FoundationDB is a key/value store, and well-known for being fast, scalable and incredibly reliable. It passed many of the distributed database tests unlike many competitors. Here's a good HN thread when Apple open-sourced it a few years ago: https://news.ycombinator.com/item?id=16877395

CockroachDB implements postgres-compatible SQL layer on top of their own key/value store called Pebble. This is similar to TiDB which implements mysql on top of their TiKV key/value store.

The latter databases offer easier programming with SQL interfaces, but since you can model just about anything on top of key/value store, there are already "layers" that can implement similar abstractions on top of FDB like the Record Layer[1], Document Layer [2], and even a SQL layer [3] at one point.

1) https://www.foundationdb.org/blog/announcing-record-layer/ 2) https://foundationdb.github.io/fdb-document-layer/ 3) https://github.com/jaytaylor/sql-layer