From Metadata Partners:
Datomic is a database of flexible, time-based facts, supporting queries and joins, with elastic scalability, and ACID transactions. Datomic can leverage highly-available, distributed storage services, and puts declarative power into the hands of application developers. Datomic software consists of the peer library and the transactor. These components connect to one of storage service options. It brings declarative data manipulation into the application, and the data with it. Get time, process, and perception: process (writes) required coordination, perception (reads) require none, the past doesn't change. It can leverage immutability, and a sound model of state. It allows hierarchy, multi-valued attributes, minimal schema, reliable operation on unreliable, ephemeral cloud instances, and time.
Features include in free version system includes integrated transactor-local storage, peer library includes a memory database and Datomic Datalog, and transactor supports 2 simultaneous peers.