+                // All other leases should have the same cltt.
+                EXPECT_EQ(leases6_[i]->cltt_, existing_lease->cltt_);
+
+                // Leases with even indexes were added to the database with modified
+                // valid lifetime. Thus the local copy of each such lease should have
+                // this modified valid lifetime. The lease #0 should be updated from
+                // the partner because of the partner's cltt was set to later time.
+                if ((i != 0) && (i % 2) == 0) {
+                    EXPECT_EQ(leases6_[i]->valid_lft_ - 1, existing_lease->valid_lft_);
+
+                } else {
+                    // All other leases should have been fetched from the partner and
+                    // inserted with no change.
+                    EXPECT_EQ(leases6_[i]->valid_lft_, existing_lease->valid_lft_);
+                }
+            }
+        }
+    }
+}
+
+// This test verifies that IPv6 leases can be fetched from the peer and inserted
+// or updated in the local lease database.
+TEST_F(HAServiceTest, asyncSyncLeases6Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=6 type=memfile persist=false"));
+
+    // Create IPv6 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases6());
+
+    for (size_t i = 0; i < leases6_.size(); ++i) {
+        // For every even lease index we add this lease to the database to exercise
+        // the scenario when a lease is already in the database and may be updated
+        // by the lease synchronization procedure.
+        if ((i % 2) == 0) {
+            // Add a copy of the lease to make sure that by modifying the lease
+            // contents we don't affect the lease in the database.
+            Lease6Ptr lease_to_add(new Lease6(*leases6_[i]));
+            // Modify valid lifetime of the lease in the database so we can
+            // later use this value to verify if the lease has been updated.
+            --lease_to_add->valid_lft_;
+            LeaseMgrFactory::instance().addLease(lease_to_add);
+        }
+    }
+
+    // Modify cltt of the first lease. This lease should be updated as a result
+    // of synchronization process because cltt is checked and the lease is
+    // updated if the cltt of the fetched lease is later than the cltt of the
+    // existing lease.
+    ++leases6_[0]->cltt_;
+
+    // For the second lease, set the wrong subnet identifier. This should be
+    // rejected and this lease shouldn't be inserted into the database.
+    // Other leases should be inserted/updated just fine.
+    ++leases6_[1]->subnet_id_ = 0;
+
+    // Modify the partner's lease cltt so it is earlier than the local lease.
+    // Therefore, this lease update should be rejected.
+    --leases6_[2]->cltt_;
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Convert leases to the JSON format, the same as used by the lease_cmds
+    // hook library. Configure our test HTTP servers to return those
+    // leases in this format.
+    ElementPtr response_arguments = Element::createMap();
+
+    // Leases are fetched in pages, so the lease4-get-page should be
+    // sent multiple times. We need to configure the server side to
+    // return leases in 3-element chunks.
+    createPagedSyncResponses6();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage,
+                          HAServerType::DHCPv6);
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT, []() {
+        // Stop running the IO service if we see a lease in the lease
+        // database which is expected to be inserted as a result of lease
+        // syncing.
+        return (!LeaseMgrFactory::instance().getLeases6(SubnetID(10)).empty());
+    }));
+
+    // Check if all leases have been stored in the local database.
+    for (size_t i = 0; i < leases6_.size(); ++i) {
+        if (i == 1) {
+            // This lease was purposely malformed and thus shouldn't be
+            // inserted into the database.
+            EXPECT_FALSE(LeaseMgrFactory::instance().getLease6(Lease::TYPE_NA,
+                                                               leases6_[i]->addr_))
+                << "lease " << leases6_[i]->addr_.toText()
+                << " was inserted into the database, but it shouldn't";
+        } else {
+            // Other leases should be inserted/updated.
+            Lease6Ptr existing_lease = LeaseMgrFactory::instance().getLease6(Lease::TYPE_NA,
+                                                                             leases6_[i]->addr_);
+            ASSERT_TRUE(existing_lease) << "lease " << leases6_[i]->addr_.toText()
+                                        << " not in the lease database";
+
+            if (i == 2) {
+                // The existing lease should have unmodified timestamp because the
+                // update is expected to be rejected. Same for valid lifetime.
+                EXPECT_LT(leases6_[i]->cltt_, existing_lease->cltt_);
+                EXPECT_NE(leases6_[i]->valid_lft_, existing_lease->valid_lft_);
+
+            } else {
+                // All other leases should have the same cltt.
+                EXPECT_EQ(leases6_[i]->cltt_, existing_lease->cltt_);
+
+                // Leases with even indexes were added to the database with modified
+                // valid lifetime. Thus the local copy of each such lease should have
+                // this modified valid lifetime. The lease #0 should be updated from
+                // the partner because of the partner's cltt was set to later time.
+                if ((i != 0) && (i % 2) == 0) {
+                    EXPECT_EQ(leases6_[i]->valid_lft_ - 1, existing_lease->valid_lft_);
+
+                } else {
+                    // All other leases should have been fetched from the partner and
+                    // inserted with no change.
+                    EXPECT_EQ(leases6_[i]->valid_lft_, existing_lease->valid_lft_);
+                }
+            }
+        }
+    }
+}
+
+// Test that there is no exception thrown during IPv6 leases synchronization
+// when server returns a wrong answer.
+TEST_F(HAServiceTest, asyncSyncLeases6WrongAnswer) {
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=6 type=memfile persist=false"));
+
+    // Create IPv6 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases6());
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Set empty response. This should cause the HA service to log an
+    // error but not crash.
+    ElementPtr response_arguments = Element::createMap();
+
+    factory2_->getResponseCreator()->setArguments(response_arguments);
+    factory3_->getResponseCreator()->setArguments(response_arguments);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage,
+                          HAServerType::DHCPv6);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(1000));
+}
+
+// Test that there is no exception thrown during IPv6 leases synchronization
+// when servers are offline.
+TEST_F(HAServiceTest, asyncSyncLeases6ServerOffline) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    TestHAService service(io_service_, network_state_, config_storage,
+                          HAServerType::DHCPv6);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service for 1 second.
+    ASSERT_NO_THROW(runIOService(1000));
+}
+
+// Test that there is no exception thrown during IPv6 leases synchronization
+// when server requires not provided authentication.
+TEST_F(HAServiceTest, asyncSyncLeases6Unauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create lease manager.
+    ASSERT_NO_THROW(LeaseMgrFactory::create("universe=6 type=memfile persist=false"));
+
+    // Create IPv6 leases which will be fetched from the other server.
+    ASSERT_NO_THROW(generateTestLeases6());
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Setting the heartbeat delay to 0 disables the recurring heartbeat.
+    // We just want to synchronize leases and not send the heartbeat.
+    config_storage->setHeartbeatDelay(0);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage,
+                          HAServerType::DHCPv6);
+
+    // Start fetching leases asynchronously.
+    ASSERT_NO_THROW(service.asyncSyncLeases());
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(1000));
+}
+
+// This test verifies that the ha-sync command is processed successfully for the
+// DHCPv4 server.
+TEST_F(HAServiceTest, processSynchronize4) {
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate success.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Lease database synchronization"
+                " complete.");
+
+    // All leases should have been inserted into the database.
+    for (size_t i = 0; i < leases4_.size(); ++i) {
+        Lease4Ptr existing_lease = LeaseMgrFactory::instance().getLease4(leases4_[i]->addr_);
+        ASSERT_TRUE(existing_lease) << "lease " << leases4_[i]->addr_.toText()
+                                    << " not in the lease database";
+    }
+
+    // The following commands should have been sent to the server2: dhcp-disable,
+    // lease4-get-page and ha-sync-complete-notify.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease4-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify", ""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-enable", ""));
+}
+
+// This test verifies that the ha-sync command is processed successfully for the
+// DHCPv4 server.
+TEST_F(HAServiceTest, processSynchronize4Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate success.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Lease database synchronization"
+                " complete.");
+
+    // All leases should have been inserted into the database.
+    for (size_t i = 0; i < leases4_.size(); ++i) {
+        Lease4Ptr existing_lease = LeaseMgrFactory::instance().getLease4(leases4_[i]->addr_);
+        ASSERT_TRUE(existing_lease) << "lease " << leases4_[i]->addr_.toText()
+                                    << " not in the lease database";
+    }
+
+    // The following commands should have been sent to the server2: dhcp-disable,
+    // lease4-get-page and ha-sync-complete-notify.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease4-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify", ""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-enable", ""));
+}
+
+// This test verifies that an error is reported when sending a dhcp-disable
+// command causes an error.
+TEST_F(HAServiceTest, processSynchronizeDisableError) {
+    // Setup the server2 to return an error to dhcp-disable commands.
+    factory2_->getResponseCreator()->setControlResult("dhcp-disable",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should only receive dhcp-disable command. Remaining three should
+    // not be sent.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("lease4-get-page",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that an error is reported when sending any not
+// authenticated command causes a not authorized error.
+TEST_F(HAServiceTest, processSynchronizeUnauthorized) {
+    // Instruct server2 to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should only receive dhcp-disable commands.
+    EXPECT_TRUE(factory2_->getResponseCreator()->getReceivedRequests().empty());
+}
+
+// This test verifies that an error is reported when sending a lease4-get-page
+// command causes an error.
+TEST_F(HAServiceTest, processSynchronizeLease4GetPageError) {
+    // Setup the server2 to return an error to dhcp-disable commands.
+    factory2_->getResponseCreator()->setControlResult("lease4-get-page",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should receive dhcp-disable, lease4-get-page and dhcp-enable
+    // commands. The lease4-get-page command results in an error (configured at
+    // the beginning of this test), but the dhcp-enable command should be sent
+    // to enable the DHCP service after the error anyway. The
+    // ha-sync-complete-notify must not be sent after the error. It should only
+    // be sent after successful synchronization.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease4-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+}
+
+// This test verifies that an error is reported when sending a dhcp-enable
+// command causes an error.
+TEST_F(HAServiceTest, processSynchronizeEnableError) {
+    // Setup the server2 to return an error to dhcp-disable commands.
+    factory2_->getResponseCreator()->setControlResult("dhcp-enable",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Return the unsupported command status for this command to enforce
+    // sending the dhcp-enable command.
+    factory2_->getResponseCreator()->setControlResult("ha-sync-complete-notify",
+                                                      CONTROL_RESULT_COMMAND_UNSUPPORTED);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should receive four commands of which ha-sync-complete-notify
+    // was unsupported.
+
+    auto requests = factory2_->getResponseCreator()->getReceivedRequests();
+    ASSERT_GE(requests.size(), 3);
+
+    // The dhcp-disable should be the first request.
+    auto request = factory2_->getResponseCreator()->findRequest("dhcp-disable","20");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(requests[0], request);
+
+    // There may be multiple lease4-get-page commands but the first one should
+    // follow the dhcp-disable request.
+    request = factory2_->getResponseCreator()->findRequest("lease4-get-page", "");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(requests[1], request);
+
+    // The ha-sync-complete-notify should be last but one.
+    request = factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify", "");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(requests[requests.size()-2], request);
+
+    // The dhcp-enable should be last.
+    request = factory2_->getResponseCreator()->findRequest("dhcp-enable", "");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(requests[requests.size()-1], request);
+}
+
+// This test verifies that dhcp-enable command is not sent to the partner after
+// receiving an error to the ha-sync-complete-notify command.
+TEST_F(HAServiceTest, processSynchronizeNotifyError) {
+    // Return an error to the ha-sync-complete-notify command.
+    factory2_->getResponseCreator()->setControlResult("ha-sync-complete-notify",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize4(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    auto requests = factory2_->getResponseCreator()->getReceivedRequests();
+
+    ASSERT_GE(requests.size(), 3);
+
+    // The dhcp-disable should be the first request.
+    auto request = factory2_->getResponseCreator()->findRequest("dhcp-disable","20");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(requests[0], request);
+
+    // There may be multiple lease4-get-page commands but the first one should
+    // follow the dhcp-disable request.
+    request = factory2_->getResponseCreator()->findRequest("lease4-get-page","");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(requests[1], request);
+
+    // The ha-sync-complete-notify should be last.
+    request = factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify","");
+    ASSERT_TRUE(request);
+    EXPECT_EQ(*(requests.rbegin()), request);
+
+    // The dhcp-enable should not be sent after ha-sync-complete-notify failure.
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that the ha-sync command is processed successfully for the
+// DHCPv6 server.
+TEST_F(HAServiceTest, processSynchronize6) {
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate success.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Lease database synchronization"
+                " complete.");
+
+    // All leases should have been inserted into the database.
+    for (size_t i = 0; i < leases6_.size(); ++i) {
+        Lease6Ptr existing_lease = LeaseMgrFactory::instance().getLease6(Lease::TYPE_NA,
+                                                                         leases6_[i]->addr_);
+        ASSERT_TRUE(existing_lease) << "lease " << leases6_[i]->addr_.toText()
+                                    << " not in the lease database";
+    }
+
+    // The following commands should have been sent to the server2: dhcp-disable,
+    // lease6-get-page and ha-sync-complete-notify.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+}
+
+// This test verifies that the ha-sync command is processed successfully for the
+// DHCPv6 server.
+TEST_F(HAServiceTest, processSynchronize6Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate success.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Lease database synchronization"
+                " complete.");
+
+    // All leases should have been inserted into the database.
+    for (size_t i = 0; i < leases6_.size(); ++i) {
+        Lease6Ptr existing_lease = LeaseMgrFactory::instance().getLease6(Lease::TYPE_NA,
+                                                                         leases6_[i]->addr_);
+        ASSERT_TRUE(existing_lease) << "lease " << leases6_[i]->addr_.toText()
+                                    << " not in the lease database";
+    }
+
+    // The following commands should have been sent to the server2: dhcp-disable,
+    // lease6-get-page and ha-sync-complete-notify.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+}
+
+// This test verifies that an error is reported when sending a dhcp-disable
+// command causes an error.
+TEST_F(HAServiceTest, processSynchronize6DisableError) {
+    // Setup the server2 to return an error to dhcp-disable commands.
+    factory2_->getResponseCreator()->setControlResult("dhcp-disable",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should only receive dhcp-disable command.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("lease6-get-page",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that an error is reported when sending any not
+// authenticated command causes a not authorized error.
+TEST_F(HAServiceTest, processSynchronize6Unauthorized) {
+    // Instruct server2 to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should only receive dhcp-disable commands.
+    EXPECT_TRUE(factory2_->getResponseCreator()->getReceivedRequests().empty());
+}
+
+// This test verifies that an error is reported when sending a lease6-get-page
+// command causes an error.
+TEST_F(HAServiceTest, processSynchronizeLease6GetPageError) {
+    // Setup the server2 to return an error to dhcp-disable commands.
+    factory2_->getResponseCreator()->setControlResult("lease6-get-page",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should receive all commands. The dhcp-disable was successful, so
+    // the dhcp-enable command must be sent to re-enable the service after failure.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+}
+
+// This test verifies that an error is reported when sending a dhcp-enable
+// command causes an error.
+TEST_F(HAServiceTest, processSynchronize6EnableError) {
+    // Setup the server2 to return an error to dhcp-disable commands.
+    factory2_->getResponseCreator()->setControlResult("dhcp-enable",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Return the unsupported command status for this command to enforce
+    // sending the dhcp-enable command.
+    factory2_->getResponseCreator()->setControlResult("ha-sync-complete-notify",
+                                                      CONTROL_RESULT_COMMAND_UNSUPPORTED);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    // The server2 should receive four commands of which ha-sync-complete-notify
+    // was unsupported.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that dhcp-enable command is not sent to the partner after
+// receiving an error to the ha-sync-complete-notify command.
+TEST_F(HAServiceTest, processSynchronize6NotifyError) {
+    // Return an error to the ha-sync-complete-notify command.
+    factory2_->getResponseCreator()->setControlResult("ha-sync-complete-notify",
+                                                      CONTROL_RESULT_ERROR);
+
+    // Run HAService::processSynchronize and gather a response.
+    ConstElementPtr rsp;
+    runProcessSynchronize6(rsp);
+
+    // The response should indicate an error
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR);
+
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-disable","20"));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("lease6-get-page",""));
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("ha-sync-complete-notify",""));
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that the DHCPv4 service can be disabled on the remote server.
+TEST_F(HAServiceTest, asyncDisableDHCPService4) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-disable command with max-period of 10 seconds.
+    // When the transaction is finished, the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncDisableDHCPService("server3", 10,
+                                                    [this](const bool success,
+                                                           const std::string& error_message,
+                                                           const int) {
+        EXPECT_TRUE(success);
+        EXPECT_TRUE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+
+    // The second server should not receive the command.
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-disable","10"));
+    // The third server should receive the dhcp-disable command with the max-period
+    // value of 10.
+    EXPECT_TRUE(factory3_->getResponseCreator()->findRequest("dhcp-disable","10"));
+}
+
+// This test verifies that the DHCPv4 service can be disabled on the remote server.
+TEST_F(HAServiceTest, asyncDisableDHCPService4Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-disable command with max-period of 10 seconds.
+    // When the transaction is finished, the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncDisableDHCPService("server3", 10,
+                                                    [this](const bool success,
+                                                           const std::string& error_message,
+                                                           const int) {
+        EXPECT_TRUE(success);
+        EXPECT_TRUE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+
+    // The second server should not receive the command.
+    EXPECT_FALSE(factory2_->getResponseCreator()->findRequest("dhcp-disable","10"));
+    // The third server should receive the dhcp-disable command with the max-period
+    // value of 10.
+    EXPECT_TRUE(factory3_->getResponseCreator()->findRequest("dhcp-disable","10"));
+}
+
+// This test verifies that there is no exception thrown as a result of dhcp-disable
+// command when the server is offline.
+TEST_F(HAServiceTest, asyncDisableDHCPService4ServerOffline) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-disable command with max-period of 10 seconds.
+    // When the transaction is finished, the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncDisableDHCPService("server2", 10,
+                                                    [this](const bool success,
+                                                           const std::string& error_message,
+                                                           const int) {
+        EXPECT_FALSE(success);
+        EXPECT_FALSE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+}
+
+// This test verifies that an error is returned when the remote server
+// returns control status error.
+TEST_F(HAServiceTest, asyncDisableDHCPService4ControlResultError) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Set the servers to return error code in response to the dhcp-enable
+    // command.
+    factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+    factory3_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-disable command with max-period of 10 seconds.
+    // When the transaction is finished, the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncDisableDHCPService("server3", 10,
+                                                    [this](const bool success,
+                                                           const std::string& error_message,
+                                                           const int) {
+        EXPECT_FALSE(success);
+        EXPECT_FALSE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+}
+
+// This test verifies that an error is returned when the remote server
+// requires not provided authentication.
+TEST_F(HAServiceTest, asyncDisableDHCPService4ControlResultUnauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-disable command with max-period of 10 seconds.
+    // When the transaction is finished, the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncDisableDHCPService("server3", 10,
+                                                    [this](const bool success,
+                                                           const std::string& error_message,
+                                                           const int) {
+        EXPECT_FALSE(success);
+        EXPECT_FALSE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+}
+
+// This test verifies that the DHCPv4 service can be enabled on the remote server.
+TEST_F(HAServiceTest, asyncEnableDHCPService4) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-enable command. When the transaction is finished,
+    // the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncEnableDHCPService("server2",
+                                                   [this](const bool success,
+                                                          const std::string& error_message,
+                                                          const int) {
+        EXPECT_TRUE(success);
+        EXPECT_TRUE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+
+    // The second server should receive the dhcp-enable.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+    // The third server should not receive the command.
+    EXPECT_FALSE(factory3_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that the DHCPv4 service can be enabled on the remote server.
+TEST_F(HAServiceTest, asyncEnableDHCPService4Authorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-enable command. When the transaction is finished,
+    // the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncEnableDHCPService("server2",
+                                                   [this](const bool success,
+                                                          const std::string& error_message,
+                                                          const int) {
+        EXPECT_TRUE(success);
+        EXPECT_TRUE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+
+    // The second server should receive the dhcp-enable.
+    EXPECT_TRUE(factory2_->getResponseCreator()->findRequest("dhcp-enable",""));
+    // The third server should not receive the command.
+    EXPECT_FALSE(factory3_->getResponseCreator()->findRequest("dhcp-enable",""));
+}
+
+// This test verifies that there is no exception thrown as a result of dhcp-enable
+// command when the server is offline.
+TEST_F(HAServiceTest, asyncEnableDHCPService4ServerOffline) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-enable command. When the transaction is finished,
+    // the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncEnableDHCPService("server2",
+                                                   [this](const bool success,
+                                                          const std::string& error_message,
+                                                          const int) {
+        EXPECT_FALSE(success);
+        EXPECT_FALSE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+}
+
+// This test verifies that an error is returned when the remote server
+// returns control status error.
+TEST_F(HAServiceTest, asyncEnableDHCPService4ControlResultError) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Set the servers to return error code in response to the dhcp-enable
+    // command.
+    factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+    factory3_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-enable command. When the transaction is finished,
+    // the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncEnableDHCPService("server2",
+                                                   [this](const bool success,
+                                                          const std::string& error_message,
+                                                          const int) {
+        EXPECT_FALSE(success);
+        EXPECT_FALSE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+}
+
+// This test verifies that an error is returned when the remote server
+// requires not provided authentication.
+TEST_F(HAServiceTest, asyncEnableDHCPService4ControlResultUnauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+        listener3_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Send dhcp-enable command. When the transaction is finished,
+    // the IO service gets stopped.
