// Copyright (C) 2017-2024 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 libdhcp_lease_cmds Kea Lease Commands Hooks Library

@section libdhcp_lease_cmdsIntro Introduction

Welcome to Kea Lease Commands Hooks Library. This documentation is addressed to
developers who are interested in the internal operation of the Lease Commands
library. This file provides information needed to understand and perhaps extend
this library.

This documentation is stand-alone: you should have read and understood <a
href="https://reports.kea.isc.org/dev_guide/">Kea Developer's Guide</a> and in
particular its section about hooks.

@section lease_cmds Lease Commands Overview

Lease Commands (or lease_cmds) is a Hook library that can be loaded by Kea to
extend it with additional mechanisms.

The primary purpose of this library is to provide commands that manage leases.
As such, the whole library is structured around command handlers. When the
library is loaded it registers a number of handlers for new commands.  When a
command is issued (be it directly via control channel or indirectly via REST
interface from control agent), the code receives a JSON command with
parameters. Those are parsed and then actual operation commences.  This
operation always interacts with an instantiation of isc::dhcp::LeaseMgr
instance, which is Kea's way of storing leases. At the time of writing this text
(Aug. 2017), Kea supports four types of lease managers: memfile, MySQL or
PostgreSQL. The lease commands provided by this library provide a unified
interface for those backends.

As with other hooks, this one also keeps its code in a separate namespace which
corresponds to the file name of the library: isc::lease_cmds.

@section lease_cmdsCode Lease Commands Code Overview

The library operation starts with Kea calling the load() function (file
load_unload.cc).  It instantiates an isc::lease_cmds::LeaseCmds object.
The constructor of that object registers all of the lease commands. For a list,
see @ref isc::lease_cmds::LeaseCmds class documentation.  This class uses Pimpl
design pattern, thus the real implementation is hidden in isc::lease_cmds::LeaseCmdsImpl.

Almost every command has its own handler, except few that share the same handler
between v4 and v6 due to its similarity. For example
isc::lease_cmds::LeaseCmdsImpl::leaseAddHandler handles lease4-add and lease6-add
commands. Although the details differ between handlers, the general approach
is the same. First, it starts with parameters sanitization and then some
interaction with isc::dhcp::LeaseMgr is conducted.

For commands that do something with a specific lease (lease4-get, lease6-get,
lease4-del, lease6-del), there is a @ref isc::lease_cmds::LeaseCmdsImpl::Parameters
class that contains parsed elements.

For details see documentation and code of the following handlers:
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseAddHandler (lease4-add, lease6-add)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease6BulkApplyHandler(lease6-bulk-apply)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetHandler (lease4-get, lease6-get)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetAllHandler(lease4-get-all, lease6-get-all)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetPageHandler(lease4-get-page, lease6-get-page)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetByHwAddressHandler(lease4-get-by-hw-address)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetByClientIdHandler(lease4-get-by-client-id)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetByDuidHandler(lease6-get-by-duid)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseGetByHostnameHandler(lease4-get-by-hostname, lease6-get-by-hostname)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease4DelHandler (lease4-del)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease6DelHandler (lease6-del)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease4UpdateHandler (lease4-update)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease6UpdateHandler (lease6-update)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease4WipeHandler (lease4-wipe)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease6WipeHandler (lease6-wipe)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease4ResendDdnsHandler(lease4-resend-ddns)
- @ref isc::lease_cmds::LeaseCmdsImpl::lease6ResendDdnsHandler(lease6-resend-ddns)
- @ref isc::lease_cmds::LeaseCmdsImpl::leaseWriteHandler (lease4-write, lease6-write)

@section lease_cmdsDesigns Lease Commands Design choices

The lease manipulation commands were implemented to provide a convenient interface
for sysadmins. The primary goal was to offer a way to interact with the live
lease database in unified way, regardless of the actual backend being used.

For some backends (MySQL and PostgreSQL) it is possible to interact directly
with the backend while Kea is running and possibly change its content. This
ability is both powerful and dangerous. In particular, only rudimentary
checks are enforced by the DB schemas (e.g. not possible to have two leases
for the same address). However, it does not prevent sysadmins from making
more obscure errors, like inserting leases for subnets that do not exist
or configuring an address that is topologically outside of the subnet to which
it should belong. These kind of checks are only possible by DHCP-aware
code, which this library provides.

Some of the queries may require a seemingly odd set of parameters. For example,
lease6-get query requires at least DUID, subnet-id and IAID to retrieve a lease
by DUID. The need for a sysadmin to know and specify an IAID is troublesome.
However, the guiding principle here was to use whatever queries were already
exposed by the lease manager and not introduce new indexes, unless absolutely
necessary. This ensures that there is no performance degradation when the
library is loaded. The only exception for that was lease4-wipe and lease6-wipe
commands that remove all leases from specific subnet. As there were no
queries that could retrieve or otherwise enumerate leases for a specific subnet,
a new query type and a new index had to be added.

@section lease_cmdsMTCompatibility Multi-Threading Compatibility

The Lease Commands Hook library is compatible with multi-threading.
All commands protect the resource they touch so with split spaces
a race with the allocation engine should not be possible when
called by a high availability server with a sane configuration.
When a race is detected a critical section is used.

Note an expired lease reclamation is called only from the periodic
process or by a command. In both cases it is executed by the main
thread so the same thread as lease commands.

*/
