Kristina Chodorow covers scaling with the MongoDB. Mentioned on Alexander Popescu’s MyNoSQL blog.
Alexander is concerned about the complexity of the autosharding solution.
But high availability requires more than understanding the capabilities of a single database solution.
A firm understanding of the concerns in Philip A Bernstein and Eric Newcomer’s Principles of Transaction Processing and Jim Gray and Andreas Reuter’s Transaction Processing: Concepts and Techniques is a good starting point.
Whether you are planning high availability for a topic map or another application.