+    ASSERT_NO_THROW(service.asyncEnableDHCPService("server2",
+                                                   [this](const bool success,
+                                                          const std::string& error_message,
+                                                          const int) {
+        EXPECT_FALSE(success);
+        EXPECT_FALSE(error_message.empty());
+        io_service_->stop();
+    }));
+
+    // Run IO service to actually perform the transaction.
+    ASSERT_NO_THROW(runIOService(TEST_TIMEOUT));
+}
+
+// This test verifies that the "ha-scopes" command is processed correctly.
+TEST_F(HAServiceTest, processScopes) {
+    // Create HA configuration.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Create HA service using this configuration.
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Enable "server1" and "server2" scopes.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processScopes({ "server1", "server2" }));
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "New HA scopes configured.");
+
+    // Verify that "server1" and "server2" scopes are enabled.
+    EXPECT_TRUE(service.query_filter_.amServingScope("server1"));
+    EXPECT_TRUE(service.query_filter_.amServingScope("server2"));
+    EXPECT_FALSE(service.query_filter_.amServingScope("server3"));
+
+    // Enable "server2" scope only.
+    ASSERT_NO_THROW(rsp = service.processScopes({ "server2" }));
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "New HA scopes configured.");
+    EXPECT_FALSE(service.query_filter_.amServingScope("server1"));
+    EXPECT_TRUE(service.query_filter_.amServingScope("server2"));
+    EXPECT_FALSE(service.query_filter_.amServingScope("server3"));
+
+    // Clear scopes.
+    ASSERT_NO_THROW(rsp = service.processScopes({ }));
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "New HA scopes configured.");
+    EXPECT_FALSE(service.query_filter_.amServingScope("server1"));
+    EXPECT_FALSE(service.query_filter_.amServingScope("server2"));
+    EXPECT_FALSE(service.query_filter_.amServingScope("server3"));
+
+    // Unsupported scope.
+    ASSERT_NO_THROW(rsp = service.processScopes({ "server1", "unsupported", "server3" }));
+    checkAnswer(rsp, CONTROL_RESULT_ERROR, "invalid server name specified 'unsupported'"
+                " while enabling/disabling HA scopes");
+    // Even though the "server1" is a valid scope name, it should not be
+    // enabled because we expect scopes enabling to be atomic operation,
+    // i.e. all or nothing.
+    EXPECT_FALSE(service.query_filter_.amServingScope("server1"));
+    EXPECT_FALSE(service.query_filter_.amServingScope("server2"));
+    EXPECT_FALSE(service.query_filter_.amServingScope("server3"));
+}
+
+// This test verifies that the ha-continue command is processed successfully.
+TEST_F(HAServiceTest, processContinue) {
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // State machine is to be paused in the waiting state.
+    ASSERT_NO_THROW(config_storage->getStateMachineConfig()->
+                    getStateConfig(HA_WAITING_ST)->setPausing("always"));
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Pause the state machine.
+    EXPECT_NO_THROW(service.transition(HA_WAITING_ST, HAService::NOP_EVT));
+    EXPECT_TRUE(service.isModelPaused());
+
+    // Process ha-continue command that should unpause the state machine.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processContinue());
+
+    // The server should have responded.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "HA state machine continues.");
+
+    // State machine should have been unpaused as a result of processing the
+    // command.
+    EXPECT_FALSE(service.isModelPaused());
+
+    // Response should include no arguments.
+    EXPECT_FALSE(rsp->get("arguments"));
+
+    // Sending ha-continue command again which should return success but
+    // slightly different textual status.
+    ASSERT_NO_THROW(rsp = service.processContinue());
+
+    // The server should have responded.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "HA state machine is not paused.");
+
+    // The state machine should not be paused.
+    EXPECT_FALSE(service.isModelPaused());
+
+    // Response should include no arguments.
+    EXPECT_FALSE(rsp->get("arguments"));
+}
+
+// This test verifies that the ha-maintenance-notify command is processed
+// successfully and transitions the state machine to the in-maintenance state
+// and that it is possible to revert to the previous state.
+// It also verifies that this command fails to transition the state machine
+// for some selected states for which it is unsupported.
+TEST_F(HAServiceTest, processMaintenanceNotify) {
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    std::set<int> valid_states = {
+        HA_WAITING_ST,
+        HA_READY_ST,
+        HA_SYNCING_ST,
+        HA_PARTNER_DOWN_ST,
+        HA_LOAD_BALANCING_ST,
+        HA_HOT_STANDBY_ST,
+    };
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Test transition from the states for which it is allowed.
+    for (auto state : valid_states) {
+        EXPECT_NO_THROW(service.transition(state, HAService::NOP_EVT));
+
+        // Process ha-maintenance-notify command that should transition the
+        // state machine to the in-maintenance state.
+        ConstElementPtr rsp;
+        ASSERT_NO_THROW(rsp = service.processMaintenanceNotify(false));
+
+        // The server should have responded.
+        ASSERT_TRUE(rsp);
+        checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Server is in-maintenance state.");
+
+        // The state machine should have been transitioned to the in-maintenance state.
+        EXPECT_EQ(HA_IN_MAINTENANCE_ST, service.getCurrState());
+
+        // Try to cancel the maintenance.
+        ASSERT_NO_THROW(rsp = service.processMaintenanceNotify(true));
+
+        // The server should have responded.
+        ASSERT_TRUE(rsp);
+        checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Server maintenance canceled.");
+
+        // The state machine should have been transitioned to the state it was in
+        // prior to transitioning to the in-maintenance state.
+        EXPECT_EQ(state, service.getCurrState());
+    }
+
+    std::set<int> invalid_states = {
+        HA_TERMINATED_ST,
+        HA_BACKUP_ST,
+        HA_PARTNER_IN_MAINTENANCE_ST
+    };
+
+    // Make sure that the transition from the other states is not allowed.
+    for (auto state : invalid_states) {
+        EXPECT_NO_THROW(service.transition(state, HAService::NOP_EVT));
+        EXPECT_NO_THROW(service.runModel(HAService::NOP_EVT));
+
+        ConstElementPtr rsp;
+        ASSERT_NO_THROW(rsp = service.processMaintenanceNotify(false));
+
+        ASSERT_TRUE(rsp);
+        checkAnswer(rsp, TestHAService::HA_CONTROL_RESULT_MAINTENANCE_NOT_ALLOWED,
+                    "Unable to transition the server from the "
+                    + stateToString(state) + " to in-maintenance state.");
+    }
+}
+
+// This test verifies the case when the server receiving the ha-maintenance-start
+// command successfully transitions to the partner-in-maintenance state and its
+// partner transitions to the in-maintenance state.
+TEST_F(HAServiceTest, processMaintenanceStartSuccess) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    HAService service(io_service_, network_state_, config_storage);
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-start command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceStart());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner of our server is online and should have responded with
+    // the success status. Therefore, this server should have transitioned
+    // to the partner-in-maintenance state.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Server is now in the partner-in-maintenance state"
+                " and its partner is in-maintenance state. The partner can be now safely"
+                " shut down.");
+
+    EXPECT_EQ(HA_PARTNER_IN_MAINTENANCE_ST, service.getCurrState());
+}
+
+// This test verifies the case when the server receiving the ha-maintenance-start
+// command successfully transitions to the partner-in-maintenance state and its
+// partner transitions to the in-maintenance state.
+TEST_F(HAServiceTest, processMaintenanceStartSuccessAuthorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    HAService service(io_service_, network_state_, config_storage);
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-start command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceStart());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner of our server is online and should have responded with
+    // the success status. Therefore, this server should have transitioned
+    // to the partner-in-maintenance state.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Server is now in the partner-in-maintenance state"
+                " and its partner is in-maintenance state. The partner can be now safely"
+                " shut down.");
+
+    EXPECT_EQ(HA_PARTNER_IN_MAINTENANCE_ST, service.getCurrState());
+}
+
+// This test verifies the case that the server transitions to the partner-down
+// state after receiving the ha-maintenance-start command. This is the case
+// when the communication with the partner server fails while this command
+// is received. It is assumed that the partner server is already terminated
+// for maintenance.
+TEST_F(HAServiceTest, processMaintenanceStartPartnerDown) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Start the server, but don't start the partner. This simulates
+    // the case that the partner is already down for maintenance.
+    ASSERT_NO_THROW({
+        listener_->start();
+    });
+
+    HAService service(io_service_, network_state_, config_storage);
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-start command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceStart());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner of our server is online and should have responded with
+    // the success status. Therefore, this server should have transitioned
+    // to the partner-in-maintenance state.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS,
+                "Server is now in the partner-down state as its"
+                " partner appears to be offline for maintenance.");
+
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service.getCurrState());
+}
+
+// This test verifies the case when the server is receiving
+// ha-maintenance-start command and tries to notify the partner
+// which returns an error.
+TEST_F(HAServiceTest, processMaintenanceStartPartnerError) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Simulate an error returned by the partner.
+    factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    HAService service(io_service_, network_state_, config_storage);
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-start command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceStart());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS,
+                "Server is now in the partner-down state as its"
+                " partner appears to be offline for maintenance.");
+
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service.getCurrState());
+}
+
+// This test verifies the case when the server is receiving
+// ha-maintenance-start command and tries to notify the partner
+// which requires not provided authentication.
+TEST_F(HAServiceTest, processMaintenanceStartPartnerUnauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    HAService service(io_service_, network_state_, config_storage);
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-start command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceStart());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS,
+                "Server is now in the partner-down state as its"
+                " partner appears to be offline for maintenance.");
+
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service.getCurrState());
+}
+
+// This test verifies the case when the server is receiving
+// ha-maintenance-start command and tries to notify the partner
+// which returns a special result indicating that it can't enter
+// the in-maintenance state.
+TEST_F(HAServiceTest, processMaintenanceStartNotAllowed) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Simulate an error returned by the partner.
+    factory2_->getResponseCreator()->
+        setControlResult(TestHAService::HA_CONTROL_RESULT_MAINTENANCE_NOT_ALLOWED);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    HAService service(io_service_, network_state_, config_storage);
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-start command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceStart());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR,
+                "Unable to transition to the partner-in-maintenance state."
+                " The partner server responded with the following message"
+                " to the ha-maintenance-notify command: response returned"
+                " (error code 1001).");
+
+    // The partner's state precludes entering the in-maintenance state. Thus, this
+    // server must not change its state either.
+    EXPECT_EQ(HA_WAITING_ST, service.getCurrState());
+}
+
+// This test verifies the case when the server receiving the ha-maintenance-cancel
+// command successfully transitions out of the partner-in-maintenance state.
+TEST_F(HAServiceTest, processMaintenanceCancelSuccess) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    ASSERT_NO_THROW(service.verboseTransition(HA_PARTNER_IN_MAINTENANCE_ST));
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-cancel command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceCancel());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner of our server is online and should have responded with
+    // the success status. Therefore, this server should have transitioned
+    // to the partner-in-maintenance state.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Server maintenance successfully canceled.");
+
+    EXPECT_EQ(HA_WAITING_ST, service.getCurrState());
+}
+
+// This test verifies the case when the server receiving the ha-maintenance-cancel
+// command successfully transitions out of the partner-in-maintenance state.
+TEST_F(HAServiceTest, processMaintenanceCancelSuccessAuthorized) {
+    // Update config to provide authentication.
+    user2_ = "foo";
+    password2_ = "bar";
+    user3_ = "test";
+    password3_ = "1234";
+
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+    setBasicAuth(config_storage);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    ASSERT_NO_THROW(service.verboseTransition(HA_PARTNER_IN_MAINTENANCE_ST));
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-cancel command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceCancel());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner of our server is online and should have responded with
+    // the success status. Therefore, this server should have transitioned
+    // to the partner-in-maintenance state.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "Server maintenance successfully canceled.");
+
+    EXPECT_EQ(HA_WAITING_ST, service.getCurrState());
+}
+
+// This test verifies that the maintenance is not canceled in case the
+// partner returns an error.
+TEST_F(HAServiceTest, processMaintenanceCancelPartnerError) {
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Simulate an error returned by the partner.
+    factory2_->getResponseCreator()->setControlResult(CONTROL_RESULT_ERROR);
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    ASSERT_NO_THROW(service.verboseTransition(HA_PARTNER_IN_MAINTENANCE_ST));
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-cancel command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceCancel());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner should have responded with an error.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR,
+                "Unable to cancel maintenance. The partner server responded"
+                " with the following message to the ha-maintenance-notify"
+                " command: response returned (error code 1).");
+
+    // The state of this server should not change.
+    EXPECT_EQ(HA_PARTNER_IN_MAINTENANCE_ST, service.getCurrState());
+}
+
+// This test verifies that the maintenance is not canceled in case the
+// partner requires not provided authentication.
+TEST_F(HAServiceTest, processMaintenanceCancelPartnerUnauthorized) {
+    // Instruct servers to require authentication.
+    factory2_->getResponseCreator()->addBasicAuth("foo", "bar");
+    factory3_->getResponseCreator()->addBasicAuth("test", "1234");
+
+    // Create HA configuration for 3 servers. This server is
+    // server 1.
+    HAConfigPtr config_storage = createValidConfiguration();
+
+    // Start the servers.
+    ASSERT_NO_THROW({
+        listener_->start();
+        listener2_->start();
+    });
+
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    ASSERT_NO_THROW(service.verboseTransition(HA_PARTNER_IN_MAINTENANCE_ST));
+
+    // The tested function is synchronous, so we need to run server side IO service
+    // in background to not block the main thread.
+    auto thread = runIOServiceInThread();
+
+    // Process ha-maintenance-cancel command.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processMaintenanceCancel());
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+    io_service_->get_io_service().reset();
+    io_service_->poll();
+
+    // The partner should have responded with an error.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_ERROR,
+                "Unable to cancel maintenance. The partner server responded"
+                " with the following message to the ha-maintenance-notify"
+                " command: Unauthorized (error code 1).");
+
+    // The state of this server should not change.
+    EXPECT_EQ(HA_PARTNER_IN_MAINTENANCE_ST, service.getCurrState());
+}
+
+// This test verifies that the ha-reset command is processed successfully.
+TEST_F(HAServiceTest, processHAReset) {
+    HAConfigPtr config_storage = createValidConfiguration();
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Transition the server to the load-balancing state.
+    EXPECT_NO_THROW(service.transition(HA_LOAD_BALANCING_ST, HAService::NOP_EVT));
+
+    // Process ha-reset command that should cause the server to transition
+    // to the waiting state.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processHAReset());
+
+    // The server should have responded.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "HA state machine reset.");
+
+    // Response should include no arguments.
+    EXPECT_FALSE(rsp->get("arguments"));
+
+    // The server should be in the waiting state.
+    EXPECT_EQ(HA_WAITING_ST, service.getCurrState());
+}
+
+// This test verifies that the ha-reset command is processed successfully when
+// the server is already in the waiting state.
+TEST_F(HAServiceTest, processHAResetWaiting) {
+    HAConfigPtr config_storage = createValidConfiguration();
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Transition the server to the waiting state.
+    EXPECT_NO_THROW(service.transition(HA_WAITING_ST, HAService::NOP_EVT));
+
+    // Process ha-reset command that should not change the state of the
+    // server because the server is already in the waiting state. It
+    // should not fail though.
+    ConstElementPtr rsp;
+    ASSERT_NO_THROW(rsp = service.processHAReset());
+
+    // The server should have responded.
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS, "HA state machine already in WAITING state.");
+
+    // Response should include no arguments.
+    EXPECT_FALSE(rsp->get("arguments"));
+
+    // The server should be in the waiting state.
+    EXPECT_EQ(HA_WAITING_ST, service.getCurrState());
+}
+
+// This test verifies that the ha-sync-complete-notify command is processed
+// successfully, the server keeps the DHCP service disabled in the partner-down
+// state and enables the service when it is in another state.
+TEST_F(HAServiceTest, processSyncCompleteNotify) {
+    HAConfigPtr config_storage = createValidConfiguration();
+    TestHAService service(io_service_, network_state_, config_storage);
+
+    // Transition the server to the partner-down state.
+    EXPECT_NO_THROW(service.transition(HA_PARTNER_DOWN_ST, HAService::NOP_EVT));
+
+    // Simulate disabling the DHCP service for synchronization.
+    EXPECT_NO_THROW(service.network_state_->disableService(NetworkState::Origin::HA_COMMAND));
+
+    ConstElementPtr rsp;
+    EXPECT_NO_THROW(rsp = service.processSyncCompleteNotify());
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS,
+                "Server successfully notified about the synchronization completion.");
+
+    // The server should remain in the partner-down state.
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service.getCurrState());
+
+    // The service should be disabled until the server transitions to the
+    // normal state.
+    EXPECT_FALSE(service.network_state_->isServiceEnabled());
+
+    // It is possible that the connection from this server to the partner
+    // is still broken. In that case, the HA_SYNCED_PARTNER_UNAVAILABLE_EVT
+    // is emitted and the server should enable DHCP service to continue
+    // serving the clients in the partner-down state.
+    EXPECT_NO_THROW(service.postNextEvent(HAService::HA_SYNCED_PARTNER_UNAVAILABLE_EVT));
+    EXPECT_NO_THROW(service.runModel(HAService::NOP_EVT));
+    EXPECT_TRUE(service.network_state_->isServiceEnabled());
+
+    // Transition the server to the load-balancing state.
+    EXPECT_NO_THROW(service.transition(HA_LOAD_BALANCING_ST, HAService::NOP_EVT));
+
+    // Disable the service again.
+    EXPECT_NO_THROW(service.network_state_->disableService(NetworkState::Origin::HA_COMMAND));
+
+    EXPECT_NO_THROW(rsp = service.processSyncCompleteNotify());
+
+    ASSERT_TRUE(rsp);
+    checkAnswer(rsp, CONTROL_RESULT_SUCCESS,
+                "Server successfully notified about the synchronization completion.");
+
+    // The server should remain in the load-balancing state.
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service.getCurrState());
+
+    // This time the service should be enabled because the server is in
+    // the state in which it sends the lease updates.
+    EXPECT_TRUE(service.network_state_->isServiceEnabled());
+}
+
+/// @brief HA partner to the server under test.
+///
+/// This is a wrapper class around @c HttpListener which simulates a
+/// partner server. It provides convenient methods to start, stop the
+/// partner (its listener) and to transition the partner between various
+/// HA states. Depending on the state and whether the partner is started
+/// or stopped, different answers are returned in response to the
+/// ha-heartbeat commands.
+class HAPartner {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// Creates the partner instance from a listner and the corresponding
+    /// response factory. It automatically transitions the partner to the
+    /// "waiting" state unless otherwise specified with the third parameter.
+    ///
+    /// @param listner pointer to the listner to be used.
+    /// @param factory pointer to the response factory to be used. This
+    /// must be the same factory that the listener is using.
+    /// @param initial_state initial state for the partner. Default is to
+    /// transition the partner to the "waiting" state which is the default
+    /// state for each starting server.
+    HAPartner(const HttpListenerPtr& listener,
+              const TestHttpResponseCreatorFactoryPtr& factory,
+              const std::string& initial_state = "waiting")
+        : listener_(listener), factory_(factory), running_(false),
+          static_date_time_(), static_scopes_(),
+          static_unsent_update_count_(0) {
+        transition(initial_state);
+    }
+
+    /// @brief Sets control result to be returned as a result of the
+    /// communication with the partner.
+    ///
+    /// @param control_result new control result value.
+    void setControlResult(const int control_result) {
+        factory_->getResponseCreator()->setControlResult(control_result);
+    }
+
+    /// @brief Sets static date-time value to be used in responses.
+    ///
+    /// @param static_date_time fixed date-time value.
+    void setDateTime(const std::string& static_date_time) {
+        static_date_time_ = static_date_time;
+    }
+
+    /// @brief Sets static scopes to be used in responses.
+    ///
+    /// @param scopes Fixed scopes set.
+    void setScopes(const std::set<std::string>& scopes) {
+        static_scopes_ = scopes;
+    }
+
+    /// @brief Enable response to commands required for leases synchronization.
+    ///
+    /// Enables dhcp-disable, dhcp-enable and lease4-get-page commands. The last
+    /// of them returns a bunch of test leases.
+    void enableRespondLeaseFetching() {
+        // Create IPv4 leases which will be fetched from the other server.
+        std::vector<Lease4Ptr> leases4;
+        ASSERT_NO_THROW(generateTestLeases(leases4));
+
+        // Convert leases to the JSON format, the same as used by the lease_cmds
+        // hook library. Configure our test HTTP servers to return those
+        // leases in this format.
+        ElementPtr response_arguments = Element::createMap();
+        response_arguments->set("leases", getLeasesAsJson(leases4));
+
+        factory_->getResponseCreator()->setArguments("lease4-get-page", response_arguments);
+    }
+
+    /// @brief Starts up the partner.
+    void startup() {
+        if (!running_) {
+            listener_->start();
+            running_ = true;
+        }
+    }
+
+    /// @brief Shuts down the partner.
+    ///
+    /// It may be used to simulate partner's crash as well as graceful
+    /// shutdown.
+    void shutdown() {
+        if (running_) {
+            listener_->stop();
+            running_ = false;
+        }
+    }
+
+    /// @brief Transitions the partner to the specified state.
+    ///
+    /// The state is provided in the textual form and the function doesn't
+    /// validate whether it is correct or not.
