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Exercise #1: Handling Errors

During this exercise, you will:

  • raise and handle exceptions in Temporal Workflows and Activities
  • Use non-retryable errors to fail an Activity
  • Locate the details of a failure in Temporal Workflows and Activities in the Event History

Make your changes to the code in the practice subdirectory (look for TODO comments that will guide you to where you should make changes to the code). If you need a hint or want to verify your changes, look at the complete version in the solution subdirectory.

Setup

You'll need two terminal windows for this exercise.

  1. In all terminals, change to the exercises/handling-errors/practice directory using the following command: 
    cd exercises/handling-errors/practice

Part A: raise a non-retryable ApplicationError to fail an Activity

In this part of the exercise, you will raise a non-retryable Application Failure (represented in Python as ApplicationError) that will fail your Activities.

Application Failures are used to communicate application-specific failures in Workflows and Activities. In Activities, raising an ApplicationError will cause the Activity to fail. However, this unless this Activity is specified as non-retryable, it will retry according to the Retry Policy. To have an Activity fail when an ApplicationError is raised, set it as non-retryable. Any other exception that is raised in a Python activity is automatically converted to an ApplicationError upon being raised.

  1. Open the shared.py file and familiarize yourself with the dataclasses you will be using during the exercise. Pay particular attention to the custom error definitions.
  2. Open the activities.py file in your text editor.
  3. In the send_bill Activity, notice how an error is raised if the chargeAmount is less than 0. If the calculated amount to charge the customer is negative, a non-retryable ApplicationError is raised. It is important to use a non-retryable failure here, as you want to fail the Activity if the amount was calculated to be negative. In this error, we pass a reason for the failure and the type of error that is being converted to an ApplicationError.
  4. Go to process_credit_card Activity, where you will raise an ApplicationError if the credit card fails its validation step. In this Activity, you will raise an error if the entered credit card number does not have 16 digits. Use the ApplicationError code from the previous step as a reference. You should set the type as CreditCardProcessingError.
  5. Save your file.

Part B: Catch the ActivityError

In this part of the exercise, you will catch the ApplicationError that was raised from the processCreditCard Activity and handle it.

  1. Open the workflow.py in your text editor.
  2. Locate the line with that beings with: credit_card_confirmation = await workflow.execute_activity_method.
    1. This code calls the process_credit_card Activity, and if a non-retryable ApplicationError is raised, the Workflow will fail. However, it is possible to catch this failure and either handle it, or continue to propagate it up.
    2. Wrap this line in a try/catch block. However, you will not catch ApplicationError. Since the ApplicationError in the Activity is designated as non-retryable, by the time it reaches the Workflow it is converted to an ActvityError.
    3. Within the catch block, add a logging statement stating that the Activity has failed.
    4. After the logging statement, raise another ApplicationError passing in a message and setting CreditCardProcessingError as the type. This will cause the Workflow to fail, as you were unable to bill the customer.
  3. Save your file.

Part C: Run the Workflow

In this part of the exercise, you will run your Workflow and see both your Workflow and Activity succeed and fail.

In the starter.py file, a valid credit card number as part of an order has been provided to run this Workflow.

First, run the Workflow successfully:

  1. In the same terminal, start the Worker by running: 
    python worker.py
  2. In another terminal, start the Workflow by executing starter.py: 
    python starter.py
  3. In the Web UI, verify that the Workflow ran successfully to completion.

Next, you'll modify the starter data to cause the Workflow to fail:

  1. Open shared.py and modify the code 
     credit_card_info = CreditCardInfo(
     holderName="Lisa Anderson", number="2424242424242424"
 )
    by deleting a digit from the String. Save this file.
  2. Stop the worker using CMD-C on Mac or Ctrl-C on Windows/Linux
  3. Restart the Worker by running: 
    python worker.py
  4. In another terminal, start the Workflow by executing starter.py: 
    python starter.py
  5. You should see the Workflow fail in the terminal where you executed starter.py. Also check the WebUI and view the failure there.

This is the end of the exercise.