Using memcache as a session store?

Question

I currently have a huge problem. Two days ago my site running on one server was too much, so I purchased two more and had them clustered (rsync and load balanced).

I

Answer 1

There are two parts to the question:

How to deal with requests going to different servers?

This is down to your load balancer/reverse-proxy. It's common to make clients stick to one server, usually by IP address or a transparent cookie set by the proxy. However, it's not necessary to have client stickiness for the sake of sessions if you have a distributed session store, which brings us to memcache.

How to use memcache for session storage on multiple servers?

memcache has a proper shared-nothing distributed architecture, so most of the intelligence is at the client end. So what you should do is go ahead and use the memcache session storage, but instead of pointing at one server, point it at ALL of them. This is covered in the docs. In your php.ini you should set session.save_path to the list of memcached servers, for example server1:11211, server2:11211.

Be aware that there are two distinct memcache client libraries available in PHP called memcache and memcached and they have different syntax for this property.

Because of the way that memcache works, you don't care where your session data is stored - it's taken care of for you.

As NathanD points out, memcache is volatile and loses data on a restart, and when you have multiple servers this would mean that some (but not all) of your users would be logged out if one was restarted. If one server dies completely your session storage will stay working. Users whose session data was on the dead server will be kicked off, but they can log back in and carry on without that server being present.