+    ///
+    /// @param state new partner state.
+    void transition(const std::string& state) {
+        ElementPtr response_arguments = Element::createMap();
+        response_arguments->set("state", Element::create(state));
+        if (!static_date_time_.empty()) {
+            response_arguments->set("date-time", Element::create(static_date_time_));
+        }
+        if (!static_scopes_.empty()) {
+            auto json_scopes = Element::createList();
+            for (auto scope : static_scopes_) {
+                json_scopes->add(Element::create(scope));
+            }
+            response_arguments->set("scopes", json_scopes);
+        }
+        if (static_unsent_update_count_ >= 0) {
+            response_arguments->set("unsent-update-count", Element::create(static_unsent_update_count_));
+        }
+        factory_->getResponseCreator()->setArguments(response_arguments);
+    }
+
+    /// @brief Sets static value of unsent update count.
+    ///
+    /// @param unsent_update_count new value of the unsent update count.
+    /// Specify a negative value to exclude the count from the response.
+    void setUnsentUpdateCount(int64_t unsent_update_count) {
+        static_unsent_update_count_ = unsent_update_count;
+    }
+
+private:
+
+    /// @brief Instance of the listener wrapped by this class.
+    HttpListenerPtr listener_;
+    /// @brief Instance of the response factory used by the listener.
+    TestHttpResponseCreatorFactoryPtr factory_;
+
+    /// @brief IPv4 leases to be used in the tests.
+    std::vector<Lease4Ptr> leases4_;
+
+    /// @brief Boolean flag indicating if the partner is running.
+    bool running_;
+
+    /// @brief Static date-time value to be returned.
+    std::string static_date_time_;
+
+    /// @brief Static scopes to be reported.
+    std::set<std::string> static_scopes_;
+
+    /// @brief Static count of lease updates not sent by the partner
+    /// because the other server was unavailable.
+    int64_t static_unsent_update_count_;
+};
+
+/// @brief Shared pointer to a partner.
+typedef boost::shared_ptr<HAPartner> HAPartnerPtr;
+
+/// @brief Test fixture class for the HA service state machine.
+class HAServiceStateMachineTest : public HAServiceTest {
+public:
+    /// @brief Constructor.
+    HAServiceStateMachineTest()
+        : HAServiceTest(), state_(),
+          partner_(new HAPartner(listener2_, factory2_)) {
+    }
+
+    /// @brief Creates common HA service instance from the provided configuration.
+    ///
+    /// The custom @c state_ object is created and it replaces the default
+    /// @c state_ object of the HA service.
+    ///
+    /// @param config pointer to the configuration to be used by the service.
+    /// @param server_type server type, i.e. DHCPv4 or DHCPv6.
+    void startService(const HAConfigPtr& config,
+                      const HAServerType& server_type = HAServerType::DHCPv4) {
+        config->setHeartbeatDelay(1);
+        config->setSyncPageLimit(1000);
+        createSTService(network_state_, config, server_type);
+        // Replace default communication state with custom state which exposes
+        // protected members and methods.
+        state_.reset(new NakedCommunicationState4(io_service_, config));
+        service_->communication_state_ = state_;
+        // Move the state machine from initial state to "waiting" state.
+        service_->runModel(HAService::NOP_EVT);
+    }
+
+    /// @brief Runs IO service until specified event occurs.
+    ///
+    /// This method runs IO service until state machine is run as a result
+    /// of receiving a response to an IO operation. IO operations such as
+    /// lease updates, heartbeats etc. trigger state machine changes.
+    /// We can capture certain events to detect when a response to the heartbeat
+    /// or other control commands  is received. This is useful to return control
+    /// to a test to verify that the state machine remains in the expected state
+    /// after receiving such response.
+    ///
+    /// @param event an event which should trigger IO service to stop.
+    void waitForEvent(const int event) {
+        ASSERT_NE(event, HAService::NOP_EVT);
+
+        service_->postNextEvent(HAService::NOP_EVT);
+
+        // Run IO service until the event occurs.
+        runIOService(TEST_TIMEOUT, [this, event]() {
+            return (service_->getLastEvent() == event);
+        });
+
+        service_->postNextEvent(HAService::NOP_EVT);
+    }
+
+    /// @brief Convenience method checking if HA service is currently running
+    /// recurring heartbeat.
+    ///
+    /// @return true if the heartbeat is run.
+    bool isDoingHeartbeat() {
+        return (state_->isHeartbeatRunning());
+    }
+
+    /// @brief Convenience method checking if HA service has detected communications
+    /// interrupted condition.
+    ///
+    /// @return true if the communications interrupted condition deemed, false
+    /// otherwise.
+    bool isCommunicationInterrupted() {
+        return (state_->isCommunicationInterrupted());
+    }
+
+    /// @brief Convenience method checking if communication failure has been
+    /// detected by the HA service based on the analysis of the DHCP traffic.
+    ///
+    /// @return true if the communication failure is deemed, false otherwise.
+    bool isFailureDetected() {
+        return (state_->failureDetected());
+    }
+
+    /// @brief Simulates a case when communication with the partner has failed
+    /// for a time long enough to assume communications interrupted condition.
+    ///
+    /// This case is simulated by modifying the last poking time far into the
+    /// past.
+    void simulateNoCommunication() {
+        state_->modifyPokeTime(-1000);
+    }
+
+    /// @brief Simulates reception of unanswered DHCP queries by the partner.
+    ///
+    /// This case is simulated by creating a large number of queries with
+    /// secs field set to high value.
+    void simulateDroppedQueries() {
+        // Create 100 packets. Around 50% of them should be assigned to the
+        // partner if load balancing is performed.
+        const unsigned queries_num = 100;
+        for (unsigned i = 0; i < queries_num; ++i) {
+            // Create query with random HW address.
+            Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+            // Set large secs field value.
+            query4->setSecs(0x00EF);
+            // This function, besides checking if the query is in scope,
+            // updates unanswered message counters. If the counters exceed
+            // a configured value the communication failure is assumed.
+            static_cast<void>(service_->inScope(query4));
+        }
+        // The state machine needs to react to the dropped queries. Therefore
+        // we run the machine now.
+        service_->runModel(HAService::NOP_EVT);
+    }
+
+    /// @brief Simulates rejected lease updates by the partner.
+    ///
+    /// Too many rejected lease updates should transition the server to the
+    /// terminated state.
+    /// @param leases_num number of simulated rejected leases.
+    void simulateRejectedLeaseUpdates(const unsigned leases_num = 100) {
+        // If the state machine is about to transition to another state,
+        // let's make sure it performs this transition.
+        service_->runModel(HAService::NOP_EVT);
+        // Simulate the rejected lease updates.
+        for (auto i = 0; i < leases_num; ++i) {
+            // Create query with random HW address.
+            Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+            static_cast<void>(state_->reportRejectedLeaseUpdate(query4, 100));
+        }
+        // The state machine needs to react to the rejected leases.
+        service_->runModel(HAService::NOP_EVT);
+    }
+
+    /// @brief Checks transitions dependent on the partner's state.
+    ///
+    /// This method uses @c partner_ object to control the state of the partner.
+    /// This method must not be used to test transitions from the syncing state
+    /// because this state includes synchronous IO operations. There is a
+    /// separate test for the transitions from the syncing state.
+    ///
+    /// @param my_state initial state of this server.
+    /// @param partner_state state of the partner.
+    /// @param final_state expected state to transition to.
+    void testTransition(const MyState& my_state, const PartnerState& partner_state,
+                        const FinalState& final_state) {
+        // We need to shutdown the partner only if the partner is to be in the
+        // 'unavailable state'.
+        if (partner_state.state_ != HA_UNAVAILABLE_ST) {
+            // This function is not meant for testing transitions from the syncing
+            // state when partner is available.
+            ASSERT_NE(my_state.state_, HA_SYNCING_ST);
+            partner_->setControlResult(CONTROL_RESULT_SUCCESS);
+
+        } else {
+            partner_->setControlResult(CONTROL_RESULT_ERROR);
+        }
+
+        // Transition this server to the desired initial state.
+        service_->transition(my_state.state_, HAService::NOP_EVT);
+        // Transition the partner to the desired state.
+        partner_->transition(service_->getStateLabel(partner_state.state_));
+        // Run the heartbeat.
+        waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+        // Make sure that this server ended up in the expected state.
+        EXPECT_EQ(final_state.state_, service_->getCurrState())
+            << "expected transition to the '"
+            << service_->getStateLabel(final_state.state_)
+            << "' state for the partner state '" << service_->getStateLabel(partner_state.state_)
+            << "', but transitioned to the '"
+            << service_->getStateLabel(service_->getCurrState())
+            << "' state";
+
+        // If the partner is unavailable we also have to verify the case when
+        // we detect that the partner is considered offline (after running the
+        // whole failure detection algorithm). The server in the in-maintenance
+        // state is excluded from this because it must not transition out of this
+        // state until an administrator makes some action.
+        if ((my_state.state_ != HA_IN_MAINTENANCE_ST) &&
+            (partner_state.state_ == HA_UNAVAILABLE_ST)) {
+            // Transition this server back to the initial state.
+            service_->transition(my_state.state_, HAService::NOP_EVT);
+            // Simulate lack of communication between the servers.
+            simulateNoCommunication();
+            // Send the heartbeat again.
+            waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+
+            // The load balancing server or the standby server is monitoring the stream
+            // of packets directed to the partner to detect delays in responses. The
+            // primary server in the hot standby configuration doesn't do it, because
+            // the partner is not meant to process any queries until it detects that
+            // the primary server is down. This is only done in states in which the
+            // DHCP service is enabled. Otherwise, the server doesn't receive DHCP
+            // queries it could analyze.
+            if (service_->network_state_->isServiceEnabled() &&
+                ((service_->config_->getHAMode() == HAConfig::LOAD_BALANCING) ||
+                 service_->config_->getThisServerConfig()->getRole() == HAConfig::PeerConfig::STANDBY)) {
+                // The server should remain in its current state until we also detect
+                // that the partner is not answering the queries.
+                ASSERT_EQ(final_state.state_, service_->getCurrState())
+                    << "expected transition to the '"
+                    << service_->getStateLabel(final_state.state_)
+                    << "' state for the partner state '" << service_->getStateLabel(partner_state.state_)
+                    << "', but transitioned to the '"
+                    << service_->getStateLabel(service_->getCurrState())
+                    << "' state";
+
+                // Back to the original state again.
+                service_->transition(my_state.state_, HAService::NOP_EVT);
+                // This time simulate no responses from the partner to the DHCP clients'
+                // requests. This should cause the server to transition to the partner
+                // down state regardless of the initial state.
+                simulateDroppedQueries();
+                waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+                EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState())
+                    << "expected transition to the 'partner-down' state, but transitioned"
+                    " to the '" << service_->getStateLabel(service_->getCurrState())
+                    << "' state";
+
+            // The primary server in the hot-standby configuration should transition to
+            // the partner-down state when there is no communication with the partner
+            // over the control channel.
+            } else {
+                EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState())
+                    << "expected transition to the 'partner-down' state, but transitioned"
+                    " to the '" << service_->getStateLabel(service_->getCurrState())
+                    << "' state";
+            }
+        }
+    }
+
+    /// @brief Checks transitions from the syncing state.
+    ///
+    /// This method uses @c partner_ object to control the state of the partner.
+    ///
+    /// @param final_state expected final server state.
+    void testSyncingTransition(const FinalState& final_state) {
+        // Transition to the syncing state.
+        service_->transition(HA_SYNCING_ST, HAService::NOP_EVT);
+        partner_->transition("ready");
+        state_->stopHeartbeat();
+
+        testSynchronousCommands([this]() {
+            service_->runModel(HAService::NOP_EVT);
+        });
+
+        state_->stopHeartbeat();
+
+        EXPECT_EQ(final_state.state_, service_->getCurrState())
+            << "expected transition to the '"
+            << service_->getStateLabel(final_state.state_)
+            << "' state, but transitioned to the '"
+            << service_->getStateLabel(service_->getCurrState())
+            << "' state";
+    }
+
+    /// @brief Tests transition from any state to "terminated".
+    ///
+    /// @param my_state initial server state.
+    void testTerminateTransition(const MyState& my_state) {
+        // Set the partner's time way in the past so as the clock skew gets high.
+        partner_->setDateTime("Sun, 06 Nov 1994 08:49:37 GMT");
+        partner_->transition("ready");
+        // Transition this server to the desired initial state.
+        service_->transition(my_state.state_, HAService::NOP_EVT);
+        // Run the heartbeat.
+        waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+        // The server should get into terminated state because of the high
+        // clock skew.
+        EXPECT_EQ(HA_TERMINATED_ST, service_->getCurrState())
+            << "expected transition to the 'terminated' state"
+            << "', but transitioned to the '"
+            << service_->getStateLabel(service_->getCurrState())
+            << "' state";
+    }
+
+    /// @brief Tests transition between states when passive-backup mode is
+    /// in use.
+    ///
+    /// @param my_state initial server state.
+    /// @param final_state expected final state.
+    void testPassiveBackupTransition(const MyState& my_state,
+                                     const FinalState& final_state) {
+        service_->transition(my_state.state_, HAService::NOP_EVT);
+        service_->runModel(TestHAService::NOP_EVT);
+
+        EXPECT_EQ(final_state.state_, service_->getCurrState())
+            << "expected transition to the '"
+            << service_->getStateLabel(final_state.state_)
+            << "', but transitioned to the '"
+            << service_->getStateLabel(service_->getCurrState())
+            << "' state";
+    }
+
+    /// @brief Test that the server is serving expected scopes while being in a
+    /// certain state.
+    ///
+    /// @param my_state state of the server.
+    /// @param scopes vector of scopes which the server is expected to handle in this
+    /// state.
+    /// @param dhcp_enabled Indicates whether DHCP service is expected to be enabled
+    /// or disabled in the given state.
+    /// @param event Event to be passed to the tested handler.
+    void expectScopes(const MyState& my_state, const std::vector<std::string>& scopes,
+                      const bool dhcp_enabled, const int event = TestHAService::NOP_EVT) {
+
+        // If expecting no scopes, let's enable some scope to make sure that the
+        // code changes this setting.
+        if (scopes.empty()) {
+            service_->query_filter_.serveScope("server1");
+
+        } else {
+            // If expecting some scopes, let's initially clear the scopes to make
+            // sure that the code sets it.
+            service_->query_filter_.serveNoScopes();
+        }
+
+        // Also, let's preset the DHCP server state to the opposite of the expected
+        // state.
+        if (dhcp_enabled) {
+            service_->network_state_->disableService(NetworkState::Origin::HA_COMMAND);
+
+        } else {
+            service_->network_state_->enableService(NetworkState::Origin::HA_COMMAND);
+        }
+
+        // Transition to the desired state.
+        service_->postNextEvent(event);
+        service_->verboseTransition(my_state.state_);
+        // Run the handler.
+        service_->runModel(TestHAService::NOP_EVT);
+        // First, check that the number of handlded scope is equal to the number of
+        // scopes specified as an argument.
+        ASSERT_EQ(scopes.size(), service_->query_filter_.getServedScopes().size())
+                << "test failed for state '" << service_->getStateLabel(my_state.state_)
+                << "'";
+
+        // Now, verify that each specified scope is handled.
+        for(auto scope : scopes) {
+            EXPECT_TRUE(service_->query_filter_.amServingScope(scope))
+                << "test failed for state '" << service_->getStateLabel(my_state.state_)
+                << "'";
+        }
+        // Verify if the DHCP service is enabled or disabled.
+        EXPECT_EQ(dhcp_enabled, service_->network_state_->isServiceEnabled())
+            << "test failed for state '" << service_->getStateLabel(my_state.state_)
+            << "'";
+    }
+
+    /// @brief Transitions the server to the specified state and checks if the
+    /// HA service would send lease updates in this state.
+    ///
+    /// @param my_state this server's state
+    /// @param peer_config configuration of the server to which lease updates are
+    /// to be sent.
+    /// @return true if the lease updates would be sent, false otherwise.
+    bool expectLeaseUpdates(const MyState& my_state,
+                            const HAConfig::PeerConfigPtr& peer_config) {
+        service_->verboseTransition(my_state.state_);
+        return (service_->shouldSendLeaseUpdates(peer_config));
+    }
+
+    /// @brief Transitions the server to the specified state and checks if the
+    /// HA service would queue lease updates in this state.
+    ///
+    /// @param my_state this server's state
+    /// @param peer_config configuration of the server to which lease updates are
+    /// to be sent or queued.
+    /// @return true if the lease updates would be queued, false otherwise.
+    bool expectQueueLeaseUpdates(const MyState& my_state,
+                                 const HAConfig::PeerConfigPtr& peer_config) {
+        service_->verboseTransition(my_state.state_);
+        return (service_->shouldQueueLeaseUpdates(peer_config));
+    }
+
+    /// @brief Transitions the server to the specified state and checks if the
+    /// HA service is sending heartbeat in this state.
+    ///
+    /// @param my_state this server's state
+    /// @return true if the heartbeat is sent in this state, false otherwise.
+    bool expectHeartbeat(const MyState& my_state) {
+        service_->verboseTransition(my_state.state_);
+        service_->runModel(TestHAService::NOP_EVT);
+        return (isDoingHeartbeat());
+    }
+
+    /// @brief Transitions the server to the specified state and checks that it
+    /// clears the rejected leases counter.
+    ///
+    /// @param my_state this server's state
+    /// @return true if the rejected leases counter has been cleared.
+    bool expectClearRejectedLeases(const MyState& my_state) {
+        simulateRejectedLeaseUpdates(1);
+        service_->verboseTransition(my_state.state_);
+        service_->runModel(TestHAService::NOP_EVT);
+        return (state_->getRejectedLeaseUpdatesCount() == 0);
+    }
+
+    /// @brief Pointer to the communication state used in the tests.
+    NakedCommunicationState4Ptr state_;
+    /// @brief Pointer to the partner used in some tests.
+    HAPartnerPtr partner_;
+};
+
+
+// Test the following scenario:
+// 1. I show up in waiting state and look around
+// 2. My partner doesn't respond over control channel
+// 3. I start analyzing partner's packets and see that
+//    it doesn't respond.
+// 4. I transition to partner down state.
+// 5. Partner finally shows up and eventually transitions to the ready state.
+// 6. I see the partner being ready, so I synchronize myself and eventually
+//    transition to the load-balancing state.
+// 7. Next, the partner crashes again.
+// 8. I detect partner's crash and transition back to partner down.
+// 9. While being in the partner down state, I find that the partner
+//    is available and it is doing load balancing.
+// 10. I stay in the partner-down state to force the partner to transition
+//     to the waiting state and synchronize its database.
+TEST_F(HAServiceStateMachineTest, waitingParterDownLoadBalancingPartnerDown) {
+    HAConfigPtr config_storage = createValidConfiguration();
+    // Disable syncing leases to avoid transitions via the syncing state.
+    // In this state it is necessary to perform synchronous tasks.
+    config_storage->setSyncLeases(false);
+    startService(config_storage);
+
+    // Start the server: offline ---> WAITING state.
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: no heartbeat response for a long period of time.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    simulateNoCommunication();
+    ASSERT_TRUE(isDoingHeartbeat());
+
+    // WAITING state: communication interrupted. In this state we don't analyze
+    // packets ('secs' field) because the DHCP service is disabled.
+    // WAITING ---> PARTNER DOWN
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+
+    // PARTNER DOWN state: still no response from the partner.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+
+    // Partner shows up and (eventually) transitions to READY state.
+    HAPartner partner(listener2_, factory2_);
+    partner.setScopes({ "server1", "server2" });
+    partner.setUnsentUpdateCount(10);
+    partner.transition("ready");
+    partner.startup();
+
+    // PARTNER DOWN state: receive a response from the partner indicating that
+    // the partner is in READY state.
+    // PARTNER DOWN ---> WAITING
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // WAITING state: synchronization is disabled to simplify this test. The
+    // server should transition straight to the ready state.
+    // WAITING --> READY
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // READY --> LOAD BALANCING
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // Ensure that the state handler is invoked.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+
+    // Check the reported info about servers.
+    ConstElementPtr ha_servers = service_->processStatusGet();
+    ASSERT_TRUE(ha_servers);
+
+    // Hard to know what is the age of the remote data. Therefore, let's simply
+    // grab it from the response.
+    ASSERT_EQ(Element::map, ha_servers->getType());
+    auto remote = ha_servers->get("remote");
+    ASSERT_TRUE(remote);
+    EXPECT_EQ(Element::map, remote->getType());
+    auto age = remote->get("age");
+    ASSERT_TRUE(age);
+    EXPECT_EQ(Element::integer, age->getType());
+    auto age_value = age->intValue();
+    EXPECT_GE(age_value, 0);
+
+    // Now append it to the whole structure for comparison.
+    std::ostringstream s;
+    s << age_value;
+
+    std::string expected = "{"
+        "    \"local\": {"
+        "        \"role\": \"primary\","
+        "        \"scopes\": [ \"server1\" ], "
+        "        \"state\": \"load-balancing\""
+        "    }, "
+        "    \"remote\": {"
+        "        \"age\": " + s.str() + ","
+        "        \"in-touch\": true,"
+        "        \"role\": \"secondary\","
+        "        \"last-scopes\": [ \"server1\", \"server2\" ],"
+        "        \"last-state\": \"ready\","
+        "        \"communication-interrupted\": false,"
+        "        \"connecting-clients\": 0,"
+        "        \"unacked-clients\": 0,"
+        "        \"unacked-clients-left\": 0,"
+        "        \"analyzed-packets\": 0"
+        "    }"
+        "}";
+    EXPECT_TRUE(isEquivalent(Element::fromJSON(expected), ha_servers));
+
+    // Crash the partner and see whether our server can return to the partner
+    // down state.
