OpenSourcery

I attended several sessions at Drupalcon on performance and scalability. One common thread was the need to offload static files to dedicated servers or a content delivery network (CDN). The use of a CDN is also part of the well-known YSlow test for measuring front-end performance. With CDN providers getting cheaper by the day, content delivery is not just for big media companies anymore! A bit more explanation is required to understand why this is complicated in Drupal, and what's being done about it. As a bonus — I'll share a trick for getting a useful chunk of your files on a CDN in less than 3 minutes.

The state of content delivery in Drupal

With the advent of cheap, auto-magical mirroring CDNs, this should be easy right? One of the challenges is that links to files can come from several disparate Drupal subsystems:

• Links to Drupal's filesystem are created with file_create_url(), which doesn't filter links through custom_url_rewrite_outbound(). There are several approaches in the works for D7 to address this; I've worked on one of the patches, though I'm not yet sure how the reworked File API impacts this.
• Some files, like CSS, come from functions such as drupal_get_css() that do their own string URL manipulations.
• Files may be linked from inline image tags added by content editors, either manually or via a WYSIWYG editor.
• Modules and themes may add their own inline images: print '<img src="' . $base_path . path_to_theme() . '/images/logo_bw.png" />' isn't uncommon.

At Drupalcon, Wim Leers presented his work on the CDN Integration module, and it seemed very promising to me. There are plans to support both push and pull CDN providers, and even possibly things like video transcoding servers. However, it's neither production-ready yet, nor available for Drupal 6.

So what are we to do? To cover all these bases, we might imagine a core hack or two, an implementation of hook_filter, custom_url_rewrite_outbound, and more. But I promised you a win in three minutes, so...

• Grab the low-hanging fruit (CSS files)
• Abuse the theme layer!

/* This goes in your theme's template.php, and will modify the $styles
 * variable that gets passed to your page template.  CSS background
 * images will also be pulled from your CDN.
 */
function mytheme_preprocess_page(&$vars, $hook) {
  if ($url = variable_get('cdn_url', '')) {
    $vars['styles'] = str_replace('href="/', 'href="' . $url, $vars['styles']);
  }
}
 
/* This goes in your server's settings.php,
 * and assumes you have a "mirror bucket" set up at this URL
 */
$conf = array(
  'cdn_url' => 'http://mirror-e7.your-cdn-provider.net/',
);