Memcached is a simple key/value memory store that uses keys to perform fast lookups of values stored in memory. Most languages have simple bindings that make this very easy from a programming perspective. This also means that it's not quite as simple to configure as APC.
Installation and Configuration
I recommend using your choice of installation mechanism (or your distribution's choice; mine's emerge) to install Memcached.
The configuration usually consists of an RC configuration parameter file that specifies how much memory to allocate and a port to listen on. In Gentoo this file is located in /etc/conf.d/ as usual but look for it in /etc/sysconfig on RHEL -like distributions.
Update the configuration to listen on the interface of your choice, use the amount of memory that's appropriate for your application and system, &c. That's all there is to Memcached and you're now ready to start the service.
Configuring Wordpress to Use Memcached
I've previously mentioned the W3 Total Cache Wordpress plugin but unfortunately this plugin doesn't allow you to easily configure Memcached usage through the web interface. It defaults to using the Memcached server located at 127.0.0.1:11211 which isn't appropriate for a remote installation of Memcached. We need to modify DOCROOT/wp-content/w3-total-cache-config.php so that the Memcached servers point to our remote server's location before enabling it in the interface. The following sed command can make this change for us:
Configuring MediaWiki to Use Memcached
Just like Wordpress, MediaWiki needs the PHP bindings for Memcached pre-installed.
Setting up MediaWiki with Memcached is a simple matter and only requires an edit of its LocalSettings.php file. We need to add the following section to enable Memcached:
## Shared memory settings $wgMainCacheType = CACHE_MEMCACHED; $wgParserCacheType = CACHE_MEMCACHED; $wgMessageCacheType = CACHE_MEMCACHED; $wgMemCachedServers = array("giskard.alunduil.com:11211"); $wgSessionsInMemcached = true;
These settings not only enable Memcached but also specifies the set of Memcached servers to use and whether or not to put sessions into Memcached.
If you do use multiple Memcached servers and want to apply a weighting factor you can do this with nested PHP arrays:
Using Memcached can greatly reduce the load placed on databases behind webapps or other data storage and retrieval systems. This usually improves the perceived load on the web application itself but not necessarily.