// Copyright (C) 2014-2020 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
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/**

 @page configBackend Kea Configuration Backends

@section configBackendIntro Introduction

Kea started as a sub-project in BIND10 that used a program (called
bindctl) to deliver configuration information to its modules. This
potentially allowed for modules to get their configuration information
in a variety of ways using what were known as configuration backends.
After BIND10 was cancelled, the Kea project briefly tried to maintain
backward compatibility with the BIND10 framework, but the effort
was discontinued due to lack of interest.

Currently the Kea team does not plan to develop any additional
configuration backends. Instead, effort is being focused on enhancing
the current control channel (see @ref ctrlSocket) to be as flexible
as possible. If you are thinking about developing new ways to
configure Kea, the recommendation is to write an external piece of
software that will communicate with Kea using this channel.

@section configBackendHistoric Alternate Configuration Backends

While this section currently has no practical value, it may become useful
one day to develop a minimalistic, stripped down Kea version that does
not have any command interface at all. This could prove useful for running
Kea in embedded regime.

The following steps are needed for the DHCPv4 server to be able to
process a new method of configuration. (It is assumed that the
modified component is DHCPv4. Similar approach applies to the other
components: DHCPv6 or DHCP-DDNS):

-# Write your own implementation of isc::dhcp::ControlledDhcpv4Srv::init(),
   isc::dhcp::ControlledDhcpv4Srv::init() and isc::dhcp::ControlledDhcpv4Srv::cleanup(),
   and put it in the src/bin/dhcp4 directory (e.g. as foo_controller.cc).
-# Modify src/bin/dhcp4/Makefile.am to include your file (e.g. foo_controller.cc) in
   the build.
-# Modify the AC_ARG_WITH(kea-config,...) macro in configure.ac to include an
   entry for your configuration backend.
-# Add your own AM_CONDITIONAL(CONFIG_BACKEND_FOO, ...) and
   AC_DEFINE(CONFIG_BACKEND_FOO, ...) macros to configure.ac (following the
   above-mentioned AC_ARG_WITH macro) to set the C++ macro for your backend.
-# Modify the sanity check in configure.ac to allow your configuration backend name.

Optionally you can also:

-# Implement unit tests for your backend in the src/bin/dhcp4/tests directory.
-# Modify src/bin/dhcp4/tests/Makefile.am to include the file(s) containing the
   unit tests.

@section configBackendJSONDesign The JSON Configuration Backend

The following are some details of the JSON backend framework.

-# Each backend uses the common code for configuration and command
   processing callbacks. They all assume that JSON formatted parameters are sent
   and they are expected to return well formatted JSON responses. The exact
   format of configuration and commands is module-specific.<br/><br/>
-# A command handler handles the reading the configuration from a
   file. Its main responsibility is to load the configuration and process
   it. The JSON backend must call that handler when starting up the server.
   This is implemented in configure() in the kea_controller.cc files
   in src/bin/dhcp4 and src/bin/dhcp6 directories.<br/><br/>
-# The current JSON parser in @ref
   isc::data::Element::fromJSON() has been extended to allow optional
   preprocessing.  For now, that capability simply removes  whole-line
   comments starting with the hash character, but it is expected to grow over
   time (in-line comments and file inclusions are the obvious envisaged
   additions). This is implemented in @ref isc::data::Element::fromJSONFile.<br/><br/>
-# The current format of the BIND10 configuration file (BIND 10 stored its
   configuration in (installation directory) /var/bind10/b10-config.db) has been
   retained as the configuration file format. Its actual naming is now arbitrary
   and left up to the user (it is passed as a parameter to the -c command line
   option). From the implementation perspective, this is slight change
   from the BIND10 days, as back then a subset of the configuration was received by
   the daemon processes. Nowadays the whole configuration is passed. To take a
   specific example, the following is how b10-config.db looked many years ago:
   @code
   {
     "Init": { ... }
     "Dhcp4": {
       "subnet4" { subnet definitions here },
       "option-data" { option data here },
       "interfaces": [ "eth0" ],
       ...
    },
     "Dhcp6": {
       "subnet6" { subnet definitions here },
       "option-data" { option data here },
       "interfaces": [ "eth0" ],
       ...
     },
     "Logging": {
       "Loggers": [{"name": *, "severity": "DEBUG" }]
      }
   }
   @endcode
   The Kea components used to receive only relevant parts of it (e.g. Kea4
   received configuration data that only contained the content of the Dhcp4 element).
   Now each component receives all of it: the code
   iterates over the top level elements and picks the appropriate
   tree (or get the element by name). That approach makes the common configuration
   (such as the logging initialization code) very easy to share among Kea4, Kea6 and
   DHCP-DDNS.<br/><br/>
-# The .spec files used in BIND 10 by the control program to validate commands
   have been removed in 1.4.<br/><br/>
-# A shell script has been added (as src/bin/keactrl/keactrl) to
   start, stop and reconfigure the daemons. Its only
   job is to pass the configuration file to each daemon and remember its PID file, so
   that sending signals is possible (for configuration reload or shutdown). It is also
   able to print out a status.<br/><br/>
-# The capability to share the same configuration file between servers and
   agents, and the Logging toplevel entry, were removed in 1.7.10.
*/