--- title: Find and fix a stuck observer --- A read model looks stale, or a reactor's side effect never fired. In an event-sourced system that usually means an **observer** — a projection, reducer, or reactor — hit an event it couldn't process and **paused that partition**. The rest of the system keeps running; only the failing event source is stuck. Here's how to find it and get it moving again. ## 1. Spot the stuck observer Start with the health check — it counts failed partitions across the whole store in one shot: ```bash cratis chronicle diagnose ``` If it reports failed partitions, list the observers and look at their state and sequence position: ```bash cratis chronicle observers list --output plain ``` An observer whose sequence number is behind the event log tail, or whose state isn't active, is your suspect. ## 2. Read the failure List the failed partitions to see *which* event source failed and on which observer: ```bash cratis chronicle failed-partitions list ``` Then read the actual error — the failing sequence number, the exception, and the attempt history: ```bash cratis chronicle failed-partitions show --detailed ``` `` is the event source id (for example `user-42`). The `--detailed` flag gives you the full stack trace; add `-o json` if you want to pipe it somewhere. ## 3. Fix the cause, then retry The failure is almost always a bug in the observer's code — a null it didn't expect, a missing case. Fix that in your application and redeploy. Then resume the partition **without** losing its state: ```bash cratis chronicle observers retry-partition ``` Retry re-processes the event that failed. For a transient error or a corrected bug, this is the right recovery path — the observer picks up where it stopped. ## 4. If the state is corrupt, replay instead If the partition's read-model state is wrong (not just stuck) — say a bug wrote bad data before it threw — retrying won't fix what's already there. Rebuild just that partition from sequence zero: ```bash cratis chronicle observers replay-partition ``` This discards the partition's accumulated state and rebuilds it from history. Other partitions are untouched. ## Done when ```bash cratis chronicle diagnose ``` reports no failed partitions and the observer's sequence number has caught up to the tail. If a whole observer is broken (a schema change, not one bad partition), see [Replay a projection](/cli/scenarios/replay-a-projection/) for a full rebuild.