Multisite override modules

Question

If I have a module in sites/all/modules named dc_front and then I have a module in sites/default/modules named dc_front, which one gets used when it is enabled?

It appears the module in default gets used.

Is this expected and reliable behavior?

Answer 1

drupal_system_listing() is the function that handles this.

According to its inline documentation:

// Duplicate files found in later search directories take precedence over
// earlier ones, so we want them to overwrite keys in our resulting
// $files array.
// The exception to this is if the later file is from a module or theme not
// compatible with Drupal core. This may occur during upgrades of Drupal
// core when new modules exist in core while older contrib modules with the
// same name exist in a directory such as sites/all/modules/.

And as far as I can tell from the function, it searches directories in the following order:

1. /profiles
2. /sites/all
3. /sites/[domain]

So each directory in the list above would override matches from previous directories.

Having modules in the /sites/default directory is not normal, so I'm not sure what the expected behavior would be. My guess is that it depends alphabetically on your [domain] directory name.