Cm_RedisSession

A Redis-based session handler for Magento with optimistic locking.

Features:

• Falls back to mysql handler if it can't connect to redis. Mysql handler falls back to file handler.
• When a session's data exceeds the compression threshold the session data will be compressed.
• Compression libraries supported are 'gzip', 'lzf' and 'snappy'. Lzf and Snappy are much faster than gzip.
• Compression can be enabled, disabled, or reconfigured on the fly with no loss of session data.
• Expiration is handled by Redis. No garbage collection needed.
• Logs when sessions are not written due to not having or losing their lock.
• Limits the number of concurrent lock requests before a 503 error is returned.

Locking Algorithm Properties:

• Only one process may get a write lock on a session.
• A process may lose it's lock if another process breaks it, in which case the session will not be written.
• The lock may be broken after BREAK_AFTER seconds and the process that gets the lock is indeterminate.
• Only MAX_CONCURRENCY processes may be waiting for a lock for the same session or else a 503 error is returned.

Installation

1. Install module using modman:

    modman clone git://github.com/colinmollenhour/Cm_RedisSession.git
   

2. Configure via app/etc/local.xml adding a global/redis_session section with the appropriate configuration if needed. See the "Configuration Example" below.

3. Refresh the config cache to allow the module to be installed by Magento.

4. Test the configuration by running the migrateSessions.php script in --test mode.

    sudo php .modman/Cm_RedisSession/migrateSessions.php --test
   

5. Change the global/session_save configuration to "db" in app/etc/local.xml. The "db" value is the MySQL handler, but Cm_RedisSession overrides it to avoid modifying core files.

6. Migrate the old sessions to Redis. See the "Migration" section below for details. The migration script will clear the config cache after migration is complete to activate the config change made in step 5.

Configuration Example

<config>
    <global>
        ...
        <session_save>db</session_save>
        <redis_session>                   <!-- All options seen here are the defaults -->
            <host>127.0.0.1</host>            <!-- Specify an absolute path if using a unix socket -->
            <port>6379</port>
            <timeout>2.5</timeout>            <!-- This is the Redis connection timeout, not the locking timeout -->
            <db>0</db>
            <compression_threshold>2048</compression_threshold>  <!-- Set to 0 to disable compression -->
            <compression_lib>gzip</compression_lib>              <!-- gzip, lzf or snappy -->
        </redis_session>
        ...
    </global>
    ...
</config>

Migration

A script is included to make session migration from files storage to Redis with minimal downtime very easy. Use a shell script like this for step 6 of the "Installation" section.

cd /var/www              # Magento installation root
touch maintenance.flag   # Enter maintenance mode
sleep 2                  # Allow any running processes to complete
# This will copy sessions into redis and clear the config cache so local.xml changes will take effect
sudo php .modman/Cm_RedisSession/migrateSessions.php -y
rm maintenance.flag      # All done, exit maintenance mode

Depending on your server setup this may require some changes. Old sessions are not deleted so you can run it again if there are problems. The migrateSessions.php script has a --test mode which you definitely should use before the final migration. Also, the --test mode can be used to compare compression performance and ratios. Last but not least, the --test mode will tell you roughly how much space your compressed sessions will consume so you know roughly how to configure maxmemory if needed. All sessions have an expiration so volatile-lru or allkeys-lru are both good maxmemory-policy settings.

Compression

Session data compresses very well so using compression is a great way to increase your capacity without dedicating a ton of RAM to Redis. Compression can be disabled by setting the compression_threshold to 0. The default compression threshold is 2048 bytes so any session data equal to or larger than this size will be compressed with the chosen compression_lib which is 'gzip' by default. However, both lzf and snappy offer much faster compression with comparable compression ratios so I definitely recommend using one of these if you have root. lzf is easy to install via pecl:

sudo pecl install lzf

NOTE: If using suhosin with session data encryption enabled (default is suhosin.session.encrypt = on), two things:

1. You will probably get very poor compression ratios.
2. Lzf fails to compress the encrypted data in my experience. No idea why..

If any compression lib fails to compress the session data an error will be logged in system.log and the session will still be saved without compression. If you have suhosin.session.encrypt on I would either recommend disabling it (unless you are on a shared host since Magento does it's own session validation already) or disable compression or at least don't use lzf with encryption enabled.

Using with Cm_Cache_Backend_Redis

Using Cm_RedisSession alongside Cm_Cache_Backend_Redis should be no problem at all. The main thing to keep in mind is that if both the cache and the sessions are using the same database, flushing the cache backend would also flush the sessions! So, don't use the same 'db' number for both if running only one instance of Redis. However, using a separate Redis instance for each is recommended to make sure that one or the other can't run wild consuming space and cause evictions for the other. For example, configure two instances each with 100M maxmemory rather than one instance with 200M maxmemory.