Child Workflow Executions

In the examples so far, we have started workflow execution directly from an application. However, a workflow execution may be started from within a workflow by calling the workflow entry point method on the generated client. When a workflow execution is started from the context of another workflow execution, it is called a child workflow execution. This allows you to refactor complex workflows into smaller units and potentially share them across different workflows. For example, you can create a payment processing workflow and call it from an order processing workflow.

Semantically, the child workflow execution behaves the same as a standalone workflow except for the following differences:

1. When the parent workflow terminates due to an explicit action by the user—for example, by calling the TerminateWorkflowExecution Amazon SWF API, or it is terminated due to a timeout—then the fate of the child workflow execution will be determined by a child policy. You can set this child policy to terminate, cancel, or abandon (keep running) child workflow executions.

2. The output of the child workflow (return value of the entry point method) can be used by the parent workflow execution just like the Promise<T> returned by an asynchronous method. This is different from standalone executions where the application must get the output by using Amazon SWF APIs.

In the following example, the OrderProcessor workflow creates a PaymentProcessor child workflow:

@Workflow
@WorkflowRegistrationOptions(defaultExecutionStartToCloseTimeoutSeconds = 60,
              defaultTaskStartToCloseTimeoutSeconds = 10)
public interface OrderProcessor {

    @Execute(version = "1.0")
    void processOrder(Order order);
}

public class OrderProcessorImpl implements OrderProcessor {
    PaymentProcessorClientFactory factory
         = new PaymentProcessorClientFactoryImpl();

    @Override
    public void processOrder(Order order) {
        float amount = order.getAmount();
        CardInfo cardInfo = order.getCardInfo();

        PaymentProcessorClient childWorkflowClient = factory.getClient();
        childWorkflowClient.processPayment(amount, cardInfo);
    }

}

@Workflow
@WorkflowRegistrationOptions(defaultExecutionStartToCloseTimeoutSeconds = 60,
                 defaultTaskStartToCloseTimeoutSeconds = 10)
public interface PaymentProcessor {

    @Execute(version = "1.0")
    void processPayment(float amount, CardInfo cardInfo);

}

public class PaymentProcessorImpl implements PaymentProcessor {
    PaymentActivitiesClient activitiesClient = new PaymentActivitiesClientImpl();

    @Override
    public void processPayment(float amount, CardInfo cardInfo) {
        Promise<PaymentType> payType = activitiesClient.getPaymentType(cardInfo);
        switch(payType.get()) {
        case Visa:
            activitiesClient.processVisa(amount, cardInfo);
            break;
        case Amex:
            activitiesClient.processAmex(amount, cardInfo);
            break;
      default:
         throw new UnSupportedPaymentTypeException();
        }
    }

}

@Activities(version = "1.0")
@ActivityRegistrationOptions(defaultTaskScheduleToStartTimeoutSeconds = 3600,
                             defaultTaskStartToCloseTimeoutSeconds = 3600)
public interface PaymentActivities {

    PaymentType getPaymentType(CardInfo cardInfo);

    void processVisa(float amount, CardInfo cardInfo);

    void processAmex(float amount, CardInfo cardInfo);

}