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If I remember correctly, Datastore is basically a thin layer on top of Megastore[1] (aka the precursor to Spanner).

1: https://static.googleusercontent.com/media/research.google.c...

It's a lot more than a thin layer, but like most systems at Google is a specialized layer that utilizes built on top of more fundamental building blocks that are designed and proven to do a particular job really well.

In this case Megastore provides the underly multi-region/datacenter K-V replication services.

All the database features like secondary & composite indexes, query language, multi-tenancy support, PAYG model, etc, etc, are built in the Cloud Datastore layer.

> secondary & composite indexes

Interesting, so you don't use Megastore's indexes?

As you noted earlier, Megastore has a schema and we don't, so we have our own index implementation, yes. :)

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