Adding upstream version 18.2.2.upstream/18.2.2

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

1 files changed, 900 insertions, 0 deletions

diff --git a/src/mon/PaxosService.h b/src/mon/PaxosService.h
new file mode 100644
index 000000000..de3145b9b
--- /dev/null
+++ b/src/mon/PaxosService.h
@@ -0,0 +1,900 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2004-2006 Sage Weil <sage@newdream.net>
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software 
+ * Foundation.  See file COPYING.
+ * 
+ */
+
+#ifndef CEPH_PAXOSSERVICE_H
+#define CEPH_PAXOSSERVICE_H
+
+#include "include/Context.h"
+#include "Paxos.h"
+#include "Monitor.h"
+#include "MonitorDBStore.h"
+
+/**
+ * A Paxos Service is an abstraction that easily allows one to obtain an
+ * association between a Monitor and a Paxos class, in order to implement any
+ * service.
+ */
+class PaxosService {
+  /**
+   * @defgroup PaxosService_h_class Paxos Service
+   * @{
+   */
+ public:
+  /**
+   * The Monitor to which this class is associated with
+   */
+  Monitor &mon;
+  /**
+   * The Paxos instance to which this class is associated with
+   */
+  Paxos &paxos;
+  /**
+   * Our name. This will be associated with the class implementing us, and will
+   * be used mainly for store-related operations.
+   */
+  std::string service_name;
+  /**
+   * If we are or have queued anything for proposal, this variable will be true
+   * until our proposal has been finished.
+   */
+  bool proposing;
+
+  bool need_immediate_propose = false;
+
+protected:
+  /**
+   * Services implementing us used to depend on the Paxos version, back when
+   * each service would have a Paxos instance for itself. However, now we only
+   * have a single Paxos instance, shared by all the services. Each service now
+   * must keep its own version, if so they wish. This variable should be used
+   * for that purpose.
+   */
+  version_t service_version;
+
+ private:
+  /**
+   * Event callback responsible for proposing our pending value once a timer 
+   * runs out and fires.
+   */
+  Context *proposal_timer;
+  /**
+   * If the implementation class has anything pending to be proposed to Paxos,
+   * then have_pending should be true; otherwise, false.
+   */
+  bool have_pending; 
+
+  /**
+   * health checks for this service
+   *
+   * Child must populate this during encode_pending() by calling encode_health().
+   */
+  health_check_map_t health_checks;
+protected:
+  /**
+   * format of our state in leveldb, 0 for default
+   */
+  version_t format_version;
+
+public:
+  const health_check_map_t& get_health_checks() const {
+    return health_checks;
+  }
+
+  /**
+   * @defgroup PaxosService_h_callbacks Callback classes
+   * @{
+   */
+  /**
+   * Retry dispatching a given service message
+   *
+   * This callback class is used when we had to wait for some condition to
+   * become true while we were dispatching it.
+   *
+   * For instance, if the message's version isn't readable, according to Paxos,
+   * then we must wait for it to become readable. So, we just queue an
+   * instance of this class onto the Paxos::wait_for_readable function, and
+   * we will retry the whole dispatch again once the callback is fired.
+   */
+  class C_RetryMessage : public C_MonOp {
+    PaxosService *svc;
+  public:
+    C_RetryMessage(PaxosService *s, MonOpRequestRef op_) :
+      C_MonOp(op_), svc(s) { }
+    void _finish(int r) override {
+      if (r == -EAGAIN || r >= 0)
+	svc->dispatch(op);
+      else if (r == -ECANCELED)
+        return;
+      else
+	ceph_abort_msg("bad C_RetryMessage return value");
+    }
+  };
+
+  class C_ReplyOp : public C_MonOp {
+    Monitor &mon;
+    MonOpRequestRef op;
+    MessageRef reply;
+  public:
+    C_ReplyOp(PaxosService *s, MonOpRequestRef o, MessageRef r) :
+      C_MonOp(o), mon(s->mon), op(o), reply(r) { }
+    void _finish(int r) override {
+      if (r >= 0) {
+	mon.send_reply(op, reply.detach());
+      }
+    }
+  };
+
+  /**
+   * @}
+   */
+
+  /**
+   * @param mn A Monitor instance
+   * @param p A Paxos instance
+   * @param name Our service's name.
