Memcache doesn't work like that, items don't expire or get deleted to make room for new items. Memcache can expire items according to how you configure it, or not expire items at all. I haven't looked at any of the memcache session stores for ruby, but in most cases memcache is used with some sort of permanent backing storage, because memcache just uses memory and if you stop memcached you lose everything in the cache.
Chris
