Did you check this?

https://docs.nats.io/nats-streaming-concepts/intro

"..Message/event persistence - NATS Streaming offers configurable message persistence: in-memory, flat files or database. The storage subsystem uses a public interface that allows contributors to develop their own custom implementations."

and

"At-least-once-delivery - NATS Streaming offers message acknowledgements between publisher and server (for publish operations) and between subscriber and server (to confirm message delivery). Messages are persisted by the server in memory or secondary storage (or other external storage) and will be redelivered to eligible subscribing clients as needed."