Memcached provides a cache expiration time option, which specifies how long objects are retained in the cache. Assuming all writes are through the cache I fail to understand why one would ever want to remove an object from the cache. In other words, if all write operations update the cache before the DB, then the cache can never contain a stale object, so why remove it?
One possible argument is that the cache will grow indefinitely if objects are never removed, but memcached allows you to specify a maximum size. Once this size is reached, memcached uses a least-recently-used (LRU) algorithm to determine which items to remove. To summarise, if a sensible maximum size has been configured, and all writes are through the cache, why would you want to expire objects after a certain length of time?
Thanks, Don