I had to pull off my sleeves, open WP code and see by myself because all I finally found was this article (well I must admit that it was very helpful):
http://wordpress.org/support/topic/273381

To explain, in short, how Wordpress crons are working, here is a shortened call stack:

• Web User Request
• => WP API initialisation
• ==> wp_cron()  // if authorized to run, the cron array is loaded (from a get_option call)
• ===> spawn_cron()     // this is called if a job pending execution is found in the cron array.
• ====> wp_remote_post(’wp-cron.php?doing_wp_cron’)     // the asynchronous call

wp_cron() is the function that initiate the process of executing the jobs if one needs to be executed.  And wp-cron.php is the one that will, at the end, execute those jobs.

In order to run the jobs from a real cron, it is simple as scheduling a wget call to wp-cron.php and disabling PHP calls to wp_cron() function.  Damn, if I knew that from beginning…  When you open the wp_cron() function declaration, you will notice that first line checks for a constant definition: DISABLE_WP_CRON.  So you define(’DISABLE_WP_CRON’, true) in your wp-config.php and that’s it.

Well, if you are using Wordpress MU, that’s not totally it. Remember the comments in the stack call above? The cron array is loaded from a function call to get_option.  In Wordpress MU, get_option reads data from wp_xx_options which is per blog database table.  So it has a set of scheduled tasks per blog.  So you need to call wp-cron.php in the context of each blog.  So it’s starting to bug me… I don’t want to do maintenance tasks everytime a blog is created in Wordpress MU!  To prevent this, I created a small php script that I called wp-cron-multi-blog.php that is calling wp-cron.php for each active blogs defined in Wordpress MU.

Here is a wrap-up of the information without the bells and whistles.  To run WP tasks from a real scheduled cron you should do this:

in wp-config.php, add the following line :

define('DISABLE_WP_CRON', true);

for a standard Wordpress installation, configure a crontab that:

• wget http://www.server.com/wp-cron.php?doing_wp_cron

for a Wordpress MU installation, configure a crontab per blog like:

• wget http://www.server.com/wp-cron.php?doing_wp_cron
• wget http://www.server.com/whatever/wp-cron.php?doing_wp_cron
• wget http://www.server.com/what-a-blogger/wp-cron.php?doing_wp_cron
• ...

or, a single crontab calling my wp-cron-multi-blog.php (placed in root installation of WP):

• wget http://www.server.com/wp-cron-multi-blog.php

A final note about wp-cron-multi-blog.php.  If your Wordpress MU installation has a lot of blogs and each of them tends to have many time consuming tasks (ex: cache cleaning), it may be a good idea to edit this script in order to limit the number of blogs to run for at one time.

Hope this helps.

Pascal.

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