You can think of FDB as a distributed storage engine. It has the same low level data model as the engines you mention, but has distributed transactions, fault t

You can think of FDB as a distributed storage engine. It has the same low level data model as the engines you mention, but has distributed transactions, fault tolerance, automatic data partitioning, operational tooling, etc built in. So if you build an "application level database" or library on top of it, it is automatically a distributed database.