RedisCluster Cache Store for Moodle™

A Moodle cache store plugin for RedisCluster.

Requirements

• A Redis Cluster (version 4.0 or better)
• PhpRedis extension (version 4.0 or better)
• php-igbinary if using igbinary as the serialization method (recommended)

Features

Cache store

This is the main use of this plugin - providing a way to use a Redis Cluster as a Moodle™ cache.

Configurable options:

• failover mode: how phpredis handles reads/writes, one of:
  • none
  • error (reads from a slave on error)
  • distribute (distributes reads over masters/slaves)
• serializer: igbinary or php
• compression: compresses data stored in redis - (de)compression occuring within phpredis

Session handler

Moodle™ can be configured to use the Redis Cluster (or a different one) as the session store by setting:

$CFG->session_handler_class = '\cachestore_rediscluster\session';

Configuration options can be set with:

$CFG->session_rediscluster = [
    'server' => '192.168.1.100:6379',
    'serversecondary' => '192.168.1.101:6379:,
    'prefix' => "mdlsession_{$CFG->dbname}:",
    'acquire_lock_timeout' => 60,
    'lock_expire' => 600,
    'max_waiters' => 10,
    'waiting_exclusions' => [],
];

The only required setting in the above array is server.

The following options govern session locking:

• acquire_lock_timeout: How long to wait for a lock to be released before giving up
• lock_expire: How long before a lock is released automatically
• max_waiters: How many threads can a session have waiting for a lock
• waiting_exclusions: Rules about scripts don't count towards the max_waiter count

Max waiters lets you define how many how many php threads a single user is allowed to have waiting for a session lock. Requests that don't take a session lock are unaffected.

By default, max_waiters is set to 10. Set it to 0 to use default Moodle™ behaviour.

Scripts with the NO_SESSION_LOCK define set to true ignore the max waiter behaviour.

Requests that match a rule defined in the waiting_exclusions array ignore the max waiter behaviour and also do not count towards the number of active requests - avoiding blocking other scripts (other scripts are still restricted by the session lock).

Rule format:

$CFG->session_rediscluster = [
    'waiting_exclusions' => [
        // Example allowing admin index page.
        [
            // Must include a script, relative to root path and including the leading slash.
            'script' => '/admin/index.php',
        ],

        // Example allowing calls to the tiny_autosave_reset_session service.
        [
            'script' => '/lib/ajax/service.php',
            // When script is `/lib/ajax/service.php` we can include a `method`. This will check
            // the requested method matches along with the script.
            'method' => 'tiny_autosave_reset_session',
        ],
    ],
];

auth_saml2 session handler

If you use the auth_saml2 plugin, you can configure it to use Redis Cluster for session storage by setting:

$CFG->auth_saml2_store = '\\cachestore_rediscluster\\auth_saml2_store';

Configuration options can then be set with:

$CFG->auth_saml2_rediscluster = [
    // The same options as are available in the standard session handler are supported here.
];

License

Copyright (c) 2021 Open LMS (https://www.openlms.net)

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.