+   */
+  PaxosService(Monitor &mn, Paxos &p, std::string name) 
+    : mon(mn), paxos(p), service_name(name),
+      proposing(false),
+      service_version(0), proposal_timer(0), have_pending(false),
+      format_version(0),
+      last_committed_name("last_committed"),
+      first_committed_name("first_committed"),
+      full_prefix_name("full"), full_latest_name("latest"),
+      cached_first_committed(0), cached_last_committed(0)
+  {
+  }
+
+  virtual ~PaxosService() {}
+
+  /**
+   * Get the service's name.
+   *
+   * @returns The service's name.
+   */
+  const std::string& get_service_name() const { return service_name; }
+
+  /**
+   * Get the store prefixes we utilize
+   */
+  virtual void get_store_prefixes(std::set<std::string>& s) const {
+    s.insert(service_name);
+  }
+  
+  // i implement and you ignore
+  /**
+   * Informs this instance that it should consider itself restarted.
+   *
+   * This means that we will cancel our proposal_timer event, if any exists.
+   */
+  void restart();
+  /**
+   * Informs this instance that an election has finished.
+   *
+   * This means that we will invoke a PaxosService::discard_pending while
+   * setting have_pending to false (basically, ignore our pending state) and
+   * we will then make sure we obtain a new state.
+   *
+   * Our state shall be updated by PaxosService::_active if the Paxos is
+   * active; otherwise, we will wait for it to become active by adding a 
+   * PaxosService::C_Active callback to it.
+   */
+  void election_finished();
+  /**
+   * Informs this instance that it is supposed to shutdown.
+   *
+   * Basically, it will instruct Paxos to cancel all events/callbacks and then
+   * will cancel the proposal_timer event if any exists.
+   */
+  void shutdown();
+
+private:
+  /**
+   * Update our state by updating it from Paxos, and then creating a new
+   * pending state if need be.
+   *
+   * @remarks We only create a pending state we our Monitor is the Leader.
+   *
+   * @pre Paxos is active
+   * @post have_pending is true if our Monitor is the Leader and Paxos is
+   *	   active
+   */
+  void _active();
+
+public:
+  /**
+   * Propose a new value through Paxos.
+   *
+   * This function should be called by the classes implementing 
+   * PaxosService, in order to propose a new value through Paxos.
+   *
+   * @pre The implementation class implements the encode_pending function.
+   * @pre have_pending is true
+   * @pre Our monitor is the Leader
+   * @pre Paxos is active
+   * @post Cancel the proposal timer, if any
+   * @post have_pending is false
+   * @post propose pending value through Paxos
+   *
+   * @note This function depends on the implementation of encode_pending on
+   *	   the class that is implementing PaxosService
+   */
+  void propose_pending();
+
+  /**
+   * Let others request us to propose.
+   *
+   * At the moment, this is just a wrapper to propose_pending() with an
+   * extra check for is_writeable(), but it's a good practice to dissociate
+   * requests for proposals from direct usage of propose_pending() for
+   * future use -- we might want to perform additional checks or put a
+   * request on hold, for instance.
+   */
+  void request_proposal() {
+    ceph_assert(is_writeable());
+
+    propose_pending();
+  }
+  /**
+   * Request service @p other to perform a proposal.
+   *
+   * We could simply use the function above, requesting @p other directly,
+   * but we might eventually want to do something to the request -- say,
+   * set a flag stating we're waiting on a cross-proposal to be finished.
+   */
+  void request_proposal(PaxosService *other) {
+    ceph_assert(other != NULL);
+    ceph_assert(other->is_writeable());
+
+    other->request_proposal();
+  }
+
+  /**
+   * Dispatch a message by passing it to several different functions that are
+   * either implemented directly by this service, or that should be implemented
+   * by the class implementing this service.
+   *
+   * @param m A message
+   * @returns 'true' on successful dispatch; 'false' otherwise.
+   */
+  bool dispatch(MonOpRequestRef op);
+
+  void refresh(bool *need_bootstrap);
+  void post_refresh();
+
+  /**
+   * @defgroup PaxosService_h_override_funcs Functions that should be
+   *					     overridden.
+   *
+   * These functions should be overridden at will by the class implementing
+   * this service.
+   * @{
+   */
+  /**
+   * Create the initial state for your system.
+   *
+   * In some of ours the state is actually set up elsewhere so this does
+   * nothing.
+   */
+  virtual void create_initial() = 0;
+
+  /**
+   * Query the Paxos system for the latest state and apply it if it's newer
+   * than the current Monitor state.
+   */
+  virtual void update_from_paxos(bool *need_bootstrap) = 0;
+
+  /**
+   * Hook called after all services have refreshed their state from paxos
+   *
+   * This is useful for doing any update work that depends on other
+   * service's having up-to-date state.
