README.md

Exercise #3: Rollback with the Saga Pattern

During this exercise, you will:

• Orchestrate Activities using a Saga pattern to implement compensating transactions
• Handle failures with rollback logic

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 & Prerequisites

1. In all terminals, change to the exercises/sagas/practice directory using the following command:

   cd exercises/sagas/practice

Part A: Review your new rollback Activities and custom Error

This Exercise uses the same structure as in the previous Exercises — meaning that it will fail at the very end on process_credit_card if you provide it with a bad credit card number.

Three new Activities have been created to demonstrate rollback actions.

• update_inventory is a new step that would run normally (whether or not the Workflow encounters an error).
• revert_inventory has also been added as a compensating action for update_inventory.
• refund_customer has been added as a compensating action for send_bill.

1. Review these new Activities at the end of the activities.py file. None of them make actual inventory or billing changes, because the intent of this Activity is to show Temporal features, but you should be able to see where you could add functionality here.
2. Close the file.

Part B: Add your new rollback Activities to your Workflow

Now you will implement a compensating action using Activities in your Temporal Workflow.

1. Open workflow.py.
2. Note that a list, compensations, has been added at the top to keep track of each Activities compensating action.
3. The update_inventory Activity invocation has been added to your Workflow after validating an order, before the send_bill Activity is called. The compensating action was added to the compensations list in the line above. Study this and use it for the next step.
4. Locate the invocation for the process_credit_card method. Add the appropriate dict to the compensations list, containing the compensating Activity and input. Use the previous step as a reference.

Part C: Test the Rollback of Your Activities

Now that you've implemented the Saga pattern, it's time to see it in action. You'll first execute the Workflow successfully, then introduce an error causing a rollback.

1. In one terminal, start the Worker by running:

   python worker.py

2. In another terminal, start the Workflow by executing:

   python starter.py

3. The Workflow should complete successfully. Verify its status is Completed in the Web UI.
4. Kill the worker using ^C
5. Open shared.py
6. Locate the following line:

    credit_card_info = CreditCardInfo(
        holderName="Lisa Anderson", number="4242424242424242"
    )

   And delete the last 2 from the end. This will cause the process_credit_card Activity to fail, triggering a compensation.
7. Restart the Worker by running:

   python worker.py

8. In another terminal, start the Workflow again by executing:

   python starter.py

9. A short time after executing, you should see a stack trace appear stating that the Activity failed.
10. Check the Workflow Execution in the Web UI. The Workflow will still be marked Failed, as the error that was raised in the Activity is re-thrown after the compensation is complete. Locate the following:

• Where did the Activity Task fail? What was the error message?
• Where did the compensations take place?
  • Hint: Look for Customer Refunded and Reverted changes to inventory.

You have now implemented the Saga pattern using Temporal.

This is the end of the exercise.