"PHP cache slam defense using (memcached) semaphore to prevent dogpile effect (aka clobbering updates, stampending herd or Slashdot effect).

Problem: too many requests hit your website at the same time to regenerate same content slamming your database. It might happen after the cache was expired.

Solution: first request generates new content while all the subsequent requests get (stale) content from cache until it's refreshed by the first request."