Archival republish. From this blog’s Nikola era; lightly copyedited.
Introduction
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
If you don’t already have the PHP bindings for Memcached installed, install them with your package manager or with PECL.
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:
sed -i -e 's/127.0.0.1:11211/HOST:PORT/g' DOCROOT/wp-content/w3-total-cache-config.phpConfiguring 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 specify 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:
$wgMemCachedServers = array(array("host:port", weight), array("host2:port2", weight));Conclusion
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.