使用 Objectify 的 Appengine 内存缓存过期

Question

I am using objectify on appengine with the java runtime. I am also using memcache via the @Cached annotation for several entities. My question is about the behavior of the objectify when putting objects into the datastore. When putting an entity that has the @Cached annotation, is the memcache updated? Or any existing cached data for that entity now out of sync with the datastore. I would like to have the memcache updated when I put an object into the datastore, however I don't know if objectify does this by default or if I need to write this myself. If the memcache is updated, then I can have a much higher expiration time(or no expiration) for my data. FYI I am not using transactions.

Answer 1

When you use @Cached, Objectify handles all updates to the memcache for you in a near-transactionally safe manner. It's "near-transactional" because while it won't fall apart under contention, there are rare circumstances under which it could go out of sync - for example, if you hit a DeadlineExceededException or OutOfMemoryException and Google terminates your VM.

Long expiration times are reasonable for most types of cached data.

Answer 2

Using Objectify, your data in Memcache will never be out of sync with the Datastore (except in some older versions and in exceptional circumstances, such as a really unlucky deadline).

I Objectify will just invalidate the Memcache version (so the next "get" will go to the Datastore and write to the Memcache), rather than updating it, though I'm not sure about this bit. Either way, Objectify sorts it out for you.