FlockDB was an open-source distributed, fault-tolerant graph database for managing wide but shallow network graphs.

[3] It was initially used by Twitter to store relationships between users, e.g. followings and favorites.

FlockDB differs from other graph databases, e.g. Neo4j in that it was not designed for multi-hop graph traversal but rather for rapid set operations, not unlike the primary use-case for Redis sets.

[4] FlockDB was posted on GitHub shortly after Twitter released its Gizzard framework, which it used to query the FlockDB distributed datastore.

[1] Twitter no longer supports FlockDB.