heisel.org

September 19, 2009

Memcached and WordPress MU

This week I, with help from many, got Word­PressMU setup to use mem­cached.

The instruc­tions for doing all this are on the Inter­net, but I thought it might help others (or at least my future self) to com­pile them all in one spot.

How WordPress caches

There are two layers of caching in Word­Press: * An object level cache, sim­i­lar to Django’s low-​level cache * Page level caching, pro­vided by plu­g­ins, sim­i­lar to Django’s per-​view cache

By the time we’re done, we’ll have both using mem­cache. The object cache defaults to the file system, and most folks use WP Super Cache which caches pages to the filesys­tem as well.

Object caching

• Install the PHP mem­cache module

• Add these set­tings to your wp-config.php

global $memcached_servers;
$memcached_servers = array('default' => array('127.0.0.1:11211'));

You can add as many servers and change the port number to match your mem­cached setup. Example:

global $memcached_servers;
$memcached_servers = array('default' => array('bradfitzrocks:11211', 'bradfitzrocks2.mycompanydomain.com:90210'));

• Install the mem­cache ver­sion of WordPress’s object-cache.php to your blog’s wp-​content folder.

  I don’t want to side­track myself com­par­ing how much Django rocks com­pared to PHP or Word­Press, but I will anyway! In Django, your cache setup is a set­ting, which makes local devel­op­ment soooo much easier.

Stop here and you’ll be live with Word­Press object caching.

Page caching

• You must have fol­lowed the steps above to hook up WordPress’s object cache to memcache.

• If it’s installed, unin­stall WP Super Cache

• Install Bat­cache by putting advanced-cache.php file in your blog’s wp-​content folder, and batcache.php in your blog’s wp-content/plugins folder.

• Acti­vate the Bat­cache plugin from within the admin inter­face, if you’re on Word­PressMU use the “Activate X Site Wide” link.

• Hit reload on a page a few times and then view source and you should see some­thing like this:

<!--
	generated 12 seconds ago
	generated in 0.683 seconds
	served from batcache in 0.006 seconds
	expires in 288 seconds
-->
</head>

• Option­ally, tweak Batcache’s behav­ior by edit­ing the options in advanced-cache.php

var $max_age = 300; // Expire batcache items aged this many seconds (zero to disable batcache)
 
var $remote = 0; // Zero disables sending buffers to remote datacenters (req/sec is never sent)
 
var $times = 2; // Only batcache a page after it is accessed this many times... (two or more)
 
var $seconds = 120; // ...in this many seconds (zero to ignore this and use batcache immediately)
 
var $group = 'batcache'; // Name of memcached group. You can simulate a cache flush by changing this.
 
var $unique = array(); // If you conditionally serve different content, put the variable values here.
 
var $headers = array(); // Add headers here. These will be sent with every response from the cache.
 
var $uncached_headers = array('transfer-encoding'); // These headers will never be cached. Apply strtolower.
 
var $debug = true; // Set false to hide the batcache info <!-- comment -->
 
var $cache_control = true; // Set false to disable Last-Modified and Cache-Control headers
 
var $cancel = false; // Change this to cancel the output buffer. Use batcache_cancel();

The Bat­cache ver­sion I linked to is a GitHub fork of Andy Skelton’s orig­i­nal ver­sion. My fork includes a patch to make sure the Content-​Type header is set cor­rectly on non-​HTML pages. I’m hoping to add some more cache inval­i­da­tion rules on top of the great work Andy already did.

Many thanks to the fol­low­ing folks:

• Mohan­jith for the object cache set up
• Andy Skel­ton for cre­at­ing Bat­cache and being kind enough to drop a ver­sion on GitHub for me to fork
• Casey Bisson for describ­ing the content-​type patch for Batcache

Filed under: Programming,Technology

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