Simulate failures

~5 minutesTemporal beginnerHands-on tutorial

One of Temporal's most important features is its ability to maintain Workflow state when something fails. Simulate some failures and see how Temporal responds.

Recover from a server crash

Unlike many modern applications, Temporal automatically preserves the state of your Workflow even if the server is down.

Stop your Worker with CTRL+C.
Start the Workflow again with dotnet run --project MoneyTransferClient.
Verify the Workflow is running in the UI.
Shut down the Temporal Server with CTRL+C.
Restart it and reload the UI.

Your Workflow is still listed:

The Workflow still appears

If the Temporal Cluster goes offline, you can pick up where you left off when it comes back online.

Recover from an unknown error in an Activity

To test this out, simulate a bug in the DepositAsync() Activity method. Let your Workflow continue to run but don't start the Worker yet.

Open MoneyTransferWorker/Activities.cs and uncomment the line that calls DepositThatFailsAsync in DepositAsync. Comment out the try-catch block below it.

Save your changes and start the Worker again:

dotnet run --project MoneyTransferWorker

You'll see the Worker complete WithdrawAsync() but error on DepositAsync() - and keep retrying using the RetryPolicy:

The Activity failing

note

Traditionally, you'd implement timeout and retry logic in your service code itself. With Temporal, you specify timeout configurations in the Workflow code as Activity options.

Your Workflow is running, but only WithdrawAsync() has succeeded. With Temporal, you can debug and fix the issue while the Workflow is running.

Pretend that you found a fix. Switch the comments back so DepositAsync() calls the regular bankService.DepositAsync. Save your changes.

Stop the Worker with CTRL+C, then restart it:

dotnet run --project MoneyTransferWorker

The Worker picks up right where the Workflow was failing and executes the fixed DepositAsync() Activity. Visit the Web UI again and you'll see the Workflow has completed:

Both Workflows completed

You have just fixed a bug in a running application without losing the state of the Workflow or restarting the transaction.

Conclusion

You now know how to run a Temporal Workflow and understand some of the value Temporal offers. You explored Workflows and Activities, you started a Workflow Execution, and you ran a Worker. You also saw how Temporal recovers from failures and retries Activities.

Further exploration

Try the following before moving on:

Change the Retry Policy so it only retries 1 time. Does the Workflow place the money back into the original account?

What's next?

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