kea/doc/devel/config-backend.dox

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/**

 @page configBackend Kea Configuration Backends

@section configBackendIntro Introduction

Kea is a flexible DHCP protocol engine. It offers a selection of lease database
backends, extensibility via the hooks API and the definition of custom options.
Depending on the environment, one lease database backend may be better than
other. Similarly, because the best way of configuring the server can depend on
the environment, Kea provides different ways of obtaining configuration
information, through the Configuration Backend. Since the means by which
configuration information is received cannot be part of the configuration itself, it
has to be chosen at the compilation time (when configuring the sources).

This page explains the background to the Configuration Backend and how
it is implemented. It is aimed at people who want to develop and
maintain their own backends.

@section configBackendMotivation Motivation for Different Backends

BIND10 (the project under which the first stages of Keas were developed)
used to maintain an extensive framework that was responsible for the
configuration of components.  After BIND10 was cancelled, two projects
were created: <a href="http://kea.isc.org">Kea</a> (focused on DHCP)
and <a href="http://www.bundy-dns.de">Bundy</a> (aimed at DNS). The
Kea team decided to remove the BIND10 framework, while the Bundy team
decided to keep it. However, even though the Kea team is focused on a
backend that reads a JSON configuration file from disk, it decided to
make it easy for others to use different backends.

While ISC currently (May 2014) plans to maintain only one configuration backend
(a JSON file read from disk), there may be other organizations (e.g.
the Bundy project community) that will maintain other backends. It is quite
possible that additional backends (e.g. using LDAP or XML) will be
developed and maintained by other organizations.

@section configBackendAdding How to Add a New Configuration Backend

@todo Will be covered in ticket #3400.

@section configBackendJSONDesign The JSON Configuration Backend

The following are some considerations that shaped the design of the configuration
backend framework.

-# A new parameter called --with-kea-config will be implemented in the
   configure script. It will allow the selection at compilation time of how the
   servers will be configured. For the next 2-3 months (until around June 2014),
   there will be two values: JSON (read from file) and BIND10 (use the BIND10 framework).
   Once the file based configuration is implemented and the Kea team is ready to switch
   (i.e. enough confidence, Forge tests updated for new configuration
   mechanism), the BIND10 backend will be removed from the Kea repository. Other projects
   (e.g. Bundy) who want to maintain it, are advised to just revert the single
   commit that will bring the BIND10 framework back to their repositories.<br/><br/>
   This switchable backend concept is quite simple. There are just different
   implementations of ControlledXSrv class, so it is a matter of compiling/linking
   one file or another. Hence it is easy to remove the old backend (and for
   Bundy to keep it if they desire so). It is also easy for other
   organizations to add and maintain their own backends (e.g. LDAP).<br/><br/>
-# Each backend must use 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/>
-# After Kea 0.9 is released, a form of secure socket will be implemented through
   which commands can be sent. Whatever the design, it
   will allow the sending of configurations and commands in JSON format and
   the receiving of responses.<br/><br/>
   Once that is done, Kea will have the same capability the BIND10
   framework to send additional parameters. One obvious use case will be
   to send a new configuration file name as the parameter for "reload".<br/><br/>
-# A command handler needs to be added for 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.<br/><br/>
-# Extend the existing JSON parser. The current JSON parser in
   @ref isc::data::Element::fromJSON() needs to be extended to allow optional preprocessing.
   For now that capability will simply remove whole-line comments staring with the hash
   character, but it is expected
   to grow over time (in-line comments and file inclusions are the obvious
   envisaged additions).<br/><br/>
-# Implement a common base class for the Kea4, Kea6, and D2 servers. Some operations will be
   common for all three components: logger initialization, handling and, at some future point,
   control socket. This calls for a small base class that @ref isc::dhcp::Dhcpv4Srv "Dhcpv4Srv",
   @ref isc::dhcp::Dhcpv6Srv "Dhcpv6Srv" and the @ref isc::d2::D2Controller "D2Controller" classes can use.
   It is expected that the base class
   (@ref isc::dhcp::Daemon) will be a small one but will grow over time as the code is unified.<br/><br/>
-# A way is needed to initialize stand-alone logging (i.e. each
   Kea component will initialize it on its own).<br/><br/>
-# The current format of the BIND10 configuration file, b10-config.db will be
   retained as the configuration file format.  This is slight change
   from the BIND10 days, as then a subset of the configuration was received by
   the daemon processes.<br/><br/>
   To take a specific example, the following is how b10-config.db
   looks today:<br/><br/>
   @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
   <br/>
   The Kea components used to receive only relevant parts of it (e.g. Kea4
   received config that contained content of the Dhcp4 element). Now they
   will receive all of it. The modification in the code to handle this
   is really minor: just iterate over the top level elements and pick the appropriate
   tree (or get the element by name). Also, that approach makes the logging
   initialization code very easy to share among Kea4, Kea6 and D2.<br/><br/>
-# The .spec files used in BIND 10 by the control program to validate commands
   will be retained. They will be kept and maintained even though no use of
   them is planned. At some future time syntax validation may be implemented,
   although it is out of scope for Kea 0.9 (and probably
   for 1.0 as it is pretty big task).<br/><br/>
-# Addition of a shell script to start/stop Kea4,Kea6 and D2. There will be a script that will
   start, stop and reconfigure the daemons. Its only
   job will be to pass the configuration file to each daemon and remember its PID file, so
   that sending signals will be be possible (for configuration reload or shutdown). Optionally,
   it could also print out a status based on PID, but that may be tricky to
   implement in a portable way. The minimum set of commands will be:
   -# Start the processes
      - eventually based on configuration, initially start them all
      - it could launch a nanny script which restarts them on a crash (past 0.9)
   -# Prompt the processes to reload configuration
      - for now it will be a matter of sending singal to the right process
      - this could also decide if D2 should still be running or not, and react accordingly (post 0.9)
   -# Stop the processes in an orderly fashion
   -# Perhaps return status of each process
*/