+   */
+  virtual void post_paxos_update() {}
+
+  /**
+   * Init on startup
+   *
+   * This is called on mon startup, after all of the PaxosService instances'
+   * update_from_paxos() methods have been called
+   */
+  virtual void init() {}
+
+  /**
+   * Create the pending state.
+   *
+   * @invariant This function is only called on a Leader.
+   * @remarks This created state is then modified by incoming messages.
+   * @remarks Called at startup and after every Paxos ratification round.
+   */
+  virtual void create_pending() = 0;
+
+  /**
+   * Encode the pending state into a ceph::buffer::list for ratification and
+   * transmission as the next state.
+   *
+   * @invariant This function is only called on a Leader.
+   *
+   * @param t The transaction to hold all changes.
+   */
+  virtual void encode_pending(MonitorDBStore::TransactionRef t) = 0;
+
+  /**
+   * Discard the pending state
+   *
+   * @invariant This function is only called on a Leader.
+   *
+   * @remarks This function is NOT overridden in any of our code, but it is
+   *	      called in PaxosService::election_finished if have_pending is
+   *	      true.
+   */
+  virtual void discard_pending() { }
+
+  /**
+   * Look at the query; if the query can be handled without changing state,
+   * do so.
+   *
+   * @param m A query message
+   * @returns 'true' if the query was handled (e.g., was a read that got
+   *	      answered, was a state change that has no effect); 'false' 
+   *	      otherwise.
+   */
+  virtual bool preprocess_query(MonOpRequestRef op) = 0;
+
+  /**
+   * Apply the message to the pending state.
+   *
+   * @invariant This function is only called on a Leader.
+   *
+   * @param m An update message
+   * @returns 'true' if the pending state should be proposed; 'false' otherwise.
+   */
+  virtual bool prepare_update(MonOpRequestRef op) = 0;
+  /**
+   * @}
+   */
+
+  /**
+   * Determine if the Paxos system should vote on pending, and if so how long
+   * it should wait to vote.
+   *
+   * @param[out] delay The wait time, used so we can limit the update traffic
+   *		       spamming.
+   * @returns 'true' if the Paxos system should propose; 'false' otherwise.
+   */
+  virtual bool should_propose(double &delay);
+
+  /**
+   * force an immediate propose.
+   *
+   * This is meant to be called from prepare_update(op).
+   */
+  void force_immediate_propose() {
+    need_immediate_propose = true;
+  }
+
+  /**
+   * @defgroup PaxosService_h_courtesy Courtesy functions
+   *
+   * Courtesy functions, in case the class implementing this service has
+   * anything it wants/needs to do at these times.
+   * @{
+   */
+  /**
+   * This is called when the Paxos state goes to active.
+   *
+   * On the peon, this is after each election.
+   * On the leader, this is after each election, *and* after each completed
+   * proposal.
+   *
+   * @note This function may get called twice in certain recovery cases.
+   */
+  virtual void on_active() { }
+
+  /**
+   * This is called when we are shutting down
+   */
+  virtual void on_shutdown() {}
+
+  /**
+   * this is called when activating on the leader
+   *
+   * it should conditionally upgrade the on-disk format by proposing a transaction
+   */
+  virtual void upgrade_format() { }
+
+  /**
+   * this is called when we detect the store has just upgraded underneath us
+   */
+  virtual void on_upgrade() {}
+
+  /**
+   * Called when the Paxos system enters a Leader election.
+   *
+   * @remarks It's a courtesy method, in case the class implementing this
+   *	      service has anything it wants/needs to do at that time.
+   */
+  virtual void on_restart() { }
+  /**
+   * @}
+   */
+
+  /**
+   * Tick.
+   */
+  virtual void tick() {}
+
+  void encode_health(const health_check_map_t& next,
+		     MonitorDBStore::TransactionRef t) {
+    using ceph::encode;
+    ceph::buffer::list bl;
+    encode(next, bl);
+    t->put("health", service_name, bl);
+    mon.log_health(next, health_checks, t);
+  }
+  void load_health();
+
+  /**
+   * @defgroup PaxosService_h_store_keys Set of keys that are usually used on
+   *					 all the services implementing this
+   *					 class, and, being almost the only keys
+   *					 used, should be standardized to avoid
+   *					 mistakes.
