Written by Sébastien Santoro (Dereckson) and extracted from http://www.mediawiki.org/wiki/Mentorship_programs/Possible_projects#Build_an_interwiki_notifications_framework_and_implement_it_for_InstantCommons:
"In January 2010, we introduced a setting to ease the possibility to reuse the Wikimedia Commons content on other MediaWiki installations. This feature is called “InstantCommons”.
The Wikimedia Commons community performs a continuous maintenance on wiki files, renaming and deleting media. We currently have tools to detect if the media are used on Wikimedia projects, and tools to replace automatically the names following a rename operation.
It would be interesting to allow wikis (and 3rd party software) to notify Wikimedia Commons they use pictures through a notification API. It would also be interesting if these wikis can subscribe to notifications, so we can notify them back we’ve made a destructive operation (e.g. rename or delete a media file currently in use on their website), so they can automatically or manually take appropriate measures.
This project is to develop an interwiki notifications framework, ideally also open to 3rd party sites, and to use it to implement it to enhance the InstantCommons feature."