Memcache clustering for php sessions?

Question

Here's a little background, currently i have

• 3 web servers
• one db server which also host memcache for php sessions for the 3 web servers.

I have the php configs on the 3 servers to point to the memcache server for sessions. It was working fine until alot of connections were being produced for reads etc, which then caused connection timeouts.

So I'm currently looking at clustering the memcache on each web server for sessions, my only concern is how to go about making sure that memcache on all the servers have the same information for sessions.

Someone guided me to http://github.com/trs21219/Memcached-Library because i am using codeigniter but how do i converge my php sessions onto this since memcache seems as a key-value store? Thanks in advance.

Has anyone checked out http://repcached.sourceforge.net/ and does it work?

Answer 1

I'm not sure you have the same expectations of memcache that its designers had.

First, however, memcache distribution works differently than you expect: there is no mechanism to replicate stored information. Each memcache instance is a simple key-value store, as you've noticed. The distribution is done by the client code which has a list of all configured memcache instances and does a hash of the key to direct it to one of the instances. It is possible for the client to store it everywhere and retrieve it locally, or for it to hash it multiple times for redundancy, but these are not straightforward exercises.

But the other issue is that memcache is designed for reasonably short-lived data that memcache is allowed to throw away at any time. This makes it really good for caching frequently accessed data that can be a little stale (say up to a few minutes old) but might be expensive to retrieve (such as almost a minute to generate from a query).

PHP sessions don't really qualify for this, in my experience. A database can easily support many thousands of PHP sessions with barely visible traffic, but you need a lot of memcache storage to support the same number: 50k per session and 5000 sessions means close to 256Mb, and then there is all the other data you want to put in there. Not enough storage and you get lots of unexplained logouts (as memcache discards session data when under memory pressure) and thus lots of annoyed users who have to keep logging in again.

Answer 2

There is no need to use some 3rd party libraries to organize memcached "cluster".

http://ru.php.net/manual/en/memcached.addserver.php

Just use this function to add several servers into the pool and after that data will be stored and distributed over those servers. The server for storing/retrieving the data for the specific key will be selected according to consistent key distribution option.

So in this case you don't need to worry about "how to go about making sure that memcache on all the servers have the same information for sessions"

Answer 3

We've found GREAT advantage applying MongoDB instead of MySQL for most things, including session handling. It's far faster, far smaller, far easier. We keep MySQL around for transactional needs, but everything else goes into Mongo now. We've relegated memcache to simply caching pages and other data that isn't critical if it's there or not, something like smarty does.