Are keys removed when memcache data expires?

Question

I'm currently working on adding memcache to an app running on GAE/J. I have a question about how production memcache behaves when values expire, if they're keys are also removed.

The closest I could find to answering this question is here: http://code.google.com/appengine/docs/java/memcache/overview.html#How_Cached_Data_Expires

I know I can't rely on values staying in memcache; and I don't, but what I am wondering, is if the keys which map to those values are also removed.

In other words, if I do

mycache.contains("key")

Will this still be true after a value is pushed out of the cache?

Side note: Don't think it makes a difference; but just in case, I'm using the async memcache from the memcache service and get a hold of it like this:

MemcacheServiceFactory.getAsyncMemcacheService();

I'm asking this because I'm not doing the typical

if(cache.get("key") == null)

I store null values when they come back from the datastore so I don't constantly lookup the null value. And I do take care to make sure to remove the key when it can possibly change. So just because the memcache lookup returns null, I work on the assumption that the query is in fact null; but I do ensure that when other queries which could change that happen that the null lookup gets removed. That is why I'm using cache.contains("key") as opposed to cache.get("key") == null

Answer 1

Your assumption is correct, mycache.contains("key") will return false if the value had been evicted from the memcache service. This behavior is documented in the Javadoc of the low-level Memcache API.