Implement an expiration throttle

While at one end of the caching spectrum, event based expiration of cache means we can have longer living cache lifetimes and smarter logic around cache lifetime in general; at the other end of spectrum, a new issue arises...

What if you expire content too frequently? For example, if a node's comments are very active, say a new comment every 1-5 minutes plus "likes"/recommends clicked for each comment or vote, if we expired the cache on each event, then we are more likely to get a higher number of cache misses than we'd like.

However, if we introduced a minimum cache lifetime, then we could "throttle" the number of expiration calls to cache within that minimum period lifetime. So continuing with the example above, if we set the throttle value to 3 minutes, then we could at least guarantee the cache would be used and not generated and purged virtually immediately.

I've implemented this solution using Drupal's cache expiration system. How it works is, when a URL is expired, it is set as a cid in its own cache bin with a lifetime of the throttled value. In this way, the cached item will naturally expire itself. Now when the URL derivatives are being generated, the expire module checks the cache and any items it can find in the cache, it discards from the URL expire list. This provides a light weight path expiration management system to keep track of how long since an item was last expired.