Sharding Counters – Google App Engine – Google Code
If you had a single entity that was the counter and the update rate was too fast then you would have contention as the serialized writes would stack up and start to timeout. The way to solve this problem is a little counter-intuitive if you are coming from a relational database, and the solution relies on the fact that reads from the App Engine datastore are extremely fast and cheap since entities that have been recently read or updated are cached in memory.