Look at Titan (http://thinkaurelius.github.com/titan), a new distributed OLTP graph database that has a storage layer that adds distributed transactions to pluggable backends, such as HBase and Cassandra. It's by the team the created Tinkerpop Blueprints and Gremlin, the graph traversal language.

You can read more about it in Matthias's PhD dissertation (http://www.knowledgefrominformation.com/category/publication...).

Also see Calvin:

"Calvin can run 500,000 transactions per second on 100 EC2 instances in Amazon’s US East (Virginia) data center, it can maintain strongly-consistent, up-to-date 100-node replicas in Amazon’s Europe (Ireland) and US West (California) data centers---at no cost to throughput."

"Calvin is designed to run alongside a non-transactional storage system, transforming it into a shared-nothing (near-)linearly scalable database system that provides high availability and full ACID transactions. These transactions can potentially span multiple partitions spread across the shared-nothing cluster. Calvin accomplishes this by providing a layer above the storage system that handles the scheduling of distributed transactions, as well as replication and network communication in the system. The key technical feature that allows for scalability in the face of distributed transactions is a deterministic locking mechanism that enables the elimination of distributed commit protocols."

http://cs.yale.edu/homes/thomson/publications/calvin-sigmod1...

And Omid (https://github.com/yahoo/omid) is another somewhat similar system, but it only works with HBase. Here's a comparison from the Omid team: https://groups.google.com/d/msg/omid-project/BTue2jAH1iQ/ZP3...