+   * @{
+   */
+  const std::string last_committed_name;
+  const std::string first_committed_name;
+  const std::string full_prefix_name;
+  const std::string full_latest_name;
+  /**
+   * @}
+   */
+
+ private:
+  /**
+   * @defgroup PaxosService_h_version_cache Variables holding cached values
+   *                                        for the most used versions (first
+   *                                        and last committed); we only have
+   *                                        to read them when the store is
+   *                                        updated, so in-between updates we
+   *                                        may very well use cached versions
+   *                                        and avoid the overhead.
+   * @{
+   */
+  version_t cached_first_committed;
+  version_t cached_last_committed;
+  /**
+   * @}
+   */
+
+  /**
+   * Callback list to be used whenever we are running a proposal through
+   * Paxos. These callbacks will be awaken whenever the said proposal
+   * finishes.
+   */
+  std::list<Context*> waiting_for_finished_proposal;
+
+ public:
+
+  /**
+   * Check if we are proposing a value through Paxos
+   *
+   * @returns true if we are proposing; false otherwise.
+   */
+  bool is_proposing() const {
+    return proposing;
+  }
+
+  /**
+   * Check if we are in the Paxos ACTIVE state.
+   *
+   * @note This function is a wrapper for Paxos::is_active
+   *
+   * @returns true if in state ACTIVE; false otherwise.
+   */
+  bool is_active() const {
+    return
+      !is_proposing() &&
+      (paxos.is_active() || paxos.is_updating() || paxos.is_writing());
+  }
+
+  /**
+   * Check if we are readable.
+   *
+   * This mirrors on the paxos check, except that we also verify that
+   *
+   *  - the client hasn't seen the future relative to this PaxosService
+   *  - this service isn't proposing.
+   *  - we have committed our initial state (last_committed > 0)
+   *
+   * @param ver The version we want to check if is readable
+   * @returns true if it is readable; false otherwise
+   */
+  bool is_readable(version_t ver = 0) const {
+    if (ver > get_last_committed() ||
+	!paxos.is_readable(0) ||
+	get_last_committed() == 0)
+      return false;
+    return true;
+  }
+
+  /**
+   * Check if we are writeable.
+   *
+   * We consider to be writeable iff:
+   *
+   *  - we are not proposing a new version;
+   *  - we are ready to be written to -- i.e., we have a pending value.
+   *  - paxos is (active or updating or writing or refresh)
+   *
+   * @returns true if writeable; false otherwise
+   */
+  bool is_writeable() const {
+    return is_active() && have_pending;
+  }
+
+  /**
+   * Wait for a proposal to finish.
+   *
+   * Add a callback to be awaken whenever our current proposal finishes being
+   * proposed through Paxos.
+   *
+   * @param c The callback to be awaken once the proposal is finished.
+   */
+  void wait_for_finished_proposal(MonOpRequestRef op, Context *c) {
+    if (op)
+      op->mark_event(service_name + ":wait_for_finished_proposal");
+    waiting_for_finished_proposal.push_back(c);
+  }
+  void wait_for_finished_proposal_ctx(Context *c) {
+    MonOpRequestRef o;
+    wait_for_finished_proposal(o, c);
+  }
+
+  /**
+   * Wait for us to become active
+   *
+   * @param c The callback to be awaken once we become active.
+   */
+  void wait_for_active(MonOpRequestRef op, Context *c) {
+    if (op)
+      op->mark_event(service_name + ":wait_for_active");
+
+    if (!is_proposing()) {
+      paxos.wait_for_active(op, c);
+      return;
+    }
+    wait_for_finished_proposal(op, c);
+  }
+  void wait_for_active_ctx(Context *c) {
+    MonOpRequestRef o;
+    wait_for_active(o, c);
+  }
+
+  /**
+   * Wait for us to become readable
+   *
+   * @param c The callback to be awaken once we become active.
+   * @param ver The version we want to wait on.
+   */
+  void wait_for_readable(MonOpRequestRef op, Context *c, version_t ver = 0) {
+    /* This is somewhat of a hack. We only do check if a version is readable on
+     * PaxosService::dispatch(), but, nonetheless, we must make sure that if that
+     * is why we are not readable, then we must wait on PaxosService and not on
+     * Paxos; otherwise, we may assert on Paxos::wait_for_readable() if it
+     * happens to be readable at that specific point in time.
