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 ProcessCreditCard if you provide it with a bad credit card number.

Three new Activities have been created to demonstrate rollback actions.

• updateInventory is a new step that would run normally (whether or not the Workflow encounters an error).
• revertInventory has also been added as a compensating action for updateInventory.
• refundCustomer has been added as a compensating action for sendBill.

1. Review these new Activities at the end of the PizzaActivities.java and PizzaActivitiesImpl.java 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 files.

Part B: Add your new rollback Activities to your Workflow

Now you will implement a compensating action using Activities in your Temporal Workflow using the Saga object. Unlike other SDKs, the Temporal Java SDK provides a custom Saga class for handling compensations.

1. Open PizzaWorkflowImpl.java.
2. Note that a Saga object, saga, has been added at the top of the Workflow Definition to keep track of each Activities compensating action.
3. The updateInventory Activity invocation has been added to your Workflow after validating an order, before the sendBill Activity is called. The compensating action was added to the compensations object in the line above. Study this and use it for the next step.
4. Locate the invocation for the processCreditCard method. Add the appropriate compensation to the saga object, including the necessary parameters.
5. Add the following line to the catch block that is associated with the updateInventory and processCreditCard Activities.

   saga.compensate();

6. Compile your code using mvn clean compile.

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:

   mvn exec:java -Dexec.mainClass="pizzaworkflow.PizzaWorker"

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

   mvn exec:java -Dexec.mainClass="pizzaworkflow.Starter"

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

   CreditCardInfo cardInfo = new CreditCardInfo("Lisa Anderson", "4242424242424242");

   And delete the last 2 from the end. This will cause the processCreditCard Activity to fail, triggering a compensation.
7. Compile your code using mvn clean compile
8. Restart the Worker by running:

   mvn exec:java -Dexec.mainClass="pizzaworkflow.PizzaWorker"

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

   mvn exec:java -Dexec.mainClass="pizzaworkflow.Starter"

10. A short time after executing, you should see a stack trace appear stating that the Activity failed.
11. 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.