+    partner.setControlResult(CONTROL_RESULT_ERROR);
+
+    // LOAD BALANCING state: wait for the next heartbeat to occur. This heartbeat
+    // fails causing the server to enter communication-recovery state in which the
+    // server is collecting lease updates to be sent when the communication is
+    // resumed.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_COMMUNICATION_RECOVERY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // COMMUNICATION RECOVERY state: simulate lack of communication for a longer
+    // period of time. We should remain the communication-recovery state and
+    // keep analyzing the traffic directed to partner.
+    simulateNoCommunication();
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_COMMUNICATION_RECOVERY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_TRUE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // COMMUNICATION RECOVERY state: simulate a lot of unanswered DHCP
+    // messages to the partner. This server should detect that the partner is
+    // not answering and transition to partner down state.
+    // COMMUNICATION RECOVERY ---> PARTNER DOWN
+    simulateDroppedQueries();
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_TRUE(isCommunicationInterrupted());
+    ASSERT_TRUE(isFailureDetected());
+
+    // Start the partner again and transition it to the load balancing state.
+    partner.setControlResult(CONTROL_RESULT_SUCCESS);
+    partner.transition("load-balancing");
+
+    // PARTNER DOWN state: the partner shows up in the load-balancing state.
+    // It may happen when the partner did not crash but there was a temporary
+    // communication error with it. It is possible that this server was not
+    // configured to monitor unacked clients and that's why it transitioned
+    // to the partner-down state. The partner may be configured differently.
+    // The partner was not receiving lease updates from us, so we need to
+    // force it to transition to the waiting state and synchronize. We stay
+    // in the partner-down state as long as necessary to force the partner
+    // to synchronize.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+}
+
+// Test the following scenario:
+// 1. I show up in waiting state and look around.
+// 2. My partner is offline and is not responding over the control channel.
+// 3. I can't communicate with the partner so I transition to the partner-down
+//    state.
+// 4. Partner shows up and eventually transitions to the ready state.
+// 5. I see the partner being ready, so I synchronize myself and eventually
+//    transition to the load-balancing state.
+// 6. Partner stops responding again.
+// 7. I monitor communication with the partner and eventually consider the
+//    communication to be interrupted.
+// 8. I start monitoring the DHCP traffic directed to the partner and observe
+//    delays in responses.
+// 9. I transition to the partner-down state again seeing that the certain
+//    number of clients can't communicate with the partner.
+// 10. The partner unexpectedly shows up in the hot-standby mode. I stay in
+//    the partner-down state to force the partner to transition to the waiting
+//    state and synchronize its database.
+TEST_F(HAServiceStateMachineTest, waitingParterDownHotStandbyPartnerDown) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    // Disable syncing leases to avoid transitions via the syncing state.
+    // In this state it is necessary to perform synchronous tasks.
+    valid_config->setSyncLeases(false);
+
+    // Start the server: offline ---> WAITING state.
+    startService(valid_config);
+
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: no heartbeat response for a long period of time.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    simulateNoCommunication();
+    ASSERT_TRUE(isDoingHeartbeat());
+
+    // WAITING state: communication interrupted. In this state we don't analyze
+    // packets ('secs' field) because the DHCP service is disabled.
+    // WAITING ---> PARTNER DOWN
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+
+    // PARTNER DOWN state: still no response from the partner.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+
+    // Partner shows up and (eventually) transitions to READY state.
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->setUnsentUpdateCount(10);
+    partner_->transition("ready");
+    partner_->startup();
+
+    // PARTNER DOWN state: receive a response from the partner indicating that
+    // the partner is in READY state.
+    // PARTNER DOWN ---> WAITING
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // WAITING state: synchronization is disabled to simplify this test. The
+    // server should transition straight to the ready state.
+    // WAITING --> READY
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // Primary server must transition to the hot-standby state first before
+    // standby can transition.
+    partner_->transition("hot-standby");
+
+    // READY --> HOT STANDBY
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_HOT_STANDBY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // Crash the partner and see whether our server can return to the partner
+    // down state.
+    partner_->setControlResult(CONTROL_RESULT_ERROR);
+
+    // HOT STANDBY state: wait for the next heartbeat to occur and make
+    // sure that a single heartbeat loss is not yet causing us to assume
+    // partner down condition.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_HOT_STANDBY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // HOT STANDBY state: simulate lack of communication for a longer
+    // period of time. We should remain in the hot-standby state waiting for
+    // unanswered DHCP traffic before going to partner-down state.
+    simulateNoCommunication();
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_HOT_STANDBY_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_TRUE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+
+    // HOT STANDBY state: simulate a lot of unanswered DHCP
+    // messages to the partner. This server should detect that the partner is
+    // not answering and transition to partner down state.
+    // HOT STANDBY ---> PARTNER DOWN
+    simulateDroppedQueries();
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_TRUE(isCommunicationInterrupted());
+    ASSERT_TRUE(isFailureDetected());
+
+    // Start the partner again and transition it to the hot standby state.
+    partner_->setControlResult(CONTROL_RESULT_SUCCESS);
+    partner_->transition("hot-standby");
+
+    // PARTNER DOWN state: the partner shows up in the hot-standby state.
+    // It may happen when the partner did not crash but there was a temporary
+    // communication error with it. The partner was not receiving lease updates
+    // from us, so we need to force it to transition to the waiting state and
+    // synchronize. We stay in the partner-down state as long as necessary to
+    // force the partner to synchronize.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_PARTNER_DOWN_ST, service_->getCurrState());
+    ASSERT_TRUE(isDoingHeartbeat());
+    ASSERT_FALSE(isCommunicationInterrupted());
+    ASSERT_FALSE(isFailureDetected());
+}
+
+// Test the following scenario:
+// 1. I begin in a load-balancing state.
+// 2. My partner is offline.
+// 3. I transition to the communication-recovery state.
+// 4. My partner shows up in the load-balancing state.
+// 5. I see the partner so I get back to load-balancing state as well.
+TEST_F(HAServiceStateMachineTest, loadBalancingCommRecoveryLoadBalancing) {
+    startService(createValidConfiguration());
+    service_->verboseTransition(HA_LOAD_BALANCING_ST);
+    service_->runModel(HAService::NOP_EVT);
+
+    // Simulate that the partner is not responding to my queries.
+    simulateNoCommunication();
+    waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+
+    EXPECT_EQ(HA_COMMUNICATION_RECOVERY_ST, service_->getCurrState());
+
+    HAPartner partner(listener2_, factory2_, "load-balancing");
+    partner.startup();
+
+    waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+}
+
+// Test the following scenario:
+// 1. I show up in the waiting state.
+// 2. My partner appears to be in the partner-down state.
+// 3. I proceed to the syncing state to fetch leases from the partner.
+// 4. The first attempt to fetch leases is unsuccessful.
+// 5. I remain in the syncing state until I am finally successful.
+// 6. I proceed to the ready state.
+// 7. I see that the partner is still in partner-down state, so I
+//    wait for the partner to transition to load-balancing state.
+TEST_F(HAServiceStateMachineTest, waitingSyncingReadyLoadBalancing) {
+    // Partner is present and is in the PARTNER DOWN state.
+    HAPartner partner(listener2_, factory2_, "partner-down");
+    partner.startup();
+
+    // Start the server: offline ---> WAITING state.
+    startService(createValidConfiguration());
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: receive a response from the partner indicating that the
+    // partner is in the load balancing state. I should transition to the
+    // SYNCING state to fetch leases from the partner.
+    // WAITING ---> SYNCING state
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_SYNCING_ST, service_->getCurrState());
+
+    // We better stop the heartbeat to not interfere with the synchronous
+    // commands.
+    state_->stopHeartbeat();
+
+    // The database synchronization is synchronous operation so we need to run
+    // the partner's IO service in thread (in background).
+    testSynchronousCommands([this, &partner]() {
+
+        // SYNCING state: the partner is up but it won't respond to the lease4-get-page
+        // command correctly. This should leave us in the SYNCING state until we finally
+        // can synchronize.
+        service_->runModel(HAService::NOP_EVT);
+        EXPECT_EQ(HA_SYNCING_ST, service_->getCurrState());
+
+        // We better stop the heartbeat to not interfere with the synchronous
+        // commands.
+        state_->stopHeartbeat();
+
+        // Enable the partner to correctly respond to the lease fetching and retry.
+        // We should successfully update the database and transition.
+        // SYNCING ---> READY
+        partner.enableRespondLeaseFetching();
+
+        // After previous attempt to synchronize the recorded partner state became
+        // "unavailable". This server won't synchronize until the heartbeat is
+        // sent which would indicate that the server is running. Therefore, we
+        // manually set the state of the partner to "partner-down".
+        state_->setPartnerState("partner-down");
+        service_->runModel(HAService::NOP_EVT);
+        EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+    });
+
+    // READY state: I do another heartbeat but my partner still seems to be in the
+    // partner down state, so I can't transition to load balancing just yet. I
+    // still remain the READY state.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+
+    // READY state: partner transitions to the LOAD BALANCING state seeing that I
+    // am ready. I should see this transition and also transition to that state.
+    // READY ---> LOAD BALANCING.
+    partner.transition("load-balancing");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+}
+
+// Test the following scenario:
+// 1. I am a primary server which is started at the same time as secondary.
+// 2. I determine that we're the primary server so I go ahead and start
+//    synchronizing.
+// 3. Partner should also determine that it is a secondary server so it should
+//    remain in the waiting state until we're ready.
+// 4. I synchronize the database and transition to the ready state.
+// 5. The partner should see that I am ready and should start the transition.
+// 6. I remain ready until the partner gets to the ready state.
+// 7. I transition to the load balancing state when the partner is ready.
+// 8. The partner transitions to the load balancing state which doesn't
+//    affect my state.
+TEST_F(HAServiceStateMachineTest, waitingSyncingReadyLoadBalancingPrimary) {
+    // Partner is present and is in the WAITING state.
+    HAPartner partner(listener2_, factory2_);
+    partner.enableRespondLeaseFetching();
+    partner.startup();
+
+    // Start the server: offline ---> WAITING state.
+    startService(createValidConfiguration());
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: both servers are in this state but our server is primary,
+    // so it transitions to the syncing state. The peer remains in the WAITING
+    // state until we're ready.
+    // WAITING --->SYNCING
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_SYNCING_ST, service_->getCurrState());
+
+    // We better stop the heartbeat to not interfere with the synchronous
+    // commands.
+    state_->stopHeartbeat();
+
+    // SYNCING state: this server will synchronously fetch leases from the peer.
+    // Therefore, we need to run the IO service in the thread to allow for
+    // synchronous operations to complete. Once the leases are fetched it should
+    // transition to the READY state.
+    // SYNCING ---> READY.
+    testSynchronousCommands([this]() {
+        service_->runModel(HAService::NOP_EVT);
+        EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+    });
+
+    // READY state: our partner sees that we're ready so it will start to
+    // synchronize. We remain in the READY state as long as the partner is not
+    // ready.
+    partner.transition("syncing");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+
+    // READY state: our partner appears to be ready. We can now start load
+    // balancing.
+    // READY ---> LOAD BALANCING.
+    partner.transition("ready");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+
+    // LOAD BALANCING state: our partner should eventually transition to the
+    // LOAD BALANCING state. This should not affect us doing load balancing.
+    partner.transition("load-balancing");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+}
+
+// Test the following scenario:
+// 1. I am secondary server configured for load balancing and started at the
+//    same time as the primary server.
+// 2. I determine that I am a secondary server, so I remain in the waiting
+//    state until the primary server indicates that it is ready.
+// 3. I start synchronizing my lease database when the partner is ready.
+// 4. I transition to the ready state and leave in that state until I see
+//    the primary transition to the load balancing state.
+// 5. I also transition to the load balancing state at that point.
+TEST_F(HAServiceStateMachineTest, waitingSyncingReadyLoadBalancingSecondary) {
+    // Partner is present and is in the WAITING state.
+    HAPartner partner(listener_, factory_);
+    partner.enableRespondLeaseFetching();
+    partner.startup();
+
+    // Create the configuration in which we're the secondary server doing
+    // load balancing.
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setThisServerName("server2");
+
+    // Start the server: offline ---> WAITING state.
+    startService(valid_config);
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: our partner is a primary so we remain in the WAITING state
+    // until it indicates it is ready.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: the partner is synchronizing the lease database. We still
+    // remain in the WAITING state.
+    partner.transition("syncing");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState());
+
+    // WAITING state: partner transitions to the READY state. This is a signal
+    // for us to start synchronization of the lease database.
+    // WAITING ---> SYNCING.
+    partner.transition("ready");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_SYNCING_ST, service_->getCurrState());
+
+    // We better stop the heartbeat to not interfere with the synchronous
+    // commands.
+    state_->stopHeartbeat();
+
+    // SYNCING state: this server will synchronously fetch leases from the peer.
+    // Therefore, we need to run the IO service in the thread to allow for
+    // synchronous operations to complete. Once the leases are fetched it should
+    // transition to the READY state.
+    // SYNCING ---> READY.
+    testSynchronousCommands([this]() {
+        service_->runModel(HAService::NOP_EVT);
+        EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+    });
+
+    // READY state: we remain ready until the other server transitions to the
+    // LOAD BALANCING state.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_READY_ST, service_->getCurrState());
+
+    // READY state: our primary server transitions to the LOAD BALANCING state.
+    // We can now also transition to this state.
+    // READY ---> LOAD BALANCING.
+    partner.transition("load-balancing");
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_LOAD_BALANCING_ST, service_->getCurrState());
+}
+
+// This test checks all combinations of server and partner states and the
+// resulting state to which the server transitions. This server is primary.
+// There is another test which validates state transitions from the
+// secondary server perspective.
+TEST_F(HAServiceStateMachineTest, stateTransitionsLoadBalancingPrimary) {
+    partner_->startup();
+
+    startService(createValidConfiguration());
+
+    // COMMUNICATION RECOVERY state transitions
+    {
+        SCOPED_TRACE("COMMUNICATION RECOVERY state transitions");
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+    }
+
+    // LOAD BALANCING state transitions
+    {
+        SCOPED_TRACE("LOAD BALANCING state transitions");
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+    }
+
+    // in-maintenance state transitions
+    {
+        SCOPED_TRACE("in-maintenance state transitions");
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+    }
+
+    // PARTNER DOWN state transitions
+    {
+        SCOPED_TRACE("PARTNER DOWN state transitions");
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+    // PARTNER in-maintenance state transitions
+    {
+        SCOPED_TRACE("PARTNER in-maintenance state transitions");
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+    // READY state transitions
+    {
+        SCOPED_TRACE("READY state transitions");
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_READY_ST));
+    }
+
+    // WAITING state transitions
+    {
+        SCOPED_TRACE("WAITING state transitions");
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_WAITING_ST));
+    }
+}
+
+// This test checks that the server in the load balancing mode does not
+// transition to the "syncing" state when "sync-leases" is disabled.
+TEST_F(HAServiceStateMachineTest, noSyncingTransitionsLoadBalancingPrimary) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setSyncLeases(false);
+    startService(valid_config);
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                   FinalState(HA_READY_ST));
+}
+
+// This test verifies that the load balancing server does not transition to
+// the communication recovery state when delayed-updates-limit is set
+// to 0.
+TEST_F(HAServiceStateMachineTest, noCommunicationRecoverytransitionsLoadBalancingPrimary) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setDelayedUpdatesLimit(0);
+    startService(valid_config);
+
+    testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                   FinalState(HA_LOAD_BALANCING_ST));
+}
+
+// This test checks that the server in the load balancing mode transitions to
+// the "terminated" state when the clock skew gets high.
+TEST_F(HAServiceStateMachineTest, terminateTransitionsLoadBalancingPrimary) {
+    partner_->startup();
+
+    startService(createValidConfiguration());
+
+    testTerminateTransition(MyState(HA_LOAD_BALANCING_ST));
+    testTerminateTransition(MyState(HA_PARTNER_DOWN_ST));
+    testTerminateTransition(MyState(HA_READY_ST));
+    testTerminateTransition(MyState(HA_WAITING_ST));
+}
+
+// This test checks that the server does not transition out of the waiting state
+// to the terminated state when the server is restarted but the clock skew has
+// been corrected.
+TEST_F(HAServiceStateMachineTest, terminateNoTransitionOnRestart) {
+    partner_->startup();
+    startService(createValidConfiguration());
+
+    // Set partner's time to the current time. This guarantees that the clock
+    // skew is below 60s and there is no reason for the server to transition
+    // to the terminated state.
+    partner_->setDateTime(HttpDateTime().rfc1123Format());
+    // The partner is in the terminated state to simulate sequential restart
+    // of the two servers from the terminated state.
+    partner_->transition("terminated");
+    // This server is in the waiting state which simulates the restart case.
+    service_->transition(HA_WAITING_ST, HAService::NOP_EVT);
+    // Run the heartbeat.
+    waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+    // The server should remain in the waiting state because the clock skew
+    // is low.
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState())
+        << "expected that the server remains in 'waiting' state"
+        << "', but transitioned to the '"
+        << service_->getStateLabel(service_->getCurrState())
+        << "' state";
+
+    // Now, let's set the partner's time way to the past to verify that this
+    // server transitions to the 'terminated' state if the administrator
+    // failed to sync the clocks prior to the restart.
+    partner_->setDateTime("Sun, 06 Nov 1994 08:49:37 GMT");
+    // Run the heartbeat.
+    waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+    EXPECT_EQ(HA_WAITING_ST, service_->getCurrState())
+        << "expected that the server transitions to the 'terminated' state"
+        << "', but transitioned to the '"
+        << service_->getStateLabel(service_->getCurrState())
+        << "' state";
+}
+
+// This test checks all combinations of server and partner states and the
+// resulting state to which the server transitions. This server is secondary.
+// There is another test which validates state transitions from the
+// primary server perspective.
+TEST_F(HAServiceStateMachineTest, stateTransitionsLoadBalancingSecondary) {
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setThisServerName("server2");
+    startService(valid_config);
+
+    partner_->startup();
+
+    // COMMUNICATION RECOVERY state transitions
+    {
+        SCOPED_TRACE("COMMUNICATION RECOVERY state transitions");
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+
+        testTransition(MyState(HA_COMMUNICATION_RECOVERY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+    }
+
+    // LOAD BALANCING state transitions
+    {
+        SCOPED_TRACE("LOAD BALANCING state transitions");
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_COMMUNICATION_RECOVERY_ST));
+    }
+
+    // in-maintenance state transitions
+    {
+        SCOPED_TRACE("in-maintenance state transitions");
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+    }
+
+    // PARTNER DOWN state transitions
+    {
+        SCOPED_TRACE("PARTNER DOWN state transitions");
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+    // PARTNER in-maintenance state transitions
+    {
+        SCOPED_TRACE("PARTNER in-maintenance state transitions");
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+    // READY state transitions
+    {
+        SCOPED_TRACE("READY state transitions");
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_READY_ST));
+    }
+
+    // WAITING state transitions
+    {
+        SCOPED_TRACE("WAITING state transitions");
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_WAITING_ST));
+    }
+}
+
+// This test checks that the server in the load balancing mode does not
+// transition to the "syncing" state when "sync-leases" is disabled.
+// This is the secondary server case.
+TEST_F(HAServiceStateMachineTest, noSyncingTransitionsLoadBalancingSecondary) {
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setThisServerName("server2");
+    valid_config->setSyncLeases(false);
+    startService(valid_config);
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                   FinalState(HA_READY_ST));
+}
+
+// This test checks that the secondary server in the load balancing mode
+// transitions to the "terminated" state when the clock skew gets high.
+TEST_F(HAServiceStateMachineTest, terminateTransitionsLoadBalancingSecondary) {
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setThisServerName("server2");
+    startService(valid_config);
+
+    testTerminateTransition(MyState(HA_LOAD_BALANCING_ST));
+    testTerminateTransition(MyState(HA_PARTNER_DOWN_ST));
+    testTerminateTransition(MyState(HA_READY_ST));
+    testTerminateTransition(MyState(HA_WAITING_ST));
+}
+
+// This test verifies that the backup server transitions to its own state.
+TEST_F(HAServiceStateMachineTest, stateTransitionsLoadBalancingBackup) {
+    HAConfigPtr valid_config = createValidConfiguration();
+
+    // server3 is marked as a backup server.
+    valid_config->setThisServerName("server3");
+    startService(valid_config);
+
+    // The server should transition to the backup state and stay there.
+    for (unsigned i = 0; i < 10; ++i) {
+        service_->runModel(HAService::NOP_EVT);
+        ASSERT_EQ(HA_BACKUP_ST, service_->getCurrState());
+        // In the backup state the DHCP service is disabled by default.
+        // It can only be enabled manually.
+        ASSERT_FALSE(service_->network_state_->isServiceEnabled());
+        ASSERT_EQ(0, service_->query_filter_.getServedScopes().size());
+    }
+}
+
+// This test verifies transitions from the syncing state in the load
+// balancing configuration.
