App Engine Performance Hacks You'll Probably Never Need, Part 1

For example, one very common performance optimization in web applications is batching writes. Why not send the very compact results of a few A/B tests to the database/store once every ten requests, rather than once every request?

However, GAE doesn't really provide a way to do this. You could just store the results in local memory and then persist to the datastore every 10 requests, but there is no guarantee your instance won't get shut down. You could also store to memcache, and do what the author suggests, persisting from memcache to the datastore periodically. But my understanding is that there aren't any guarantees as to what the memcache size or revocation policy is, so it's not really possible to guarantee that all data gets persisted. (Losing random data intended to be persisted may be fine for A/B testing, but probably not for many other things.)

Likewise, it would be nice to delay datastore puts as the author does, or put them in a task queue, but both of those seem to take as long as doing a datastore put in the first place.

Is this analysis wrong? Does Google's memcache implementation provide some guarantees that mean that your data won't get removed as a result of your or someone else's actions without you knowing? Is there some reasonable pattern for storing persistent data on GAE but not having to force the user to wait?