How often does memcache on Google AppEngine lose data?

Question

Memcache in general and on AppEngine in specific is unreliable in the sense that my data may be deleted from the cache for whatever reason at any point in time. However, in some cases there might be cases where a small risk may be worth the added performance using memcache could give, such as updating some data in memcache that gets saved periodically to some other, more reliable storage. Are there any numbers from Google that could give me an indication of the actual probability that a memcache entry would be lost from the cache before its expiration time, given that I keep within my quotas?

Are there any reasons other than hardware failure and administrative operations such as machines at the data centers being upgraded/moved/replaced that would cause entries to be removed from memcache prematurely?

Answer 1

Memcache, like any cache, should be used as... a cache. If you can't find something in the cache, there must be a strategy to find it in permanent storage.

In addition to the reasons you mention, Memcache and other caching approaches have limits to the amount of items they will hold (discarding usually the least recently used ones when the cache is full), and often also set other cache invalidation policies (e.g. flush everything unused for one hour).

If you don't configure and operate the cache yourself, you have NO guarantee of when and how items might be removed from the cache intentionally / by design.