+TEST_F(HAServiceStateMachineTest, syncingTransitionsLoadBalancing) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    startService(valid_config);
+    waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+
+    // The syncing state handler doesn't start synchronization until it
+    // detects that the partner is online. It may remember that from the
+    // previous heartbeat attempts. If the partner appears to be unavailable
+    // it will continue heartbeats before it synchronizes. This prevents the
+    // server from making endless attempts to synchronize without any chance
+    // to succeed. We verify that the server is not trying to synchronize
+    // by checking that the last event is not the one associated with the
+    // synchronization attempt.
+    ASSERT_NE(service_->getLastEvent(), HAService::HA_SYNCING_FAILED_EVT);
+    ASSERT_NE(service_->getLastEvent(), HAService::HA_SYNCING_SUCCEEDED_EVT);
+
+    // Run the syncing state handler.
+    testSyncingTransition(FinalState(HA_SYNCING_ST));
+
+    // We should see no synchronization attempts because the partner is
+    // offline.
+    EXPECT_NE(service_->getLastEvent(), HAService::HA_SYNCING_FAILED_EVT);
+    EXPECT_NE(service_->getLastEvent(), HAService::HA_SYNCING_SUCCEEDED_EVT);
+
+    // Startup the partner.
+    partner_->enableRespondLeaseFetching();
+    partner_->startup();
+
+    // We haven't been running heartbeats so we have to manually set the
+    // partner's state to something other than 'unavailable'.
+    state_->setPartnerState("ready");
+
+    // Retry the test.
+    testSyncingTransition(FinalState(HA_READY_ST));
+    // This time the server should have synchronized.
+    EXPECT_EQ(HAService::HA_SYNCING_SUCCEEDED_EVT, service_->getLastEvent());
+}
+
+// This test verifies that the HA state machine can be paused in certain states
+// when the server is operating in load balancing mode. The test also verifies
+// that heartbeat is active even if the state machine is paused.
+TEST_F(HAServiceStateMachineTest, stateTransitionsLoadBalancingPause) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration();
+    auto state_machine = valid_config->getStateMachineConfig();
+
+    // Set state machine pausing in various states.
+    state_machine->getStateConfig(HA_LOAD_BALANCING_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_PARTNER_DOWN_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_READY_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_SYNCING_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_TERMINATED_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_WAITING_ST)->setPausing("always");
+
+    startService(valid_config);
+
+    {
+        SCOPED_TRACE("LOAD BALANCING state transitions");
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+
+        EXPECT_TRUE(service_->unpause());
+        // An additional attempt to unpause should return false.
+        EXPECT_FALSE(service_->unpause());
+
+        testTransition(MyState(HA_LOAD_BALANCING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+    }
+
+    {
+        SCOPED_TRACE("PARTNER DOWN state transitions");
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+
+        EXPECT_TRUE(service_->unpause());
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+    }
+
+
+    {
+        SCOPED_TRACE("READY state transitions");
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_READY_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+
+        EXPECT_TRUE(service_->unpause());
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_LOAD_BALANCING_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+    }
+
+
+    {
+        SCOPED_TRACE("WAITING state transitions");
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+
+        EXPECT_TRUE(service_->unpause());
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_SYNCING_ST));
+        EXPECT_TRUE(state_->isHeartbeatRunning());
+    }
+}
+
+// This test verifies that the HA state machine can be paused in the syncing
+// state.
+TEST_F(HAServiceStateMachineTest, syncingTransitionsLoadBalancingPause) {
+    HAConfigPtr valid_config = createValidConfiguration();
+
+    auto state_machine = valid_config->getStateMachineConfig();
+
+    // Pause state machine in various states.
+    state_machine->getStateConfig(HA_LOAD_BALANCING_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_PARTNER_DOWN_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_READY_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_SYNCING_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_TERMINATED_ST)->setPausing("always");
+    state_machine->getStateConfig(HA_WAITING_ST)->setPausing("always");
+
+    startService(valid_config);
+    waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT);
+
+    // The syncing state handler doesn't start synchronization until it
+    // detects that the partner is online. It may remember that from the
+    // previous heartbeat attempts. If the partner appears to be unavailable
+    // it will continue heartbeats before it synchronizes. This prevents the
+    // server from making endless attempts to synchronize without any chance
+    // to succeed. We verify that the server is not trying to synchronize
+    // by checking that the last event is not the one associated with the
+    // synchronization attempt.
+    ASSERT_NE(service_->getLastEvent(), HAService::HA_SYNCING_FAILED_EVT);
+    ASSERT_NE(service_->getLastEvent(), HAService::HA_SYNCING_SUCCEEDED_EVT);
+
+    // Startup the partner.
+    partner_->enableRespondLeaseFetching();
+    partner_->startup();
+
+    // We haven't been running heartbeats so we have to manually set the
+    // partner's state to something other than 'unavailable'.
+    state_->setPartnerState("ready");
+
+    // Run the syncing state handler.
+    testSyncingTransition(FinalState(HA_SYNCING_ST));
+
+    // We should see no synchronization attempts because the server is paused
+    // in this state.
+    EXPECT_NE(service_->getLastEvent(), HAService::HA_SYNCING_FAILED_EVT);
+    EXPECT_NE(service_->getLastEvent(), HAService::HA_SYNCING_SUCCEEDED_EVT);
+
+    // Unpause the state machine.
+    EXPECT_TRUE(service_->unpause());
+
+    // Retry the test. It should now transition to the ready state.
+    testSyncingTransition(FinalState(HA_READY_ST));
+
+    // This time the server should have synchronized.
+    EXPECT_EQ(HAService::HA_SYNCING_SUCCEEDED_EVT, service_->getLastEvent());
+}
+
+// This test verifies that the server takes ownership of the given scopes
+// and whether the DHCP service is disabled or enabled in certain states.
+TEST_F(HAServiceStateMachineTest, scopesServingLoadBalancing) {
+    startService(createValidConfiguration());
+
+    // LOAD BALANCING and TERMINATED: serving my own scope.
+    expectScopes(MyState(HA_LOAD_BALANCING_ST), { "server1" }, true);
+    expectScopes(MyState(HA_TERMINATED_ST), { "server1" }, true);
+
+    // PARTNER DOWN and PARTNER IN MAINTENANCE: serving both scopes.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1", "server2" }, true);
+    expectScopes(MyState(HA_PARTNER_IN_MAINTENANCE_ST), { "server1", "server2" }, true);
+
+    // IN MAINTENANCE, READY & WAITING: serving no scopes.
+    expectScopes(MyState(HA_IN_MAINTENANCE_ST), { }, false);
+    expectScopes(MyState(HA_READY_ST), { }, false);
+    expectScopes(MyState(HA_WAITING_ST), { }, false);
+}
+
+// This test verifies that the server does not take ownership of the
+// partner's scope when auto-failover parameter is set to false.
+TEST_F(HAServiceStateMachineTest, scopesServingLoadBalancingNoFailover) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->getThisServerConfig()->setAutoFailover(false);
+    startService(valid_config);
+
+    // LOAD BALANCING and TERMINATED: serving my own scope.
+    expectScopes(MyState(HA_LOAD_BALANCING_ST), { "server1" }, true);
+    expectScopes(MyState(HA_TERMINATED_ST), { "server1" }, true);
+
+    // PARTNER DOWN: still serving my own scope because auto-failover is disabled.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1" }, true);
+
+    // PARTNER IN MAINTENANCE: always serving all scopes.
+    expectScopes(MyState(HA_PARTNER_IN_MAINTENANCE_ST), { "server1", "server2" }, true);
+
+    // Same for the partner-down case during maintenance.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1", "server2" }, true,
+                 HAService::HA_MAINTENANCE_START_EVT);
+
+    // IN MAINTENANCE, READY & WAITING: serving no scopes.
+    expectScopes(MyState(HA_IN_MAINTENANCE_ST), { }, false);
+    expectScopes(MyState(HA_READY_ST), { }, false);
+    expectScopes(MyState(HA_WAITING_ST), { }, false);
+}
+
+// This test verifies if the server would send lease updates to the partner
+// while being in various states. The HA configuration is load balancing.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesLoadBalancing) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getFailoverPeerConfig();
+
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_COMMUNICATION_RECOVERY_ST), peer_config));
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_LOAD_BALANCING_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), peer_config));
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_PARTNER_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_READY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_SYNCING_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_TERMINATED_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// Check that lease updates are sent to the backup server even when the
+// secondary is in the partner-down state.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesToBackup) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setWaitBackupAck(false);
+    startService(valid_config);
+
+    // Send the updates to the backup server.
+    HAConfig::PeerConfigPtr backup_config = valid_config->getPeerConfig("server3");
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), backup_config));
+}
+
+// This test verifies if the server would not send lease updates to the
+// partner if lease updates are administratively disabled.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesDisabledLoadBalancing) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setSendLeaseUpdates(false);
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getFailoverPeerConfig();
+
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_COMMUNICATION_RECOVERY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_LOAD_BALANCING_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_PARTNER_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_READY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_SYNCING_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_TERMINATED_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// Check that lease updates to the backup server are not queued.
+TEST_F(HAServiceStateMachineTest, shouldQueueLeaseUpdatesToBackup) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setWaitBackupAck(false);
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr backup_config = valid_config->getPeerConfig("server3");
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_COMMUNICATION_RECOVERY_ST), backup_config));
+}
+
+// This test verifies if the server would queue lease updates to the partner
+// while being in communication-recovery state. The HA configuration is load
+// balancing.
+TEST_F(HAServiceStateMachineTest, shouldQueueLeaseUpdatesLoadBalancing) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getFailoverPeerConfig();
+
+    EXPECT_TRUE(expectQueueLeaseUpdates(MyState(HA_COMMUNICATION_RECOVERY_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_LOAD_BALANCING_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_PARTNER_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_READY_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_SYNCING_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_TERMINATED_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// This test verifies if the server would not queue lease updates to the
+// partner if lease updates are administratively disabled.
+TEST_F(HAServiceStateMachineTest, shouldQueueLeaseUpdatesDisabledLoadBalancing) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    valid_config->setSendLeaseUpdates(false);
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getFailoverPeerConfig();
+
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_COMMUNICATION_RECOVERY_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_LOAD_BALANCING_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_PARTNER_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_READY_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_SYNCING_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_TERMINATED_ST), peer_config));
+    EXPECT_FALSE(expectQueueLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// This test verifies if the server would send heartbeat to the partner
+// while being in various states. The HA configuration is load balancing.
+TEST_F(HAServiceStateMachineTest, heartbeatLoadBalancing) {
+    HAConfigPtr valid_config = createValidConfiguration();
+    startService(valid_config);
+
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_LOAD_BALANCING_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_PARTNER_DOWN_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_PARTNER_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_READY_ST)));
+    EXPECT_FALSE(expectHeartbeat(MyState(HA_TERMINATED_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_WAITING_ST)));
+}
+
+// This test checks all combinations of server and partner states and the
+// resulting state to which the server transitions. This server is primary.
+// There is another test which validates state transitions from the
+// standby server perspective.
+TEST_F(HAServiceStateMachineTest, stateTransitionsHotStandbyPrimary) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    // HOT STANDBY state transitions
+    {
+        SCOPED_TRACE("HOT STANDBY state transitions");
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+    }
+
+    // in-maintenance state transitions
+    {
+        SCOPED_TRACE("in-maintenance state transitions");
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+    }
+
+    // PARTNER DOWN state transitions
+    {
+        SCOPED_TRACE("PARTNER DOWN state transitions");
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+    // PARTNER in-maintenance state transitions
+    {
+        SCOPED_TRACE("PARTNER in-maintenance state transitions");
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_PARTNER_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+    // READY state transitions
+    {
+        SCOPED_TRACE("READY state transitions");
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_READY_ST));
+    }
+
+    // WAITING state transitions
+    {
+        SCOPED_TRACE("WAITING state transitions");
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_WAITING_ST));
+    }
+}
+
+// This test checks that the server in the hot standby mode does not
+// transition to the "syncing" state when "sync-leases" is disabled.
+// This is the primary server case.
+TEST_F(HAServiceStateMachineTest, noSyncingTransitionsHotStandbyPrimary) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->getPeerConfig("server2")->setRole("standby");
+    valid_config->setSyncLeases(false);
+
+    startService(valid_config);
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                   FinalState(HA_READY_ST));
+}
+
+// This test checks that the primary server in the hot standby mode
+// transitions to the "terminated" state when the clock skew gets high.
+TEST_F(HAServiceStateMachineTest, terminateTransitionsHotStandbyPrimary) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    testTerminateTransition(MyState(HA_HOT_STANDBY_ST));
+    testTerminateTransition(MyState(HA_PARTNER_DOWN_ST));
+    testTerminateTransition(MyState(HA_READY_ST));
+    testTerminateTransition(MyState(HA_WAITING_ST));
+}
+
+// This test checks all combinations of server and partner states and the
+// resulting state to which the server transitions. This server is standby.
+// There is another test which validates state transitions from the
+// primary server perspective.
+TEST_F(HAServiceStateMachineTest, stateTransitionsHotStandbyStandby) {
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    // HOT STANDBY state transitions
+    {
+        SCOPED_TRACE("HOT STANDBY state transitions");
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_HOT_STANDBY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+    }
+
+    // in-maintenance state transitions
+    {
+        SCOPED_TRACE("in-maintenance state transitions");
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_IN_MAINTENANCE_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+    }
+
+    // PARTNER DOWN state transitions
+    {
+        SCOPED_TRACE("PARTNER DOWN state transitions");
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+
+        testTransition(MyState(HA_PARTNER_DOWN_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_PARTNER_DOWN_ST));
+    }
+
+
+    // READY state transitions
+    {
+        SCOPED_TRACE("READY state transitions");
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_HOT_STANDBY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_PARTNER_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_IN_MAINTENANCE_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_TERMINATED_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_READY_ST));
+
+        testTransition(MyState(HA_READY_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_READY_ST));
+    }
+
+    // WAITING state transitions
+    {
+        SCOPED_TRACE("WAITING state transitions");
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_COMMUNICATION_RECOVERY_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_LOAD_BALANCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_IN_MAINTENANCE_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                       FinalState(HA_SYNCING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_SYNCING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_TERMINATED_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_WAITING_ST),
+                       FinalState(HA_WAITING_ST));
+
+        testTransition(MyState(HA_WAITING_ST), PartnerState(HA_UNAVAILABLE_ST),
+                       FinalState(HA_WAITING_ST));
+    }
+}
+
+// This test checks that the server in the hot standby mode does not
+// transition to the "syncing" state when "sync-leases" is disabled.
+// This is the standby server case.
+TEST_F(HAServiceStateMachineTest, noSyncingTransitionsHotStandbyStandby) {
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+    valid_config->setSyncLeases(false);
+
+    startService(valid_config);
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_HOT_STANDBY_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_PARTNER_DOWN_ST),
+                   FinalState(HA_READY_ST));
+
+    testTransition(MyState(HA_WAITING_ST), PartnerState(HA_READY_ST),
+                   FinalState(HA_READY_ST));
+}
+
+// This test checks that the standby server in the hot standby mode
+// transitions to the "terminated" state when the clock skew gets high.
+TEST_F(HAServiceStateMachineTest, terminateTransitionsHotStandbyStandby) {
+    partner_.reset(new HAPartner(listener_, factory_));
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+
+    // Turn it into hot-standby configuration.
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    testTerminateTransition(MyState(HA_HOT_STANDBY_ST));
+    testTerminateTransition(MyState(HA_PARTNER_DOWN_ST));
+    testTerminateTransition(MyState(HA_READY_ST));
+    testTerminateTransition(MyState(HA_WAITING_ST));
+}
+
+// This test verifies that the server takes ownership of the given scopes
+// and whether the DHCP service is disabled or enabled in certain states.
+// This is primary server.
+TEST_F(HAServiceStateMachineTest, scopesServingHotStandbyPrimary) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    // HOT STANDBY and TERMINATED: serving my own scope
+    expectScopes(MyState(HA_HOT_STANDBY_ST), { "server1" }, true);
+    expectScopes(MyState(HA_TERMINATED_ST), { "server1" }, true);
+
+    // PARTNER DOWN and PARTNER IN MAINTENANCE: still serving my own scope.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1" }, true);
+    expectScopes(MyState(HA_PARTNER_IN_MAINTENANCE_ST), { "server1" }, true);
+
+    // IN MAINTENANCE, READY & WAITING: serving no scopes.
+    expectScopes(MyState(HA_IN_MAINTENANCE_ST), { }, false);
+    expectScopes(MyState(HA_READY_ST), { }, false);
+    expectScopes(MyState(HA_WAITING_ST), { }, false);
+}
+
+// This test verifies that auto-failover setting does not affect scopes
+// handling by the primary server in the hot-standby mode.
+TEST_F(HAServiceStateMachineTest, scopesServingHotStandbyPrimaryNoFailover) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    // Disable auto-failover.
+    valid_config->getThisServerConfig()->setAutoFailover(false);
+
+    startService(valid_config);
+
+    // HOT STANDBY and TERMINATED: serving my own scope
+    expectScopes(MyState(HA_HOT_STANDBY_ST), { "server1" }, true);
+    expectScopes(MyState(HA_TERMINATED_ST), { "server1" }, true);
+
+    // PARTNER IN MAINTENANCE & PARTNER DOWN: still serving my own scope.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1" }, true);
+    expectScopes(MyState(HA_PARTNER_IN_MAINTENANCE_ST), { "server1" }, true);
+
+    // IN MAINTENANCE, READY & WAITING: serving no scopes.
+    expectScopes(MyState(HA_IN_MAINTENANCE_ST), { }, false);
+    expectScopes(MyState(HA_READY_ST), { }, false);
+    expectScopes(MyState(HA_WAITING_ST), { }, false);
+}
+
+// This test verifies if the server would send lease updates to the partner
+// while being in various states. The HA configuration is hot standby and
+// the server is primary.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesHotStandbyPrimary) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getFailoverPeerConfig();
+
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_HOT_STANDBY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), peer_config));
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_PARTNER_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_READY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_SYNCING_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_TERMINATED_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// This test verifies if the server would send heartbeat to the partner
+// while being in various states. The HA configuration is hot standby.
+TEST_F(HAServiceStateMachineTest, heartbeatHotStandby) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_HOT_STANDBY_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_PARTNER_DOWN_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_PARTNER_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_READY_ST)));
+    EXPECT_FALSE(expectHeartbeat(MyState(HA_TERMINATED_ST)));
+    EXPECT_TRUE(expectHeartbeat(MyState(HA_WAITING_ST)));
+}
+
+// This test verifies that the server takes ownership of the given scopes
+// and whether the DHCP service is disabled or enabled in certain states.
+// This is standby server.
+TEST_F(HAServiceStateMachineTest, scopesServingHotStandbyStandby) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    // HOT STANDBY: serving no scopes because the primary is active.
+    // The DHCP is enabled to be able to monitor activity of the
+    // primary.
+    expectScopes(MyState(HA_HOT_STANDBY_ST), { }, true);
+
+    // TERMINATED: serving no scopes because the primary is active.
+    expectScopes(MyState(HA_TERMINATED_ST), { }, true);
+
+    // PARTNER IN MAINTENANCE & PARTNER DOWN: serving server1's scope.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1" }, true);
+    expectScopes(MyState(HA_PARTNER_IN_MAINTENANCE_ST), { "server1" }, true);
+
+    // IN MAINTENANCE, READY & WAITING: serving no scopes.
+    expectScopes(MyState(HA_IN_MAINTENANCE_ST), { }, false);
+    expectScopes(MyState(HA_READY_ST), { }, false);
+    expectScopes(MyState(HA_WAITING_ST), { }, false);
+}
+
+// This test verifies that the standby server does not take ownership
+// of the primary server's scope when auto-failover is set to false
+TEST_F(HAServiceStateMachineTest, scopesServingHotStandbyStandbyNoFailover) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    // Disable auto-failover.
+    valid_config->getThisServerConfig()->setAutoFailover(false);
+
+    startService(valid_config);
+
+    // HOT STANDBY: serving no scopes because the primary is active.
+    // The DHCP is enabled to be able to monitor activity of the
+    // primary.
+    expectScopes(MyState(HA_HOT_STANDBY_ST), { }, true);
+
+    // TERMINATED: serving no scopes because the primary is active.
+    expectScopes(MyState(HA_TERMINATED_ST), { }, true);
+
+    // PARTNER DOWN: still serving no scopes because auto-failover is set to false.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { }, true);
+
+    // PARTNER IN MAINTENANCE: serving partner's scopes.
+    expectScopes(MyState(HA_PARTNER_IN_MAINTENANCE_ST), { "server1" }, true);
+
+    // Same for the partner-down case during maintenance.
+    expectScopes(MyState(HA_PARTNER_DOWN_ST), { "server1" }, true,
+                 HAService::HA_MAINTENANCE_START_EVT);
+
+    // IN MAINTENANCE, READY & WAITING: serving no scopes.
