Replay a projection

You changed how a read model is built — added a field, fixed a mapping, reshaped the projection — and now the existing read model is out of date. Because the events are still the source of truth, you don’t migrate the read model: you replay the observer and let it rebuild from history.

Caution

Replay is destructive to the read model: the observer discards all accumulated state and rebuilds it from sequence zero. The event log is untouched — that’s the whole point — but the read model is unavailable or incomplete while it catches up. Prefer replay-partition when only one event source is affected.

1. Find the observer

Projections, reducers, and reactors are all observers. List them and grab the id of the one whose read model you changed:

cratis chronicle observers list --type projection

Use -q to get just the ids if you want to script it:

cratis chronicle observers list --type projection -q

2. Replay it

cratis chronicle observers replay <OBSERVER_ID>

The command asks for confirmation first — replay re-processes every event the observer subscribes to. In an automated pipeline, skip the prompt:

cratis chronicle observers replay <OBSERVER_ID> --yes

3. Watch it catch up

Replay runs in the background. Watch the health view refresh until the observer reaches the tail:

cratis chronicle diagnose --watch

Or check the observer’s sequence position directly:

cratis chronicle observers show <OBSERVER_ID>

Done when

The observer’s sequence number matches the event log tail and its read model reflects the new shape. Spot-check the rebuilt data with read-models or by reading the events it was built from.