This sample shows you how you can verify changes to workflow code are compatible with existing workflow histories.

To run, first see README.md for prerequisites. Then, run the following from this directory to start the worker:

uv run worker.py

This will start the worker. Then, in another terminal, run the following to execute a workflow:

uv run starter.py

Next, run the replayer:

uv run replayer.py

Which should produce some output like:

WorkflowReplayResults(replay_failures={})

Great! Replay worked. Of course, the reason for the exercise is to catch if you've changed workflow code in a manner which is not compatible with the existing histories. Try it. Open up worker.py and change the JustActivity workflow to sleep just before running the activity. Add await asyncio.sleep(0.1) just before the line with workflow.execute_activity.

Now run the replayer again. The results from the replay_workflows call now indeed contains a failure! Something like:

WorkflowReplayResults(replay_failures={'e6418672-323c-4868-9de4-ece8f34fec53': NondeterminismError('Workflow activation completion failed: Failure { failure: Some(Failure { message: "Nondeterminism(\\"Timer machine does not handle this event: HistoryEvent(id: 8, Some(ActivityTaskScheduled))\\")", source: "", stack_trace: "", encoded_attributes: None, cause: None, failure_info: Some(ApplicationFailureInfo(ApplicationFailureInfo { r#type: "", non_retryable: false, details: None })) }) }')})

This is telling you that the workflow is not compatible with the existing history. Phew! Glad we didn't deploy that one.