I'm still in the exploration/experimentation stage of the project, but I'm currently using a mixture of SQLite, PostgreSQL, S3, and DuckDB.

My original thought was to spin up SQLite databases as needed because they are super lightweight, well-tested, and supported by almost every programming language. If you want to set up an agent in another programming language via MCP, but you still want to be able to access the agent memory directly, you can use the same schema in a SQLite database.

I may end up using mnesia for more metadata or system-oriented data storage though. It's very well designed imo.

But one of the biggest reasons has just been the really nice integration with DuckDB. I can query all of the SQLite databases persisted in a directory and aggregate some metadata really easily.