If you are used to traditional databases, then to only append and never update your data may sound like a crazy idea. However, not only does this enable historical queries, but also enhances fault tolerance and scalability. In this presentation we briefly describe two immutable data stores (Rich Hickey's Datomic and Greg Young's EventStore) and compare their different data models using an example problem domain. Along the way we learn about CQRS, Aggregates, Projections and why you want your data to be immutable.
EventStore is a data store for applications using event sourcing and time-series data. EventStore runs on .NET and Mono.
Datomic is a database of time-based facts, with declarative queries and ACID transactions. Datomic is written in Clojure and runs on the JVM.