Apache NiFi Registry System Administrator's Guide
Also available as:

Metadata Database

The metadata database maintains the knowledge of which buckets exist, which versioned items belong to which buckets, as well as the version history for each item.

Currently, NiFi Registry supports using H2, Postgres 9.x, and MySQL (5.6, 5.7, 8.0) for the relational database engine.

NiFi Registry 0.1.0 only supports H2.