Ten years ago I worked with a database called Versant OODBMS (from Actian). I was a junior sysadmin so i was essentially administering it at a very surface level but skimming the documentation (and trying some of the samples) it was very cool that you could pick essentially any random class, implement an interface (and hence a few method) and that was it, you had a database-serializable object.
The main issue was really scaling out (as in, multiple machines) but otherwise was a really great database.
* AFAICT, this is not (yet) in active use in production YouTrack, which is still using https://github.com/JetBrains/xodus
* the technical leader of the YouTrackDB project, Andrii Lomakin, was one of the main contributors to OrientDB -- see https://medium.com/@youtrackdb
* YouTrackDB is a fork of OrientDB
* this project makes heavy use of Claude Code
* YouTrackDB does not use Kotlin
* this can be used as a network service (rather than just embedded in a single runtime process like in current production YouTrack), but I've not yet got the docker images working to try.
from the blurb
YouTrackDB is a general-purpose object-oriented graph database developed by JetBrains, used internally in production. It implements the Apache TinkerPop API with Gremlin query language support and features O(1) link traversal, schema-less/mixed/full modes, and encryption at rest. The project is a fork of OrientDB, re-architected under the com.jetbrains.youtrackdb package namespace.