What is a good resource for understanding GAE datastore architecture?

Is there a detailed analysis of Google App Engine's datastore architecture somewhere? What I am looking for is a resource that can help me:

1. Understand why a particular restriction exist in datastore (e.g transactions requiring same entity group)
2. Build a deeper understanding of storage mechanics that he开发者_高级运维lp me mentally visualize efficiency of a particular data model.

GAE documentation have some good individual articles, but I am looking for more detail treatment, a book perhaps.

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You could start with presentation from Google I/O:

• App Engine Datastore Under The Covers

The presentation on Appstats also gives some insight into how datastore (and RPC calls in general) work, as well as giving some optimization tips.