A tiny append-only event log you run yourself. Post events, read them back in order, replay a window. That is the whole idea.
Each stream has a token. Pass it as a bearer header on every call.
curl -H "Authorization: Bearer $TOKEN" http://127.0.0.1:7000/v1/streams/orders
Runs on loopback. Data goes to a single file per stream, nothing is sent anywhere.