+    expectScopes(MyState(HA_IN_MAINTENANCE_ST), { }, false);
+    expectScopes(MyState(HA_READY_ST), { }, false);
+    expectScopes(MyState(HA_WAITING_ST), { }, false);
+}
+
+// This test verifies if the server would send lease updates to the partner
+// while being in various states. The HA configuration is hot standby and
+// the server is secondary.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesHotStandbyStandby) {
+    HAConfigPtr valid_config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    valid_config->setThisServerName("server2");
+    valid_config->getPeerConfig("server2")->setRole("standby");
+
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getFailoverPeerConfig();
+
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_HOT_STANDBY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_PARTNER_DOWN_ST), peer_config));
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_PARTNER_IN_MAINTENANCE_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_READY_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_SYNCING_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_TERMINATED_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// This test verifies if the backup server doesn't send lease updates.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesBackup) {
+    HAConfigPtr valid_config = createValidConfiguration();
+
+    // Turn it into hot-standby configuration.
+    valid_config->setThisServerName("server3");
+
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getPeerConfig("server1");
+
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_BACKUP_ST), peer_config));
+    EXPECT_FALSE(expectLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// This test checks all combinations of the primary server in the passive
+// backup configuration.
+TEST_F(HAServiceStateMachineTest, stateTransitionsPassiveBackup) {
+    partner_->startup();
+
+    HAConfigPtr valid_config = createValidPassiveBackupConfiguration();
+    startService(valid_config);
+
+    testPassiveBackupTransition(MyState(HA_WAITING_ST), FinalState(HA_PASSIVE_BACKUP_ST));
+    testPassiveBackupTransition(MyState(HA_PASSIVE_BACKUP_ST), FinalState(HA_PASSIVE_BACKUP_ST));
+}
+
+// This test checks that no scopes are being served in the waiting state
+// of the passive backup configuration and that active server's scope is
+// served while in the passive-backup state.
+TEST_F(HAServiceStateMachineTest, scopesServingPassiveBackup) {
+    HAConfigPtr valid_config = createValidPassiveBackupConfiguration();
+    startService(valid_config);
+
+    EXPECT_TRUE(service_->query_filter_.amServingScope("server1"));
+    EXPECT_FALSE(service_->query_filter_.amServingScope("server2"));
+    EXPECT_TRUE(service_->network_state_->isServiceEnabled());
+}
+
+// Verifies that lease updates are sent by the server in the passive-backup state.
+TEST_F(HAServiceStateMachineTest, shouldSendLeaseUpdatesPassiveBackup) {
+    HAConfigPtr valid_config = createValidPassiveBackupConfiguration();
+    startService(valid_config);
+
+    HAConfig::PeerConfigPtr peer_config = valid_config->getPeerConfig("server2");
+
+    EXPECT_TRUE(expectLeaseUpdates(MyState(HA_WAITING_ST), peer_config));
+}
+
+// Verifies that the server transitions to the terminated state as a result
+// of too many rejected leases.
+TEST_F(HAServiceStateMachineTest, shouldTerminateDueToRejectedLeases) {
+    startService(createValidConfiguration());
+    service_->verboseTransition(HA_LOAD_BALANCING_ST);
+    service_->runModel(HAService::NOP_EVT);
+
+    // Simulate many rejected leases. It should cause the server to transition
+    // to the terminated state.
+    simulateRejectedLeaseUpdates(100);
+    EXPECT_EQ(HA_TERMINATED_ST, service_->getCurrState());
+}
+
+// Verifies that rejected leases are cleared upon entering certain states and
+// are not cleared upon entering other states in the load balancing mode.
+TEST_F(HAServiceStateMachineTest, clearRejectedLeaseUpdatesLoadBalancing) {
+    startService(createValidConfiguration());
+
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_COMMUNICATION_RECOVERY_ST)));
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_LOAD_BALANCING_ST)));
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_PARTNER_DOWN_ST)));
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_PARTNER_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_READY_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_TERMINATED_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_WAITING_ST)));
+}
+
+// Verifies that rejected leases are cleared upon entering certain states and
+// are not cleared upon entering other states in the hot standby mode.
+TEST_F(HAServiceStateMachineTest, clearRejectedLeaseUpdatesHotStandby) {
+    HAConfigPtr config_storage = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config_storage->getPeerConfig("server2")->setRole("standby");
+    startService(config_storage);
+
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_HOT_STANDBY_ST)));
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_PARTNER_DOWN_ST)));
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_PARTNER_IN_MAINTENANCE_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_READY_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_TERMINATED_ST)));
+    EXPECT_TRUE(expectClearRejectedLeases(MyState(HA_WAITING_ST)));
+}
+
+// Verifies that rejected leases are cleared upon entering certain states and
+// are not cleared upon entering other states in the passive backup mode.
+TEST_F(HAServiceStateMachineTest, clearRejectedLeaseUpdatesPassiveBackup) {
+    HAConfigPtr config_storage = createValidPassiveBackupConfiguration();
+    config_storage->getPeerConfig("server2")->setRole("backup");
+    startService(config_storage);
+
+    EXPECT_FALSE(expectClearRejectedLeases(MyState(HA_PASSIVE_BACKUP_ST)));
+}
+
+// Verifies that the server doesn't terminate when the partner is unavailable.
+TEST_F(HAServiceStateMachineTest, doNotTerminateWhenPartnerUnavailable) {
+    HAConfigPtr config_storage = createValidConfiguration();
+    ASSERT_NO_THROW_LOG(service_.reset(new TestHAService(io_service_, network_state_,
+                                                         config_storage)));
+    NakedCommunicationState4Ptr state(new NakedCommunicationState4(io_service_,
+                                                                   config_storage));
+    service_->communication_state_ = state;
+
+    // Begin in the load-balancing state and the partner is also in
+    // that state.
+    service_->verboseTransition(HA_LOAD_BALANCING_ST);
+    service_->runModel(HAService::NOP_EVT);
+    service_->communication_state_->poke();
+    service_->communication_state_->setPartnerState("load-balancing");
+
+    // Change the clock skew, so it is beyond the acceptable threshold.
+    state->clock_skew_ += boost::posix_time::time_duration(0, 1, 5);
+
+    // Send the heartbeat. We didn't setup the connection to the partner.
+    // The heartbeat should fail and our server should transition to the
+    // communication-recovery state. Earlier, the server had a bug that
+    // it would take the last known clock skew and transition to the
+    // terminated state instead. It was wrong because when the partner is
+    // unavailable we can't tell the current clock skew. The administrator
+    // may actually be fixing the time on the other server. Thus, we must
+    // not terminate.
+    ASSERT_NO_FATAL_FAILURE(waitForEvent(HAService::HA_HEARTBEAT_COMPLETE_EVT));
+    EXPECT_EQ(HA_COMMUNICATION_RECOVERY_ST, service_->getCurrState());
+}
+
+} // end of anonymous namespace
diff --git a/src/hooks/dhcp/high_availability/tests/ha_test.cc b/src/hooks/dhcp/high_availability/tests/ha_test.cc
new file mode 100644
index 0000000..7c90242
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/ha_test.cc
@@ -0,0 +1,378 @@
+// Copyright (C) 2017-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <asiolink/asio_wrapper.h>
+#include <ha_test.h>
+#include <asiolink/interval_timer.h>
+#include <cc/command_interpreter.h>
+#include <cc/data.h>
+#include <dhcp/dhcp4.h>
+#include <dhcp/dhcp6.h>
+#include <dhcp/duid.h>
+#include <dhcp/hwaddr.h>
+#include <dhcp/option.h>
+#include <dhcp/option_int.h>
+#include <dhcpsrv/cfgmgr.h>
+#include <hooks/hooks.h>
+#include <hooks/hooks_manager.h>
+#include <util/range_utilities.h>
+#include <functional>
+#include <utility>
+#include <vector>
+
+using namespace isc::asiolink;
+using namespace isc::data;
+using namespace isc::dhcp;
+using namespace isc::hooks;
+
+namespace {
+
+/// @brief Structure that holds registered hook indexes.
+struct TestHooks {
+    int hooks_index_dhcp4_srv_configured_;
+
+    /// Constructor that registers hook points for the tests.
+    TestHooks() {
+        hooks_index_dhcp4_srv_configured_ = HooksManager::registerHook("dhcp4_srv_configured");
+    }
+};
+
+TestHooks Hooks;
+
+}
+
+namespace isc {
+namespace ha {
+namespace test {
+
+HATest::HATest()
+    : io_service_(new IOService()),
+      network_state_(new NetworkState(NetworkState::DHCPv4)) {
+}
+
+HATest::~HATest() {
+}
+
+void
+HATest::startHAService() {
+    if (HooksManager::calloutsPresent(Hooks.hooks_index_dhcp4_srv_configured_)) {
+        CalloutHandlePtr callout_handle = HooksManager::createCalloutHandle();
+        callout_handle->setArgument("io_context", io_service_);
+        callout_handle->setArgument("network_state", network_state_);
+        HooksManager::callCallouts(Hooks.hooks_index_dhcp4_srv_configured_,
+                                   *callout_handle);
+    }
+}
+
+void
+HATest::runIOService(long ms) {
+    io_service_->get_io_service().reset();
+    IntervalTimer timer(*io_service_);
+    timer.setup(std::bind(&IOService::stop, io_service_), ms,
+                IntervalTimer::ONE_SHOT);
+    io_service_->run();
+    timer.cancel();
+}
+
+void
+HATest::runIOService(long ms, std::function<bool()> stop_condition) {
+    io_service_->get_io_service().reset();
+    IntervalTimer timer(*io_service_);
+    bool timeout = false;
+    timer.setup(std::bind(&HATest::stopIOServiceHandler, this, std::ref(timeout)),
+                ms, IntervalTimer::ONE_SHOT);
+
+    while (!stop_condition() && !timeout) {
+        io_service_->run_one();
+    }
+
+    timer.cancel();
+}
+
+boost::shared_ptr<std::thread>
+HATest::runIOServiceInThread() {
+    io_service_->get_io_service().reset();
+
+    bool running = false;
+    std::mutex mutex;
+    std::condition_variable condvar;
+
+    io_service_->post(std::bind(&HATest::signalServiceRunning, this, std::ref(running),
+                                std::ref(mutex), std::ref(condvar)));
+    boost::shared_ptr<std::thread>
+        th(new std::thread(std::bind(&IOService::run, io_service_.get())));
+
+    std::unique_lock<std::mutex> lock(mutex);
+    while (!running) {
+        condvar.wait(lock);
+    }
+
+    return (th);
+}
+
+void
+HATest::testSynchronousCommands(std::function<void()> commands) {
+    // Run IO service in thread.
+    auto thread = runIOServiceInThread();
+
+    // Run desired commands.
+    commands();
+
+    // Stop the IO service. This should cause the thread to terminate.
+    io_service_->stop();
+    thread->join();
+}
+
+void
+HATest::signalServiceRunning(bool& running, std::mutex& mutex,
+                             std::condition_variable& condvar) {
+    std::lock_guard<std::mutex> lock(mutex);
+    running = true;
+    condvar.notify_one();
+}
+
+void
+HATest::stopIOServiceHandler(bool& stop_flag) {
+    stop_flag = true;
+}
+
+ConstElementPtr
+HATest::createValidJsonConfiguration(const HAConfig::HAMode& ha_mode) const {
+    std::ostringstream config_text;
+    config_text <<
+        "["
+        "     {"
+        "         \"this-server-name\": \"server1\","
+        "         \"mode\": " << (ha_mode == HAConfig::LOAD_BALANCING ?
+                                  "\"load-balancing\"" : "\"hot-standby\"") << ","
+        "         \"sync-page-limit\": 3,"
+        "         \"heartbeat-delay\": 1000,"
+        "         \"max-response-delay\": 1000,"
+        "         \"max-ack-delay\": 10000,"
+        "         \"max-unacked-clients\": 10,"
+        "         \"max-rejected-clients\": 10,"
+        "         \"multi-threading\": {"
+        "             \"enable-multi-threading\": false"
+        "         },"
+        "         \"wait-backup-ack\": false,"
+        "         \"peers\": ["
+        "             {"
+        "                 \"name\": \"server1\","
+        "                 \"url\": \"http://127.0.0.1:18123/\","
+        "                 \"role\": \"primary\","
+        "                 \"auto-failover\": true"
+        "             },"
+        "             {"
+        "                 \"name\": \"server2\","
+        "                 \"url\": \"http://127.0.0.1:18124/\","
+        "                 \"role\": " << (ha_mode == HAConfig::LOAD_BALANCING ?
+                                          "\"secondary\"" : "\"standby\"") << ","
+        "                 \"auto-failover\": true"
+        "             },"
+        "             {"
+        "                 \"name\": \"server3\","
+        "                 \"url\": \"http://127.0.0.1:18125/\","
+        "                 \"role\": \"backup\","
+        "                 \"auto-failover\": false"
+        "             }"
+        "         ]"
+        "     }"
+        "]";
+
+    return (Element::fromJSON(config_text.str()));
+}
+
+ConstElementPtr
+HATest::createValidPassiveBackupJsonConfiguration() const {
+    std::ostringstream config_text;
+    config_text <<
+        "["
+        "     {"
+        "         \"this-server-name\": \"server1\","
+        "         \"mode\": \"passive-backup\","
+        "         \"multi-threading\": {"
+        "             \"enable-multi-threading\": false"
+        "         },"
+        "         \"wait-backup-ack\": false,"
+        "         \"peers\": ["
+        "             {"
+        "                 \"name\": \"server1\","
+        "                 \"url\": \"http://127.0.0.1:18123/\","
+        "                 \"role\": \"primary\""
+        "             },"
+        "             {"
+        "                 \"name\": \"server2\","
+        "                 \"url\": \"http://127.0.0.1:18124/\","
+        "                 \"role\": \"backup\""
+        "             },"
+        "             {"
+        "                 \"name\": \"server3\","
+        "                 \"url\": \"http://127.0.0.1:18125/\","
+        "                 \"role\": \"backup\""
+        "             }"
+        "         ]"
+        "     }"
+        "]";
+
+    return (Element::fromJSON(config_text.str()));
+}
+
+
+HAConfigPtr
+HATest::createValidConfiguration(const HAConfig::HAMode& ha_mode) const {
+    HAConfigPtr config_storage(new HAConfig());
+    HAConfigParser parser;
+
+    parser.parse(config_storage, createValidJsonConfiguration(ha_mode));
+    return (config_storage);
+}
+
+HAConfigPtr
+HATest::createValidPassiveBackupConfiguration() const {
+    HAConfigPtr config_storage(new HAConfig());
+    HAConfigParser parser;
+
+    parser.parse(config_storage, createValidPassiveBackupJsonConfiguration());
+    return (config_storage);
+}
+
+void
+HATest::checkAnswer(const isc::data::ConstElementPtr& answer,
+                    const int exp_status,
+                    const std::string& exp_txt) {
+    int rcode = 0;
+    isc::data::ConstElementPtr comment;
+    comment = isc::config::parseAnswer(rcode, answer);
+
+    EXPECT_EQ(exp_status, rcode)
+        << "Expected status code " << exp_status
+        << " but received " << rcode << ", comment: "
+        << (comment ? comment->str() : "(none)");
+
+    // Ok, parseAnswer interface is weird. If there are no arguments,
+    // it returns content of text. But if there is an argument,
+    // it returns the argument and it's not possible to retrieve
+    // "text" (i.e. comment).
+    if (comment->getType() != Element::string) {
+        comment = answer->get("text");
+    }
+
+    if (!exp_txt.empty()) {
+        EXPECT_EQ(exp_txt, comment->stringValue());
+    }
+}
+
+std::vector<uint8_t>
+HATest::randomKey(const size_t key_size) const {
+    std::vector<uint8_t> key(key_size);
+    util::fillRandom(key.begin(), key.end());
+    return (key);
+}
+
+
+Pkt4Ptr
+HATest::createMessage4(const uint8_t msg_type, const uint8_t hw_address_seed,
+                       const uint8_t client_id_seed, const uint16_t secs) const {
+    Pkt4Ptr message(new Pkt4(msg_type, 0x1234));
+
+    HWAddrPtr hw_address(new HWAddr(std::vector<uint8_t>(6, hw_address_seed),
+                                    HTYPE_ETHER));
+    message->setHWAddr(hw_address);
+    message->setSecs(secs);
+
+    if (client_id_seed > 0) {
+        OptionPtr opt_client_id(new Option(Option::V4, DHO_DHCP_CLIENT_IDENTIFIER,
+                                           std::vector<uint8_t>(6, client_id_seed)));
+        message->addOption(opt_client_id);
+    }
+
+    return (message);
+}
+
+Pkt4Ptr
+HATest::createQuery4(const std::string& hw_address_text) const {
+    Pkt4Ptr query4(new Pkt4(DHCPDISCOVER, 0x1234));
+    HWAddr hwaddr = HWAddr::fromText(hw_address_text);
+    query4->setHWAddr(HWAddrPtr(new HWAddr(hwaddr.hwaddr_, HTYPE_ETHER)));
+    return (query4);
+}
+
+Pkt4Ptr
+HATest::createQuery4(const std::vector<uint8_t>& hw_address,
+                     const std::vector<uint8_t>& client_id) const {
+    Pkt4Ptr query4(new Pkt4(DHCPDISCOVER, 0x1234));
+    query4->setHWAddr(HWAddrPtr(new HWAddr(hw_address, HTYPE_ETHER)));
+    if (!client_id.empty()) {
+        OptionPtr opt_client_id(new Option(Option::V4, DHO_DHCP_CLIENT_IDENTIFIER,
+                                           client_id));
+        query4->addOption(opt_client_id);
+    }
+    return (query4);
+}
+
+Pkt6Ptr
+HATest::createQuery6(const std::vector<uint8_t>& duid) const {
+    Pkt6Ptr query6(new Pkt6(DHCPV6_SOLICIT, 0x1234));
+    OptionPtr opt_duid(new Option(Option::V6, D6O_CLIENTID, duid));
+    query6->addOption(opt_duid);
+    return (query6);
+}
+
+Pkt6Ptr
+HATest::createMessage6(const uint8_t msg_type, const uint8_t duid_seed,
+                       const uint16_t elapsed_time) const {
+    Pkt6Ptr message(new Pkt6(msg_type, 0x1234));
+
+    OptionPtr opt_duid(new Option(Option::V6, D6O_CLIENTID, OptionBuffer(8, duid_seed)));
+    message->addOption(opt_duid);
+
+    OptionUint16Ptr opt_elapsed_time(new OptionUint16(Option::V6, D6O_ELAPSED_TIME,
+                                                      elapsed_time));
+    message->addOption(opt_elapsed_time);
+
+    return (message);
+}
+
+Pkt6Ptr
+HATest::createQuery6(const std::string& duid_text) const {
+    Pkt6Ptr query6(new Pkt6(DHCPV6_SOLICIT, 0x1234));
+    DUID duid = DUID::fromText(duid_text);
+    OptionPtr client_id(new Option(Option::V6, D6O_CLIENTID, duid.getDuid()));
+    query6->addOption(client_id);
+    return (query6);
+}
+
+void
+HATest::setDHCPMultiThreadingConfig(bool enabled, uint32_t thread_pool_size,
+                                    uint32_t packet_queue_size) {
+    ElementPtr mt_config = Element::createMap();
+    mt_config->set("enable-multi-threading", Element::create(enabled));
+    mt_config->set("thread-pool-size",
+                   Element::create(static_cast<int>(thread_pool_size)));
+    mt_config->set("queue-size",
+                   Element::create(static_cast<int>(packet_queue_size)));
+    CfgMgr::instance().getStagingCfg()->setDHCPMultiThreading(mt_config);
+}
+
+std::string
+HATest::makeHAMtJson(bool enable_multi_threading, bool http_dedicated_listener,
+                     uint32_t http_listener_threads,  uint32_t http_client_threads) {
+    std::stringstream ss;
+    ss << "\"multi-threading\": {"
+       << " \"enable-multi-threading\": "
+       << (enable_multi_threading ? "true" : "false") << ","
+       << " \"http-dedicated-listener\": "
+       << (http_dedicated_listener ? "true" : "false")  << ","
+       << " \"http-listener-threads\": " << http_listener_threads  << ","
+       << " \"http-client-threads\": " << http_client_threads << "}";
+    return (ss.str());
+}
+
+}  // namespace test
+}  // namespace ha
+}  // namespace isc
diff --git a/src/hooks/dhcp/high_availability/tests/ha_test.h b/src/hooks/dhcp/high_availability/tests/ha_test.h
new file mode 100644
index 0000000..0b763ca
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/ha_test.h
@@ -0,0 +1,304 @@
+// Copyright (C) 2017-2022 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <communication_state.h>
+#include <ha_config.h>
+#include <ha_config_parser.h>
+#include <asiolink/io_service.h>
+#include <cc/data.h>
+#include <dhcp/duid.h>
+#include <dhcp/hwaddr.h>
+#include <dhcp/pkt4.h>
+#include <dhcp/pkt6.h>
+#include <dhcpsrv/network_state.h>
+#include <hooks/libinfo.h>
+#include <boost/shared_ptr.hpp>
+#include <gtest/gtest.h>
+#include <cstdint>
+#include <functional>
+#include <string>
+#include <vector>
+#include <mutex>
+#include <condition_variable>
+#include <thread>
+
+namespace isc {
+namespace ha {
+namespace test {
+
+/// @brief Derivation of the @c CommunicationState class which allows
+/// for modifications of poke time.
+///
+/// @tparam @c CommunicationState4 or @c CommunicationState6.
+template<typename StateType>
+class NakedCommunicationState : public StateType {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// @param io_service pointer to the IO service object.
+    explicit NakedCommunicationState(const asiolink::IOServicePtr& io_service,
+                                     const HAConfigPtr& config)
+        : StateType(io_service, config) {
+    }
+
+    /// @brief Checks if the object was poked recently.
