Parallel CSS - AdvAgg Plugin

Upgrade to the CDN module!

The Parallel CSS - AdvAgg Plugin module is now obsolete.
The CDN module contains a superset of the Parallel CSS - AdvAgg Plugin module's functionality, with a better UI, and a Drupal 7 port readily available.
No migration path is provided, but it's very simple, see #1410318: Merge into CDN module?. More details are available in the CDN module's Advanced Help documentation. Install CDN module version 2.5 or later, which ships with #1452092: Automatically distribute certain filetypes over multiple CDNs.

Below you, can find a quick tutorial on how you can duplicate the Parallel CSS - AdvAgg Plugin module's behavior using the CDN module.

Why did/does this module exist?

Modern browsers cap the simultaneous image download rate at somewhere around 6 images per domain. This module allows your site to pull images from separate domains simultaneously, thereby removing this cap, and allowing your site to load images more quickly.

This also holds the potential to free up server use, or at least divert some of the requests to different servers so that you do not reach your sever request limits as easily.

In other words: this allows your admins to manage the source URLs your images are referencing during the CSS aggregation process.

Old set-up (pre-CDN module):

At admin/settings/advagg/parallel-css, enter the domains:

http://img1.drupal.org
http://img2.drupal.org
http://img3.drupal.org
http://img4.drupal.org
http://s1.amazonaws.com/drupal_cdn

Given the following CSS:

background:url('sites/all/themes/do/1.png');
background:url('sites/all/themes/do/2.png');
background:url('sites/all/themes/do/3.png');
background:url('sites/all/themes/do/4.png');
background:url('sites/all/themes/do/5.png');
background:url('sites/all/themes/do/6.png');
background:url('sites/all/themes/do/7.png');
background:url('sites/all/themes/do/8.png');

The resulting CSS would look like this:

background:url('http://img1.drupal.org/sites/all/themes/do/1.png');
background:url('http://img2.drupal.org/sites/all/themes/do/2.png');
background:url('http://img3.drupal.org/sites/all/themes/do/3.png');
background:url('http://img4.drupal.org/sites/all/themes/do/4.png');
background:url('http://s1.amazonaws.com/drupal_cdn/sites/all/themes/do/5.png');
background:url('http://img1.drupal.org/sites/all/themes/do/6.png');
background:url('http://img2.drupal.org/sites/all/themes/do/7.png');
background:url('http://img3.drupal.org/sites/all/themes/do/8.png');

The randomness of the assignment of an image to an image is tied to the md5 hash and number of servers available. This is to make sure that in different CSS files, 1.png will always be assigned to the same server.

New set-up (using the CDN module):

1. Make sure CSS aggregation is enabled for your Drupal site.
2. Install & enable the CDN module module.
3. Go to admin/settings/cdn/details (Drupal 6) or admin/config/development/cdn/details (Drupal 7) and enter the following CDN mapping:
   

   http://img1.drupal.org http://img2.drupal.org http://img3.drupal.org http://img4.drupal.org http://s1.amazonaws.com/drupal_cdn
   http://js.example.com|.jpg .jpeg .gif .png .ico
   

   i.e.:

   domain1 domain2 domain3|.jpg .jpeg .gif .png .ico
   

4. Same results as with this module: sit back and watch your website load quicker since your browser is downloading CSS images in parallel.