Distributed database backend for keystone

The rough plan is to write nosql driver for different backends of keystone. The backend is going to be MagnetoDB which provides an api similar to Amazon DynamoDB on top of cassandra. The main benefit from this backend is fault tolerance. In distributed environment nodes can go down any time and network partitions happen. We plan to address these issues by writing this backend.

This is going to be optional and the original mysql driver of course will be there.