+     */
+    if (op)
+      op->mark_event(service_name + ":wait_for_readable");
+
+    if (is_proposing() ||
+	ver > get_last_committed() ||
+	get_last_committed() == 0)
+      wait_for_finished_proposal(op, c);
+    else {
+      if (op)
+        op->mark_event(service_name + ":wait_for_readable/paxos");
+
+      paxos.wait_for_readable(op, c);
+    }
+  }
+
+  void wait_for_readable_ctx(Context *c, version_t ver = 0) {
+    MonOpRequestRef o; // will initialize the shared_ptr to NULL
+    wait_for_readable(o, c, ver);
+  }
+
+  /**
+   * Wait for us to become writeable
+   *
+   * @param c The callback to be awaken once we become writeable.
+   */
+  void wait_for_writeable(MonOpRequestRef op, Context *c) {
+    if (op)
+      op->mark_event(service_name + ":wait_for_writeable");
+
+    if (is_proposing())
+      wait_for_finished_proposal(op, c);
+    else if (!is_writeable())
+      wait_for_active(op, c);
+    else
+      paxos.wait_for_writeable(op, c);
+  }
+  void wait_for_writeable_ctx(Context *c) {
+    MonOpRequestRef o;
+    wait_for_writeable(o, c);
+  }
+
+  
+  /**
+   * @defgroup PaxosService_h_Trim Functions for trimming states
+   * @{
+   */
+  /**
+   * trim service states if appropriate
+   *
+   * Called at same interval as tick()
+   */
+  void maybe_trim();
+
+  /**
+   * Auxiliary function to trim our state from version @p from to version
+   * @p to, not including; i.e., the interval [from, to[
+   *
+   * @param t The transaction to which we will add the trim operations.
+   * @param from the lower limit of the interval to be trimmed
+   * @param to the upper limit of the interval to be trimmed (not including)
+   */
+  void trim(MonitorDBStore::TransactionRef t, version_t from, version_t to);
+
+  /**
+   * encode service-specific extra bits into trim transaction
+   *
+   * @param tx transaction
+   * @param first new first_committed value
+   */
+  virtual void encode_trim_extra(MonitorDBStore::TransactionRef tx,
+				 version_t first) {}
+
+  /**
+   * Get the version we should trim to.
+   *
+   * Should be overloaded by service if it wants to trim states.
+   *
+   * @returns the version we should trim to; if we return zero, it should be
+   *	      assumed that there's no version to trim to.
+   */
+  virtual version_t get_trim_to() const {
+    return 0;
+  }
+
+  /**
+   * @}
+   */
+  /**
+   * @defgroup PaxosService_h_Stash_Full
+   * @{
+   */
+  virtual bool should_stash_full();
+  /**
+   * Encode a full version on @p t
+   *
+   * @note We force every service to implement this function, since we strongly
+   *	   desire the encoding of full versions.
+   * @note Services that do not trim their state, will be bound to only create
+   *	   one full version. Full version stashing is determined/controlled by
+   *	   trimming: we stash a version each time a trim is bound to erase the
+   *	   latest full version.
+   *
+   * @param t Transaction on which the full version shall be encoded.
+   */
+  virtual void encode_full(MonitorDBStore::TransactionRef t) = 0;
+
+  /**
+   * @}
+   */
+
+  /**
+   * Cancel events.
+   *
+   * @note This function is a wrapper for Paxos::cancel_events
+   */
+  void cancel_events() {
+    paxos.cancel_events();
+  }
+
+  /**
+   * @defgroup PaxosService_h_store_funcs Back storage interface functions
+   * @{
+   */
+  /**
+   * @defgroup PaxosService_h_store_modify Wrapper function interface to access
+   *					   the back store for modification
+   *					   purposes
+   * @{
+   */
+  void put_first_committed(MonitorDBStore::TransactionRef t, version_t ver) {
+    t->put(get_service_name(), first_committed_name, ver);
+  }
+  /**
+   * Set the last committed version to @p ver
+   *
+   * @param t A transaction to which we add this put operation
+   * @param ver The last committed version number being put
+   */
+  void put_last_committed(MonitorDBStore::TransactionRef t, version_t ver) {
+    t->put(get_service_name(), last_committed_name, ver);
+
+    /* We only need to do this once, and that is when we are about to make our
+     * first proposal. There are some services that rely on first_committed
+     * being set -- and it should! -- so we need to guarantee that it is,
+     * specially because the services itself do not do it themselves. They do
+     * rely on it, but they expect us to deal with it, and so we shall.
