Google App Engine: Which is its RDBMS?

According to this: http://code.google.com/appengine/docs/whatisgoogleappengine.html it seems that GAE only uses Datastore to store data, which is equivalent with Table service on Windows Azure Platform.

Does anyone know that which RDBMS it uses? or such thing exists or not?

EDIT: Windows Azure Platform, a cloud-computing platform by Microsoft, offers 2 options to store data:

• Windows Azure Storage service, which include Table service, which is similar with GAE Datastore. Both store data in entities, and their "tables" are schemeless
• SQL Azure, which is a RDBMS on the cloud, based on SQL Server. It supports most features of SQL Server, includes T-SQL

That's why I 开发者_运维技巧think it's hard to believe that Google App Engine doesn't offer a RDBMS. I searched, but found no confirmation. That's why I asked here

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App Engine doesn't use an RDBMS, though we did just announce that we'll be supporting SQL in the near future in App Engine for Business. The App Engine datastore is based on Bigtable.

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It is not a RDBMS in the usual sense of that word. In a relational database, data is stored in relations (commonly called tables) base on candidate keys. Every relation has a primary key, for a database to be normalized, all other data in a relation is related to the primary key, the whole key and nothing but the key.

From http://code.google.com/appengine/docs/python/datastore/:

The App Engine datastore is a schemaless object datastore, with a query engine and atomic transactions. The Python interface includes a rich data modeling API and a SQL-like query language called GQL.

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I'm not sure RDBMS applies to cloud computing!