07-debuggable-state/06-logging-checkpoints-replay.md

Logging, checkpoints, and replay

Good state architecture enables powerful debugging tools such as action logs, checkpoints, and replay.

These capabilities allow developers and LLMs to inspect how the system reached a certain state and reproduce issues reliably.

Action logs

When actions are explicit and reducers are predictable, the system can keep a log of actions that occurred.

An action log helps answer:

Action logs are much more useful when action names and payloads are clear.

Checkpoints allow the system to capture a snapshot of state at a specific moment.

This can be used to:

Checkpoints are especially useful during debugging sessions.

Replay means reapplying actions from the log to reconstruct how the state evolved.

With replay, developers can:

Replay works best when reducers are pure and state transitions are deterministic.

To support reliable replay:

This ensures that replaying the same actions leads to the same state.

Design reducers and actions so the state history can be inspected and replayed. This makes debugging dramatically easier for both humans and LLMs.