+     */
+    if (!get_first_committed())
+      put_first_committed(t, ver);
+  }
+  /**
+   * Put the contents of @p bl into version @p ver
+   *
+   * @param t A transaction to which we will add this put operation
+   * @param ver The version to which we will add the value
+   * @param bl A ceph::buffer::list containing the version's value
+   */
+  void put_version(MonitorDBStore::TransactionRef t, version_t ver,
+		   ceph::buffer::list& bl) {
+    t->put(get_service_name(), ver, bl);
+  }
+  /**
+   * Put the contents of @p bl into a full version key for this service, that
+   * will be created with @p ver in mind.
+   *
+   * @param t The transaction to which we will add this put operation
+   * @param ver A version number
+   * @param bl A ceph::buffer::list containing the version's value
+   */
+  void put_version_full(MonitorDBStore::TransactionRef t,
+			version_t ver, ceph::buffer::list& bl) {
+    std::string key = mon.store->combine_strings(full_prefix_name, ver);
+    t->put(get_service_name(), key, bl);
+  }
+  /**
+   * Put the version number in @p ver into the key pointing to the latest full
+   * version of this service.
+   *
+   * @param t The transaction to which we will add this put operation
+   * @param ver A version number
+   */
+  void put_version_latest_full(MonitorDBStore::TransactionRef t, version_t ver) {
+    std::string key = mon.store->combine_strings(full_prefix_name, full_latest_name);
+    t->put(get_service_name(), key, ver);
+  }
+  /**
+   * Put the contents of @p bl into the key @p key.
+   *
+   * @param t A transaction to which we will add this put operation
+   * @param key The key to which we will add the value
+   * @param bl A ceph::buffer::list containing the value
+   */
+  void put_value(MonitorDBStore::TransactionRef t,
+		 const std::string& key, ceph::buffer::list& bl) {
+    t->put(get_service_name(), key, bl);
+  }
+
+  /**
+   * Put integer value @v into the key @p key.
+   *
+   * @param t A transaction to which we will add this put operation
+   * @param key The key to which we will add the value
+   * @param v An integer
+   */
+  void put_value(MonitorDBStore::TransactionRef t,
+		 const std::string& key, version_t v) {
+    t->put(get_service_name(), key, v);
+  }
+
+  /**
+   * @}
+   */
+
+  /**
+   * @defgroup PaxosService_h_store_get Wrapper function interface to access
+   *					the back store for reading purposes
+   * @{
+   */
+
+  /**
+   * @defgroup PaxosService_h_version_cache Obtain cached versions for this
+   *                                        service.
+   * @{
+   */
+  /**
+   * Get the first committed version
+   *
+   * @returns Our first committed version (that is available)
+   */
+  version_t get_first_committed() const{
+    return cached_first_committed;
+  }
+  /**
+   * Get the last committed version
+   *
+   * @returns Our last committed version
+   */
+  version_t get_last_committed() const{
+    return cached_last_committed;
+  }
+
+  /**
+   * @}
+   */
+
+  /**
+   * Get the contents of a given version @p ver
+   *
+   * @param ver The version being obtained
+   * @param bl The ceph::buffer::list to be populated
+   * @return 0 on success; <0 otherwise
+   */
+  virtual int get_version(version_t ver, ceph::buffer::list& bl) {
+    return mon.store->get(get_service_name(), ver, bl);
+  }
+  /**
+   * Get the contents of a given full version of this service.
+   *
+   * @param ver A version number
+   * @param bl The ceph::buffer::list to be populated
+   * @returns 0 on success; <0 otherwise
+   */
+  virtual int get_version_full(version_t ver, ceph::buffer::list& bl) {
+    std::string key = mon.store->combine_strings(full_prefix_name, ver);
+    return mon.store->get(get_service_name(), key, bl);
+  }
+  /**
+   * Get the latest full version number
+   *
+   * @returns A version number
+   */
+  version_t get_version_latest_full() {
+    std::string key = mon.store->combine_strings(full_prefix_name, full_latest_name);
+    return mon.store->get(get_service_name(), key);
+  }
+
+  /**
+   * Get a value from a given key.
+   *
+   * @param[in] key The key
+   * @param[out] bl The ceph::buffer::list to be populated with the value
+   */
+  int get_value(const std::string& key, ceph::buffer::list& bl) {
+    return mon.store->get(get_service_name(), key, bl);
+  }
+  /**
+   * Get an integer value from a given key.
+   *
+   * @param[in] key The key
+   */
+  version_t get_value(const std::string& key) {
+    return mon.store->get(get_service_name(), key);
+  }
+
+  /**
+   * @}
+   */
+  /**
+   * @}
+   */
+};
+
+#endif