In-memory event-sourced ACID-transactional replicated object graph engine. What? Can you say that again? Ok, it’s an application server that keeps all your data in RAM. It runs without a database because all the transactions are recorded in a log and used to restore the state of the application at startup or when setting up a replica. You define the object model, commands and queries using C#. Besides being very simple Memstate is also very fast and will outperform any relational database.
In the Haskell world, a similar tool is acid-state.