mySQL AND memcached for PHP sessions?

Question

For a high traffic web site we are planning to scale up to use 2 web servers in a HA setup.

One issue we will need to tackle is the management of PHP sessions.

The obvious answer is to move session handling to the DB which is easy and example code is widely available ton the internet.

On the other hand we are aware of the benefits of memcached but once a memcached node fails, users on that node will lose their session.

So we are thinking of implementing a setup where sessions are handled in memcached by default but also written in the DB. When we get a memcached MISS we would try to also retrieve it from the DB.

Does the above make sense and are there any implementation examples you are aware of?

thanks in advance

Answer 1

I refer you to Dormando's oft-cited explanation of how to store sessions in MySQL with memcached caching . The original LiveJournal post is more wordy but more thoroughly explains why storing sessions in memcached only is a bad idea.

In short:

• Read session data from memcached first, look in MySQL on a cache miss.
• Write session data to memcached on every update.
• Only write to MySQL if cache data hasn't been synced for 120 seconds or so.
• Run a periodic script that checks MySQL for expired sessions. For every expired session, update from memcached and only expire the ones that are truly expired.

Answer 2

Sessions it's a temporary thing, there is nothing to worry about if once per month memcache-server will fail and truncate sessions. I'm sure you can use just memcache for sessions, without replication in DB.

But if you still want to dump sessions to disk, as existing solution you can use Redis :

Redis works with an in-memory dataset. Depending on your use case, you can persist it either by dumping the dataset to disk

...

Redis also supports trivial-to-setup master-slave replication, with very fast non-blocking first synchronization, auto-reconnection on net split and so forth.