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The Google Datastore is built on Megastore. Megastore's data model is based on entity groups, that represent fine-grained, application-defined partitions (e.g. a user's message inbox). Transactions are supported per co-located entity group, each of which is mapped to a single row in BigTable that offers row-level atomicity. Transactions spanning multiple entity groups are not encouraged, as they require expensive two-phase commits. Megastore uses synchronous wide area replication. The replication protocol is based on Paxos consensus over positions in a shared write-ahead log.

The reason for the Datastore only allowing very limited queries is that they seek to target each query to an entity group in order to be efficient. Queries using the entity group are fast, auto-indexed and consistent. Global indexes, on the other hand, are explicitly defined and only eventually consistent (similar to DynamoDB). Any query on unindexed properties simply returns empty results and each query can only have one inequality condition [1].

[1] https://cloud.google.com/datastore/docs/concepts/queries#ine...