perhaps a NATS server colocated on each monolith server (or even embedded in your app, if it is written in golang, meaning that all communication is in-process) and use NATS KV?

Or if you just want it all to be in-memory, perhaps use some other non-distributed caching library and do the replication via NATS? Im sure there's lots of gotchas with something like that, but Marmot is an example of doing SQLite replication via NATS Jetstream

edit: actually, you can set jetstream/kv to be in-memory rather than file persistence. So, it could do the job of olric or rolling your own distributed kv via nats. https://docs.nats.io/nats-concepts/jetstream/streams#storage...