Kea  2.1.2-git
Kea Lease Commands Hooks Library

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 Kea Developer's Guide and in particular its section about hooks.

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, PostgreSQL or Cassandra. 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.

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 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 isc::lease_cmds::LeaseCmdsImpl::Parameters class that contains parsed elements.

For details see documentation and code of the following handlers:

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, PostgreSQL and Cassandra) 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.

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.