when we update the sites table, we need a reliable way to invalidate cache.

If the cache key could somehow incorporate last update time for the sites table, that might work. Essentially, the cache key needs to change.

the cache key is currently something like:

wikidatawiki:sites/SiteList#2014-03-17+Site:2013-01-23

(the dates being constants tied to the format of SiteList and Site, so when those change the cache gets bumped. but those are not the only possible reasons for invalidating this cache)

Without doing anything, the cache has an expiry of an hour. Perhaps, alternatively we could make that somewhat shorter and then just be patient?