Yes, agreed on SQLite/Postgres. But I'm going to benchmark RocksDB next and see what the performance characteristics are. I suspect the LSM tree storage engine of RocksDB might perform better since agents are so write heavy when running highly concurrent workloads. After all, you are streaming LLM tokens into disk and fanning them out to subscribed clients.
SQLite is surprisingly performant for single node applications even when comparing to Postgres. Postgres consumes a lot more memory and requires IO to hop through IPC whereas you can keep everything in process in SQLite with a shared connection pool.
I've been testing different storage engines for my agent harness and I can get up to 7.5k concurrent sessions on a single vCPU with SQLite whereas Postgres crashes or runs out connections.