+    ///
+    /// @return true if the object was poked less than 5 seconds ago,
+    /// false otherwise.
+    bool isPoked() const {
+        return (StateType::getDurationInMillisecs() < 5000);
+    }
+
+    using StateType::config_;
+    using StateType::timer_;
+    using StateType::clock_skew_;
+    using StateType::last_clock_skew_warn_;
+    using StateType::my_time_at_skew_;
+    using StateType::partner_time_at_skew_;
+    using StateType::unsent_update_count_;
+    using StateType::getRejectedLeaseUpdatesCountFromContainer;
+};
+
+/// @brief Type of the NakedCommunicationState for DHCPv4.
+typedef NakedCommunicationState<CommunicationState4> NakedCommunicationState4;
+
+/// @brief Type of the pointer to the @c NakedCommunicationState4.
+typedef boost::shared_ptr<NakedCommunicationState4> NakedCommunicationState4Ptr;
+
+/// @brief Type of the NakedCommunicationState for DHCPv6.
+typedef NakedCommunicationState<CommunicationState6> NakedCommunicationState6;
+
+/// @brief Type of the pointer to the @c NakedCommunicationState6.
+typedef boost::shared_ptr<NakedCommunicationState6> NakedCommunicationState6Ptr;
+
+/// @brief General test fixture class for all HA unittests.
+///
+/// It provides basic functions to load and unload HA hooks library.
+/// All test classes should derive from this class.
+class HATest : public ::testing::Test {
+public:
+
+    /// @brief Constructor.
+    HATest();
+
+    /// @brief Destructor.
+    virtual ~HATest();
+
+    /// @brief Calls dhcp4_srv_configured callout to set IO service pointer.
+    void startHAService();
+
+    /// @brief Runs IO service for a specified amount of time.
+    ///
+    /// @param ms number of milliseconds for which the IO service should be
+    /// run.
+    void runIOService(long ms);
+
+    /// @brief Runs IO service until timeout occurs or until provided method
+    /// returns true.
+    ///
+    /// @param ms number of milliseconds for which the IO service should be
+    /// run.
+    /// @param stop_condition pointer to the function which returns true if
+    /// when the IO service should be stopped.
+    void runIOService(long ms, std::function<bool()> stop_condition);
+
+    /// @brief Runs IO service in a thread.
+    ///
+    /// @return Shared pointer to the thread.
+    boost::shared_ptr<std::thread> runIOServiceInThread();
+
+    /// @brief Executes commands while running IO service in a thread.
+    ///
+    /// @param commands pointer to a function to be executed while IO service
+    /// is run in thread.
+    void testSynchronousCommands(std::function<void()> commands);
+
+protected:
+
+    /// @brief Signals that the IO service is running.
+    ///
+    /// @param running reference to the flag which is set to true when the
+    /// IO service starts running and executes this function.
+    /// @param mutex reference to the mutex used for synchronization.
+    /// @param condvar reference to condition variable used for synchronization.
+    void signalServiceRunning(bool& running, std::mutex& mutex,
+                              std::condition_variable& condvar);
+
+public:
+
+    /// @brief Handler for timeout during runIOService invocation.
+    ///
+    /// @param [out] stop_flag set to true when the handler is invoked.
+    void stopIOServiceHandler(bool& stop_flag);
+
+    /// @brief Return HA configuration with three servers in JSON format.
+    ///
+    /// @param ha_mode HA operation mode (default is load balancing).
+    /// @return Pointer to the unparsed configuration.
+    data::ConstElementPtr
+    createValidJsonConfiguration(const HAConfig::HAMode& ha_mode =
+                                 HAConfig::LOAD_BALANCING) const;
+
+    /// @brief Return HA configuration with one primary and two backup
+    /// servers in the JSON format.
+    ///
+    /// @return Pointer to the unparsed configuration.
+    data::ConstElementPtr
+    createValidPassiveBackupJsonConfiguration() const;
+
+    /// @brief Return HA configuration with three servers.
+    ///
+    /// @param ha_mode HA operation mode (default is load balancing).
+    /// @return Pointer to the parsed configuration.
+    HAConfigPtr createValidConfiguration(const HAConfig::HAMode& ha_mode =
+                                         HAConfig::LOAD_BALANCING) const;
+
+    /// @brief Return passive-backup configuration.
+    ///
+    /// @return Pointer to the parsed configuration.
+    HAConfigPtr createValidPassiveBackupConfiguration() const;
+
+    /// @brief Checks the status code and message against expected values.
+    ///
+    /// @param answer Element set containing an integer response and string
+    /// comment.
+    /// @param exp_status Status code against which to compare the status.
+    /// @param exp_txt Expected text (not checked if empty)
+    void checkAnswer(const isc::data::ConstElementPtr& answer,
+                     const int exp_status,
+                     const std::string& exp_txt = "");
+
+    /// @brief Creates an identifier of arbitrary size with random values.
+    ///
+    /// This function is useful in generating random client identifiers and
+    /// HW addresses for load balancing tests.
+    ///
+    /// @param key_size Size of the generated random identifier.
+    std::vector<uint8_t> randomKey(const size_t key_size) const;
+
+    /// @brief Generates simple DHCPv4 message.
+    ///
+    /// @param msg_type DHCPv4 message type to be created.
+    /// @param hw_address_seed value from which HW address will be generated.
+    /// @param client_id_seed value from which client identifier will be
+    /// generated.
+    /// @param secs value to be stored in the "secs" field of the DHCPv4 message.
+    ///
+    /// @return Pointer to the created message.
+    dhcp::Pkt4Ptr createMessage4(const uint8_t msg_type,
+                                 const uint8_t hw_address_seed,
+                                 const uint8_t client_id_seed,
+                                 const uint16_t secs) const;
+
+    /// @brief Creates test DHCPv4 query instance.
+    ///
+    /// @param hw_address_text HW address to be included in the query. It is
+    /// used in load balancing.
+    ///
+    /// @return Pointer to the DHCPv4 query instance.
+    dhcp::Pkt4Ptr createQuery4(const std::string& hw_address_text) const;
+
+    /// @brief Creates test DHCPv4 query instance.
+    ///
+    /// @param hw_address HW address to be included in the query. It is used
+    /// in load balancing.
+    /// @param client_id optional client identifier.
+    dhcp::Pkt4Ptr createQuery4(const std::vector<uint8_t>& hw_address,
+                               const std::vector<uint8_t>& client_id =
+                               std::vector<uint8_t>()) const;
+
+    /// @brief Creates test DHCPv6 query instance.
+    ///
+    /// @param duid DUI to be included in the query. It is used in load balancing.
+    dhcp::Pkt6Ptr createQuery6(const std::vector<uint8_t>& duid) const;
+
+    /// @brief Generates simple DHCPv6 message.
+    ///
+    /// @param msg_type DHCPv6 message type to be created.
+    /// @param duid value from which DUID will be generated.
+    /// @param elapsed_time value of the Elapsed Time option.
+    ///
+    /// @return Pointer to the created message.
+    dhcp::Pkt6Ptr createMessage6(const uint8_t msg_type,
+                                 const uint8_t duid_seed,
+                                 const uint16_t elapsed_time) const;
+
+    /// @brief Sets the DHCP multi-threading configuration in staging SrvConfig.
+    ///
+    /// @param enable_multi_threading value that maps to enable-multi-threading.
+    /// @param thread_pool_size value that maps to thread-pool-size.
+    /// @param queue_size value that maps to queue-size.
+    void setDHCPMultiThreadingConfig(bool enable_multi_threading,
+                                     uint32_t thread_pool_size = 0,
+                                     uint32_t queue_size = 16);
+
+    /// @brief Replace a pattern in a configuration.
+    ///
+    /// @param config Configuration to patch.
+    /// @param from String to replace.
+    /// @param repl String which replaces all occurrences of from.
+    /// @result A copy of config where all occurrences of from were replaced
+    /// by repl.
+    std::string replaceInConfig(const std::string& config,
+                                const std::string& from,
+                                const std::string& repl) {
+        std::string result(config);
+        if (from.empty()) {
+            return (result);
+        }
+        for (;;) {
+            size_t where = result.find(from);
+            if (where == std::string::npos) {
+                return (result);
+            }
+            result.replace(where, from.size(), repl);
+        }
+        return (result);
+    }
+
+    /// @brief Constructs JSON string for HA "multi-threading" element.
+    ///
+    /// Constructs a JSON string with the following content:
+    ///
+    /// ```
+    /// "multi-threading" {
+    ///     "enable-multi-threading": <bool>,
+    ///     "dedicated-http-listener": <bool>,
+    ///     "http-listener-threads": <int>,
+    ///     "http-client-threads": <int>
+    /// }"
+    /// ```
+    ///
+    /// @param enable_multi_threading value for enable-multi-threading.
+    /// @param http_dedicated_listener value for dedicated-http-listener.
+    /// @param http_listener_threads value for http-listener-threads
+    /// @param http_client_threads value for http-client-threads
+    ///
+    /// @return JSON string
+    std::string makeHAMtJson(bool enable_multi_threading,
+                             bool http_dedicated_listener,
+                             uint32_t http_listener_threads,
+                             uint32_t http_client_threads);
+
+    /// @brief Creates test DHCPv6 query instance.
+    ///
+    /// @param duid_text DUID to be included in the query. It is used in load
+    /// balancing.
+    ///
+    /// @return Pointer to the DHCPv6 query instance.
+    dhcp::Pkt6Ptr createQuery6(const std::string& duid_text) const;
+
+    /// @brief Pointer to the IO service used in the tests.
+    asiolink::IOServicePtr io_service_;
+
+    /// @brief Object holding a state of the DHCP service.
+    dhcp::NetworkStatePtr network_state_;
+};
+
+} // end of namespace isc::ha::test
+} // end of namespace isc::ha
+} // end of namespace isc
diff --git a/src/hooks/dhcp/high_availability/tests/lease_update_backlog_unittest.cc b/src/hooks/dhcp/high_availability/tests/lease_update_backlog_unittest.cc
new file mode 100644
index 0000000..a1cbe62
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/lease_update_backlog_unittest.cc
@@ -0,0 +1,91 @@
+// Copyright (C) 2020-2021 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <lease_update_backlog.h>
+#include <asiolink/io_address.h>
+#include <dhcp/duid.h>
+#include <dhcp/hwaddr.h>
+
+#include <boost/make_shared.hpp>
+#include <boost/pointer_cast.hpp>
+#include <gtest/gtest.h>
+
+using namespace isc::asiolink;
+using namespace isc::dhcp;
+using namespace isc::ha;
+
+namespace {
+
+// This test verifies that lease updates can be pushed to the queue,
+// retrieved from the queue.
+TEST(LeaseUpdateBacklogTest, pushAndPop) {
+    // Create the queue with limit of 5 lease updates.
+    LeaseUpdateBacklog backlog(5);
+
+    // Add 5 lease updates.
+    for (auto i = 0; i < 5; ++i) {
+        IOAddress address(i + 1);
+        HWAddrPtr hwaddr = boost::make_shared<HWAddr>(std::vector<uint8_t>(6, static_cast<uint8_t>(i)),
+                                                      HTYPE_ETHER);
+        Lease4Ptr lease = boost::make_shared<Lease4>(address, hwaddr, ClientIdPtr(), 60, 0, 1);
+        // Some lease updates have type "Add", some have type "Delete".
+        ASSERT_TRUE(backlog.push(i % 2 ? LeaseUpdateBacklog::ADD : LeaseUpdateBacklog::DELETE, lease));
+        EXPECT_FALSE(backlog.wasOverflown());
+    }
+
+    // Add 6th lease update. This should exceed the size limit.
+    IOAddress address("192.0.2.0");
+    HWAddrPtr hwaddr = boost::make_shared<HWAddr>(std::vector<uint8_t>(6, static_cast<uint8_t>(0xA)),
+                                                  HTYPE_ETHER);
+    Lease4Ptr lease = boost::make_shared<Lease4>(address, hwaddr, ClientIdPtr(), 60, 0, 1);
+    ASSERT_FALSE(backlog.push(LeaseUpdateBacklog::ADD, lease));
+    EXPECT_TRUE(backlog.wasOverflown());
+
+    // Try to pop all lease updates.
+    LeaseUpdateBacklog::OpType op_type;
+    for (auto i = 0; i < 5; ++i) {
+        auto lease = backlog.pop(op_type);
+        ASSERT_TRUE(lease);
+        ASSERT_EQ(i % 2 ? LeaseUpdateBacklog::ADD : LeaseUpdateBacklog::DELETE, op_type);
+    }
+
+    // When trying to pop from an empty queue it should return null pointer.
+    lease = boost::dynamic_pointer_cast<Lease4>(backlog.pop(op_type));
+    EXPECT_FALSE(lease);
+    EXPECT_TRUE(backlog.wasOverflown());
+
+    // Reset queue state.
+    ASSERT_NO_THROW(backlog.clear());
+    EXPECT_FALSE(backlog.wasOverflown());
+}
+
+// This test verifies that all lease updates can be removed.
+TEST(LeaseUpdateBacklogTest, clear) {
+    // Create the queue with limit of 5 lease updates.
+    LeaseUpdateBacklog backlog(5);
+
+    // Add 5 lease updates.
+    for (auto i = 0; i < 3; ++i) {
+        IOAddress address(i + 1);
+        HWAddrPtr hwaddr = boost::make_shared<HWAddr>(std::vector<uint8_t>(6, static_cast<uint8_t>(i)),
+                                                      HTYPE_ETHER);
+        Lease4Ptr lease = boost::make_shared<Lease4>(address, hwaddr, ClientIdPtr(), 60, 0, 1);
+        ASSERT_TRUE(backlog.push(LeaseUpdateBacklog::ADD, lease));
+    }
+
+    // Make sure all lease updates have been added.
+    EXPECT_EQ(3, backlog.size());
+
+    // Remove lease updates.
+    ASSERT_NO_THROW(backlog.clear());
+
+    // There should be no lease updates.
+    EXPECT_EQ(0, backlog.size());
+}
+
+} // end of anonymous namespace
diff --git a/src/hooks/dhcp/high_availability/tests/query_filter_unittest.cc b/src/hooks/dhcp/high_availability/tests/query_filter_unittest.cc
new file mode 100644
index 0000000..4f7f2c4
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/query_filter_unittest.cc
@@ -0,0 +1,1029 @@
+// Copyright (C) 2018-2022 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <ha_test.h>
+#include <ha_config.h>
+#include <ha_config_parser.h>
+#include <query_filter.h>
+#include <cc/data.h>
+#include <exceptions/exceptions.h>
+#include <dhcp/dhcp4.h>
+#include <dhcp/dhcp6.h>
+#include <dhcp/hwaddr.h>
+#include <util/multi_threading_mgr.h>
+
+#include <cstdint>
+#include <string>
+
+using namespace isc;
+using namespace isc::data;
+using namespace isc::dhcp;
+using namespace isc::ha;
+using namespace isc::ha::test;
+using namespace util;
+
+namespace  {
+
+/// @brief Test fixture class for @c QueryFilter class.
+class QueryFilterTest : public HATest {
+public:
+    /// @brief Constructor.
+    QueryFilterTest() {
+        MultiThreadingMgr::instance().setMode(false);
+    }
+
+    /// @brief Destructor.
+    ~QueryFilterTest() {
+        MultiThreadingMgr::instance().setMode(false);
+    }
+
+    /// @brief This test verifies that client identifier is used for load
+    /// balancing.
+    void loadBalancingClientIdThisPrimary();
+
+    /// @brief This test verifies the case when load balancing is enabled and
+    /// this server is primary.
+    void loadBalancingThisPrimary();
+
+    /// @brief This test verifies the case when load balancing is enabled and
+    /// this server is secondary.
+    void loadBalancingThisSecondary();
+
+    /// @brief This test verifies the case when load balancing is enabled and
+    /// this server is backup.
+    /// @todo Expand these tests once we implement the actual load balancing to
+    /// verify which packets are in scope.
+    void loadBalancingThisBackup();
+
+    /// @brief This test verifies the case when hot-standby is enabled and this
+    /// server is primary.
+    void hotStandbyThisPrimary();
+
+    /// @brief This test verifies the case when hot-standby is enabled and this
+    /// server is standby.
+    void hotStandbyThisSecondary();
+
+    /// @brief This test verifies the case when hot-standby is enabled and this
+    /// server is backup.
+    void hotStandbyThisBackup();
+
+    /// @brief This test verifies the case when load balancing is enabled and
+    /// this server is primary.
+    void loadBalancingThisPrimary6();
+
+    /// @brief This test verifies the case when load balancing is enabled and
+    /// this server is secondary.
+    void loadBalancingThisSecondary6();
+
+    /// @brief This test verifies the case when load balancing is enabled and
+    /// this server is backup.
+    /// @todo Expand these tests once we implement the actual load balancing to
+    /// verify which packets are in scope.
+    void loadBalancingThisBackup6();
+
+    /// @brief This test verifies the case when hot-standby is enabled and this
+    /// server is primary.
+    void hotStandbyThisPrimary6();
+
+    /// @brief This test verifies the case when hot-standby is enabled and this
+    /// server is standby.
+    void hotStandbyThisSecondary6();
+
+    /// @brief This test verifies the case when hot-standby is enabled and this
+    /// server is backup.
+    void hotStandbyThisBackup6();
+
+    /// @brief This test verifies that it is possible to explicitly enable and
+    /// disable certain scopes.
+    void explicitlyServeScopes();
+
+    /// @brief This test verifies that load balancing only affects the scope of
+    /// DHCPv4 message types that HA cares about.
+    void loadBalancingHaTypes4();
+
+    /// @brief This test verifies that load balancing only affects the scope of
+    /// DHCPv6 message types that HA cares about.
+    void loadBalancingHaTypes6();
+};
+
+void
+QueryFilterTest::loadBalancingThisPrimary() {
+    HAConfigPtr config = createValidConfiguration();
+
+    QueryFilter filter(config);
+
+    // By default the server1 should serve its own scope only. The
+    // server2 should serve its scope.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Count number of in scope packets.
+    unsigned in_scope = 0;
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // If the query is in scope, increase the counter of packets in scope.
+        if (filter.inScope(query4, scope_class)) {
+            ASSERT_EQ("HA_server1", scope_class);
+            ASSERT_NE(scope_class, "HA_server2");
+            ++in_scope;
+        }
+    }
+
+    // We should have roughly 50/50 split of in scope and out of scope queries.
+    // However, we don't know exactly how many. To be safe we simply assume that
+    // we got more than 25% of in scope and more than 25% out of scope queries.
+    EXPECT_GT(in_scope, static_cast<unsigned>(queries_num / 4));
+    EXPECT_GT(queries_num - in_scope, static_cast<unsigned>(queries_num / 4));
+
+    // Simulate failover scenario.
+    filter.serveFailoverScopes();
+
+    // In the failover case, the server1 should also take responsibility for
+    // the server2's queries.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // Every single query mist be in scope.
+        ASSERT_TRUE(filter.inScope(query4, scope_class));
+    }
+
+    // However, the one that lacks HW address and client id should be out of
+    // scope.
+    Pkt4Ptr query4(new Pkt4(DHCPDISCOVER, 1234));
+    EXPECT_FALSE(filter.inScope(query4, scope_class));
+}
+
+void
+QueryFilterTest::loadBalancingClientIdThisPrimary() {
+    HAConfigPtr config = createValidConfiguration();
+
+    QueryFilter filter(config);
+
+    // By default the server1 should serve its own scope only. The
+    // server2 should serve its scope.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Fixed HW address used in tests.
+    std::vector<uint8_t> hw_address(HWAddr::ETHERNET_HWADDR_LEN);
+
+    // Count number of in scope packets.
+    unsigned in_scope = 0;
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random client identifier.
+        Pkt4Ptr query4 = createQuery4(hw_address, randomKey(8));
+        // If the query is in scope, increase the counter of packets in scope.
+        if (filter.inScope(query4, scope_class)) {
+            ASSERT_EQ("HA_server1", scope_class);
+            ASSERT_NE(scope_class, "HA_server2");
+            ++in_scope;
+        }
+    }
+
+    // We should have roughly 50/50 split of in scope and out of scope queries.
+    // However, we don't know exactly how many. To be safe we simply assume that
+    // we got more than 25% of in scope and more than 25% out of scope queries.
+    EXPECT_GT(in_scope, static_cast<unsigned>(queries_num / 4));
+    EXPECT_GT(queries_num - in_scope, static_cast<unsigned>(queries_num / 4));
+
+    // Simulate failover scenario.
+    filter.serveFailoverScopes();
+
+    // In the failover case, the server1 should also take responsibility for
+    // the server2's queries.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random client identifier.
+        Pkt4Ptr query4 = createQuery4(hw_address, randomKey(8));
+        // Every single query mist be in scope.
+        ASSERT_TRUE(filter.inScope(query4, scope_class));
+    }
+}
+
+void
+QueryFilterTest::loadBalancingThisSecondary() {
+    HAConfigPtr config = createValidConfiguration();
+
+    // We're now a secondary server.
+    config->setThisServerName("server2");
+
+    QueryFilter filter(config);
+
+    // By default the server2 should serve its own scope only. The
+    // server1 should serve its scope.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Count number of in scope packets.
+    unsigned in_scope = 0;
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // If the query is in scope, increase the counter of packets in scope.
