PHP Sessions with Redis Cluster (using AWS Elasticache)

Posted on September 26th, 2017 in Linux System Administration,PHP,Programming by Brandon

I’ve recently been moving some of our project from a single Redis server (or server with a replica) to the more modern Redis Cluster configuration. However, when trying to set up PHP sessions to use the cluster, I found there wasn’t a lot of documentation or examples. This serves as a walk-through for setting up PHP sessions to use a redis Cluster, specifically with Elasticache on AWS.

First, create your Elasticache Redis Instance like so. Note the “Cluster Mode Enabled” is what causes redis to operate in Cluster mode.

AWS Elasticache Redis Creation

Once there servers are launched, make note of the Configuration Endpoint which should look something like:

Finally, use these settings in your php.ini file. The exact location of this file will depend on your OS, but on modern Ubuntu instances, You can place it in /etc/php/7.0/apache2/conf.d/30-redis-sessions.ini

Note the special syntax for the save_path where is has seed[]=. You only need to put the main cluster configuration endpoint here. Not all of the individual instances as other examples online appear to use.

session.save_handler = rediscluster
session.save_path = "seed[]"
session.gc_maxlifetime = 1296000

That’s it. Restart your webserver and sessions should now get saved to your Redis cluster.

IIn the even that something goes wrong, you might see something like this in your web server log files:

PHP Warning: Unknown: Failed to write session data (redis). Please verify that the current setting of session.save_path is correct (tcp:// in Unknown on line 0

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