Ilya Grigorik implements the stale-while-revalidate Cache-Control proposal with EventMachine and memcached:

The application logic is simple: if we have never seen this request, process, and cache it; if we've seen this request, and the cache is valid, then render response; if we've seen this response, but the cache is stale, render the stale version immediately, and then continue the process to update the cache. Also, to avoid the 'stampeding' effect, we've added a flag to mark a request as in-progress, to indicate that an application server is working on updating the cache.

caching