+        if (filter.inScope(query4, scope_class)) {
+            ASSERT_EQ("HA_server2", scope_class);
+            ASSERT_NE(scope_class, "HA_server1");
+            ++in_scope;
+        }
+    }
+
+    // We should have roughly 50/50 split of in scope and out of scope queries.
+    // However, we don't know exactly how many. To be safe we simply assume that
+    // we got more than 25% of in scope and more than 25% out of scope queries.
+    EXPECT_GT(in_scope, static_cast<unsigned>(queries_num / 4));
+    EXPECT_GT(queries_num - in_scope, static_cast<unsigned>(queries_num / 4));
+
+    // Simulate failover scenario.
+    filter.serveFailoverScopes();
+
+    // In this scenario, the server1 died, so the server2 should now serve
+    // both scopes.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // Every single query must be in scope.
+        ASSERT_TRUE(filter.inScope(query4, scope_class));
+    }
+}
+
+void
+QueryFilterTest::loadBalancingThisBackup() {
+    HAConfigPtr config = createValidConfiguration();
+
+    config->setThisServerName("server3");
+
+    QueryFilter filter(config);
+
+    // The backup server doesn't handle any DHCP traffic by default.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // None of the packets should be handlded by the backup server.
+        ASSERT_FALSE(filter.inScope(query4, scope_class));
+    }
+
+    // Simulate failover. Although, backup server never starts handling
+    // other server's traffic automatically, it can be manually instructed
+    // to do so. This simulates such scenario.
+    filter.serveFailoverScopes();
+
+    // The backup server now handles traffic of server 1 and server 2.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // Every single query must be in scope.
+        ASSERT_TRUE(filter.inScope(query4, scope_class));
+    }
+}
+
+void
+QueryFilterTest::hotStandbyThisPrimary() {
+    HAConfigPtr config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config->getPeerConfig("server2")->setRole("standby");
+
+    QueryFilter filter(config);
+
+    Pkt4Ptr query4 = createQuery4("11:22:33:44:55:66");
+
+    // By default, only the primary server is active.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    std::string scope_class;
+
+    // It should process its queries.
+    EXPECT_TRUE(filter.inScope(query4, scope_class));
+
+    // Simulate failover scenario, in which the active server detects a
+    // failure of the standby server. This doesn't change anything in how
+    // the traffic is distributed.
+    filter.serveFailoverScopes();
+
+    // The server1 continues to process its own traffic.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    EXPECT_TRUE(filter.inScope(query4, scope_class));
+    EXPECT_EQ("HA_server1", scope_class);
+    EXPECT_NE(scope_class, "HA_server2");
+}
+
+void
+QueryFilterTest::hotStandbyThisSecondary() {
+    HAConfigPtr config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config->getPeerConfig("server2")->setRole("standby");
+    config->setThisServerName("server2");
+
+    QueryFilter filter(config);
+
+    Pkt4Ptr query4 = createQuery4("11:22:33:44:55:66");
+
+    // The server2 doesn't process any queries by default. The whole
+    // traffic is processed by the server1.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    std::string scope_class;
+
+    EXPECT_FALSE(filter.inScope(query4, scope_class));
+    EXPECT_EQ("HA_server1", scope_class);
+    EXPECT_NE(scope_class, "HA_server2");
+
+    // Simulate failover case whereby the standby server detects a
+    // failure of the active server.
+    filter.serveFailoverScopes();
+
+    // The server2 now handles the traffic normally handled by the
+    // server1.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    EXPECT_TRUE(filter.inScope(query4, scope_class));
+    EXPECT_EQ("HA_server1", scope_class);
+    EXPECT_NE(scope_class, "HA_server2");
+}
+
+void
+QueryFilterTest::hotStandbyThisBackup() {
+    HAConfigPtr config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config->getPeerConfig("server2")->setRole("standby");
+    config->setThisServerName("server3");
+
+    QueryFilter filter(config);
+
+    Pkt4Ptr query4 = createQuery4("11:22:33:44:55:66");
+
+    // By default the backup server doesn't process any traffic.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    std::string scope_class;
+
+    EXPECT_FALSE(filter.inScope(query4, scope_class));
+
+    // Simulate failover. Although, backup server never starts handling
+    // other server's traffic automatically, it can be manually instructed
+    // to do so. This simulates such scenario.
+    filter.serveFailoverScopes();
+
+    // The backup server now handles the entire traffic, i.e. the traffic
+    // that the primary server handles.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    EXPECT_TRUE(filter.inScope(query4, scope_class));
+}
+
+void
+QueryFilterTest::loadBalancingThisPrimary6() {
+    HAConfigPtr config = createValidConfiguration();
+
+    QueryFilter filter(config);
+
+    // By default the server1 should serve its own scope only. The
+    // server2 should serve its scope.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Count number of in scope packets.
+    unsigned in_scope = 0;
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random DUID.
+        Pkt6Ptr query6 = createQuery6(randomKey(10));
+        // If the query is in scope, increase the counter of packets in scope.
+        if (filter.inScope(query6, scope_class)) {
+            ASSERT_EQ("HA_server1", scope_class);
+            ASSERT_NE(scope_class, "HA_server2");
+            ++in_scope;
+        }
+    }
+
+    // We should have roughly 50/50 split of in scope and out of scope queries.
+    // However, we don't know exactly how many. To be safe we simply assume that
+    // we got more than 25% of in scope and more than 25% out of scope queries.
+    EXPECT_GT(in_scope, static_cast<unsigned>(queries_num / 4));
+    EXPECT_GT(queries_num - in_scope, static_cast<unsigned>(queries_num / 4));
+
+    // Simulate failover scenario.
+    filter.serveFailoverScopes();
+
+    // In the failover case, the server1 should also take responsibility for
+    // the server2's queries.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt6Ptr query6 = createQuery6(randomKey(10));
+        // Every single query mist be in scope.
+        ASSERT_TRUE(filter.inScope(query6, scope_class));
+    }
+
+    // However, the one that lacks DUID should be out of scope.
+    Pkt6Ptr query6(new Pkt6(DHCPV6_SOLICIT, 1234));
+    EXPECT_FALSE(filter.inScope(query6, scope_class));
+}
+
+void
+QueryFilterTest::loadBalancingThisSecondary6() {
+    HAConfigPtr config = createValidConfiguration();
+
+    // We're now a secondary server.
+    config->setThisServerName("server2");
+
+    QueryFilter filter(config);
+
+    // By default the server2 should serve its own scope only. The
+    // server1 should serve its scope.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Count number of in scope packets.
+    unsigned in_scope = 0;
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt6Ptr query6 = createQuery6(randomKey(10));
+        // If the query is in scope, increase the counter of packets in scope.
+        if (filter.inScope(query6, scope_class)) {
+            ASSERT_EQ("HA_server2", scope_class);
+            ASSERT_NE(scope_class, "HA_server1");
+            ++in_scope;
+        }
+    }
+
+    // We should have roughly 50/50 split of in scope and out of scope queries.
+    // However, we don't know exactly how many. To be safe we simply assume that
+    // we got more than 25% of in scope and more than 25% out of scope queries.
+    EXPECT_GT(in_scope, static_cast<unsigned>(queries_num / 4));
+    EXPECT_GT(queries_num - in_scope, static_cast<unsigned>(queries_num / 4));
+
+    // Simulate failover scenario.
+    filter.serveFailoverScopes();
+
+    // In this scenario, the server1 died, so the server2 should now serve
+    // both scopes.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt6Ptr query6 = createQuery6(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // Every single query must be in scope.
+        ASSERT_TRUE(filter.inScope(query6, scope_class));
+    }
+}
+
+void
+QueryFilterTest::loadBalancingThisBackup6() {
+    HAConfigPtr config = createValidConfiguration();
+
+    config->setThisServerName("server3");
+
+    QueryFilter filter(config);
+
+    // The backup server doesn't handle any DHCP traffic by default.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Set the test size - 65535 queries.
+    const unsigned queries_num = 65535;
+    std::string scope_class;
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt6Ptr query6 = createQuery6(randomKey(10));
+        // None of the packets should be handlded by the backup server.
+        ASSERT_FALSE(filter.inScope(query6, scope_class));
+    }
+
+    // Simulate failover. Although, backup server never starts handling
+    // other server's traffic automatically, it can be manually instructed
+    // to do so. This simulates such scenario.
+    filter.serveFailoverScopes();
+
+    // The backup server now handles traffic of server 1 and server 2.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Repeat the test, but this time all should be in scope.
+    for (unsigned i = 0; i < queries_num; ++i) {
+        // Create query with random HW address.
+        Pkt6Ptr query6 = createQuery6(randomKey(10));
+        // Every single query must be in scope.
+        ASSERT_TRUE(filter.inScope(query6, scope_class));
+    }
+}
+
+void
+QueryFilterTest::hotStandbyThisPrimary6() {
+    HAConfigPtr config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config->getPeerConfig("server2")->setRole("standby");
+
+    QueryFilter filter(config);
+
+    Pkt6Ptr query6 = createQuery6("01:02:11:22:33:44:55:66");
+
+    // By default, only the primary server is active.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    std::string scope_class;
+
+    // It should process its queries.
+    EXPECT_TRUE(filter.inScope(query6, scope_class));
+
+    // Simulate failover scenario, in which the active server detects a
+    // failure of the standby server. This doesn't change anything in how
+    // the traffic is distributed.
+    filter.serveFailoverScopes();
+
+    // The server1 continues to process its own traffic.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    EXPECT_TRUE(filter.inScope(query6, scope_class));
+    EXPECT_EQ("HA_server1", scope_class);
+    EXPECT_NE(scope_class, "HA_server2");
+}
+
+void
+QueryFilterTest::hotStandbyThisSecondary6() {
+    HAConfigPtr config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config->getPeerConfig("server2")->setRole("standby");
+    config->setThisServerName("server2");
+
+    QueryFilter filter(config);
+
+    Pkt6Ptr query6 = createQuery6("01:02:11:22:33:44:55:66");
+
+    // The server2 doesn't process any queries by default. The whole
+    // traffic is processed by the server1.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    std::string scope_class;
+
+    EXPECT_FALSE(filter.inScope(query6, scope_class));
+    EXPECT_EQ("HA_server1", scope_class);
+    EXPECT_NE(scope_class, "HA_server2");
+
+    // Simulate failover case whereby the standby server detects a
+    // failure of the active server.
+    filter.serveFailoverScopes();
+
+    // The server2 now handles the traffic normally handled by the
+    // server1.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    EXPECT_TRUE(filter.inScope(query6, scope_class));
+    EXPECT_EQ("HA_server1", scope_class);
+    EXPECT_NE(scope_class, "HA_server2");
+}
+
+void
+QueryFilterTest::hotStandbyThisBackup6() {
+    HAConfigPtr config = createValidConfiguration(HAConfig::HOT_STANDBY);
+    config->getPeerConfig("server2")->setRole("standby");
+    config->setThisServerName("server3");
+
+    QueryFilter filter(config);
+
+    Pkt6Ptr query6 = createQuery6(randomKey(10));
+
+    // By default the backup server doesn't process any traffic.
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    std::string scope_class;
+
+    EXPECT_FALSE(filter.inScope(query6, scope_class));
+
+    // Simulate failover. Although, backup server never starts handling
+    // other server's traffic automatically, it can be manually instructed
+    // to do so. This simulates such scenario.
+    filter.serveFailoverScopes();
+
+    // The backup server now handles the entire traffic, i.e. the traffic
+    // that the primary server handles.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    EXPECT_TRUE(filter.inScope(query6, scope_class));
+}
+
+void
+QueryFilterTest::explicitlyServeScopes() {
+    HAConfigPtr config = createValidConfiguration();
+
+    QueryFilter filter(config);
+
+    // Initially, the scopes should be set according to the load
+    // balancing configuration.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Enable "server2" scope.
+    ASSERT_NO_THROW(filter.serveScope("server2"));
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Enable only "server2" scope.
+    ASSERT_NO_THROW(filter.serveScopeOnly("server2"));
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_TRUE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Explicitly enable selected scopes.
+    ASSERT_NO_THROW(filter.serveScopes({ "server1", "server3" }));
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_TRUE(filter.amServingScope("server3"));
+
+    // Revert to defaults.
+    ASSERT_NO_THROW(filter.serveDefaultScopes());
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Disable all scopes.
+    ASSERT_NO_THROW(filter.serveNoScopes());
+    EXPECT_FALSE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Test negative cases.
+    EXPECT_THROW(filter.serveScope("unsupported"), BadValue);
+    EXPECT_THROW(filter.serveScopeOnly("unsupported"), BadValue);
+    EXPECT_THROW(filter.serveScopes({ "server1", "unsupported" }), BadValue);
+}
+
+void
+QueryFilterTest::loadBalancingHaTypes4() {
+    HAConfigPtr config = createValidConfiguration();
+
+    QueryFilter filter(config);
+
+    // By default the server1 should serve its own scope only. The
+    // server2 should serve its scope.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Use DHCPDISCOVER to find MAC addresses in scope for server1 and server2.
+    Pkt4Ptr server1_pkt;
+    Pkt4Ptr server2_pkt;
+    const unsigned max_scope_tries = 100;
+    for (unsigned i = 0; i < max_scope_tries; ++i) {
+        // Create query with random HW address.
+        std::string scope_class;
+        Pkt4Ptr query4 = createQuery4(randomKey(HWAddr::ETHERNET_HWADDR_LEN));
+        // If the query is in scope then we're done.
+        if (filter.inScope(query4, scope_class)) {
+            ASSERT_EQ("HA_server1", scope_class);
+            server1_pkt = query4;
+            if (server2_pkt) {
+                break;
+            }
+        } else {
+            ASSERT_EQ("HA_server2", scope_class);
+            server2_pkt = query4;
+            if (server1_pkt) {
+                break;
+            }
+        }
+    }
+
+    ASSERT_TRUE(server1_pkt && server2_pkt) << "do not have both scopes in "
+                << max_scope_tries << ", load balance broken?";
+
+    // Iterate over message types.  While setting message type to zero is
+    // semantically invalid, it is useful for testing here. Similarly we exceed
+    // DHCP_TYPES_EOF just to be sure.
+    for (uint8_t msg_type = 0; msg_type < DHCP_TYPES_EOF + 2; ++msg_type) {
+        // All message types should be in scope for server1.
+        server1_pkt->setType(msg_type);
+
+        std::string scope_class;
+        bool is_in_scope = filter.inScope(server1_pkt, scope_class);
+        ASSERT_EQ("HA_server1", scope_class);
+        EXPECT_TRUE(is_in_scope);
+
+        server2_pkt->setType(msg_type);
+        scope_class.clear();
+        is_in_scope = filter.inScope(server2_pkt, scope_class);
+        switch (msg_type) {
+            case DHCPDISCOVER:
+            case DHCPREQUEST:
+            case DHCPDECLINE:
+            case DHCPRELEASE:
+            case DHCPINFORM:
+                // HA message types should be in scope for server2.
+                ASSERT_EQ("HA_server2", scope_class);
+                EXPECT_FALSE(is_in_scope);
+                break;
+            default:
+                // Non HA message types should be in scope for server1.
+                ASSERT_EQ("HA_server1", scope_class);
+                EXPECT_TRUE(is_in_scope);
+                break;
+        }
+    }
+}
+
+void
+QueryFilterTest::loadBalancingHaTypes6() {
+    HAConfigPtr config = createValidConfiguration();
+
+    QueryFilter filter(config);
+
+    // By default the server1 should serve its own scope only. The
+    // server2 should serve its scope.
+    EXPECT_TRUE(filter.amServingScope("server1"));
+    EXPECT_FALSE(filter.amServingScope("server2"));
+    EXPECT_FALSE(filter.amServingScope("server3"));
+
+    // Use DHCPV6_SOLICIT to find MAC addresses in scope for server1 and server2.
+    Pkt6Ptr server1_pkt;
+    Pkt6Ptr server2_pkt;
+    const unsigned max_scope_tries = 100;
+    for (unsigned i = 0; i < max_scope_tries; ++i) {
+        // Create query with random HW address.
+        std::string scope_class;
+
+        // Create query with random DUID.
+        Pkt6Ptr query6 = createQuery6(randomKey(10));
+        if (filter.inScope(query6, scope_class)) {
+            ASSERT_EQ("HA_server1", scope_class);
+            // In scope for server1, save it.
+            server1_pkt = query6;
+            if (server2_pkt) {
+                // Have both, we're done.
+                break;
+            }
+        } else {
+            ASSERT_EQ("HA_server2", scope_class);
+            // In scope for server2, save it.
+            server2_pkt = query6;
+            if (server1_pkt) {
+                // Have both, we're done.
+                break;
+            }
+        }
+    }
+
+    ASSERT_TRUE(server1_pkt && server2_pkt) << "do not have both scopes in "
+                << max_scope_tries << ", load balance broken?";
+
+    // Iterate over message types.  While setting message type to zero is
+    // semantically invalid, it is useful for testing here. Similarly we exceed
+    // DHCPV6_TYPES_EOF just to be sure.
+    for (uint8_t msg_type = 0; msg_type < DHCPV6_TYPES_EOF + 2; ++msg_type) {
+        // All message types should be in scope for server1.
+        server1_pkt->setType(msg_type);
+
+        std::string scope_class;
+        bool is_in_scope = filter.inScope(server1_pkt, scope_class);
+        ASSERT_EQ("HA_server1", scope_class);
+        EXPECT_TRUE(is_in_scope);
+
+        server2_pkt->setType(msg_type);
+        scope_class.clear();
+        is_in_scope = filter.inScope(server2_pkt, scope_class);
+        switch (msg_type) {
+            case DHCPV6_SOLICIT:
+            case DHCPV6_REQUEST:
+            case DHCPV6_CONFIRM:
+            case DHCPV6_RENEW:
+            case DHCPV6_REBIND:
+            case DHCPV6_RELEASE:
+            case DHCPV6_DECLINE:
+                // HA message types should be in scope for server2.
+                ASSERT_EQ("HA_server2", scope_class);
+                EXPECT_FALSE(is_in_scope);
+                break;
+            default:
+                // Non HA message types should be in scope for server1.
+                ASSERT_EQ("HA_server1", scope_class);
+                EXPECT_TRUE(is_in_scope);
+                break;
+        }
+    }
+}
+
+TEST_F(QueryFilterTest, loadBalancingClientIdThisPrimary) {
+    loadBalancingClientIdThisPrimary();
+}
+
+TEST_F(QueryFilterTest, loadBalancingClientIdThisPrimaryMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingClientIdThisPrimary();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisPrimary) {
+    loadBalancingThisPrimary();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisPrimaryMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingThisPrimary();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisSecondary) {
+    loadBalancingThisSecondary();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisSecondaryMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingThisSecondary();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisBackup) {
+    loadBalancingThisBackup();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisBackupMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingThisBackup();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisPrimary) {
+    hotStandbyThisPrimary();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisPrimaryMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hotStandbyThisPrimary();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisSecondary) {
+    hotStandbyThisSecondary();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisSecondaryMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hotStandbyThisSecondary();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisBackup) {
+    hotStandbyThisBackup();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisBackupMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hotStandbyThisBackup();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisPrimary6) {
+    loadBalancingThisPrimary6();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisPrimary6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingThisPrimary6();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisSecondary6) {
+    loadBalancingThisSecondary6();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisSecondary6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingThisSecondary6();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisBackup6) {
+    loadBalancingThisBackup6();
+}
+
+TEST_F(QueryFilterTest, loadBalancingThisBackup6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingThisBackup6();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisPrimary6) {
+    hotStandbyThisPrimary6();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisPrimary6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hotStandbyThisPrimary6();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisSecondary6) {
+    hotStandbyThisSecondary6();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisSecondary6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hotStandbyThisSecondary6();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisBackup6) {
+    hotStandbyThisBackup6();
+}
+
+TEST_F(QueryFilterTest, hotStandbyThisBackup6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    hotStandbyThisBackup6();
+}
+
+TEST_F(QueryFilterTest, explicitlyServeScopes) {
+    explicitlyServeScopes();
+}
+
+TEST_F(QueryFilterTest, explicitlyServeScopesMultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    explicitlyServeScopes();
+}
+
+TEST_F(QueryFilterTest, loadBalancingHaTypes4) {
+    loadBalancingHaTypes4();
+}
+
+TEST_F(QueryFilterTest, loadBalancingHaTypes4MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingHaTypes4();
+}
+
+TEST_F(QueryFilterTest, loadBalancingHaTypes6) {
+    loadBalancingHaTypes6();
+}
+
+TEST_F(QueryFilterTest, loadBalancingHaTypes6MultiThreading) {
+    MultiThreadingMgr::instance().setMode(true);
+    loadBalancingHaTypes6();
+}
+
+}
diff --git a/src/hooks/dhcp/high_availability/tests/run_unittests.cc b/src/hooks/dhcp/high_availability/tests/run_unittests.cc
new file mode 100644
index 0000000..3b1baf9
--- /dev/null
+++ b/src/hooks/dhcp/high_availability/tests/run_unittests.cc
@@ -0,0 +1,19 @@
+// Copyright (C) 2017-2018 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include <config.h>
+
+#include <log/logger_support.h>
+#include <gtest/gtest.h>
+
+int
+main(int argc, char* argv[]) {
+    ::testing::InitGoogleTest(&argc, argv);
+    isc::log::initLogger();
+    int result = RUN_ALL_TESTS();
+
+    return (result);
+}