Here's one:

count(page-impression) >= count(page-save-attempt) >= count(page-save-success)

Super-simple approach: a repository which consists of a single script and a 'sql' subdirectory. The subdirectory contains SQL queries, one to a file. Each query should return a boolean. When the script is launched, it iterates through these queries and executes each in turn against the database. If any query returns 0, an e-mail alert is sent. A cron job ensures the script is invoked at regular intervals.

If we had this, Steven and Dario would be able to add and remove assertions very easily, using git rm / git add and git review. I'd use puppet to configure a cron job to keep the repository state up-to-date.

[moving from MediaWiki extensions to Analytics product - see bug 61946]