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.

Answer 2

You can also write your sessions to a SQL DB. This will also give you persistent sessions across servers. See the tutorial below: http://culttt.com/2013/02/04/how-to-save-php-sessions-to-a-database/

If you insist on running the sessions in memcached (in order to have better performance, but lower consistency) you can simply rewrite his code in a couple of places

Answer 3

You can sync file-based sessions too. I do it on my two load-balanced servers. On Ubuntu and a bunch of other Linuxes, sessions are stored in /var/lib/php5.

I followed the steps over here on Superuser, mounting the PHP session folder from one server on the other.

Answer 4

You want to be careful about doing this. One thing to be careful about is to not host session info with other non-session data. It isn't the biggest deal to clear your cache when it only contains your own site's data but you do not want to wipe out people's sessions along with it.

As long as your are using the same key with memcache you should hit the same server every time. So that issue should go away.

Answer 5

You would need to set up memcache to run on one of the servers and have all of the servers use that same memcache instance for the sessions. Otherwise, if they each run their own memcache instance, you'll have the same problem as before.

Other than configuring memcache accordingly and telling PHP to use it as your session handler, you shouldn't have to make any changes to your code.

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To clarify the advice I gave here, if you group all three servers into a single pool, you won't have any problems as long as every PHP instance references those servers in the same order. memcache uses client side hashing, so you will be guaranteed that the same key is read / written on the same server. Of course, if you alter that list in any way, then sessions will become invalidated.

The memcache developers actually don't even recommend that you use memcache for storing session data because it isn't persistent, and thus if you have to restart memcache (or something happens), then all of your users will be logged out.

Answer 6

it is better to use memcachedb as a persistent storage for memcache itself or even to use the more smarter redis which I highly recommend in your case