Daemon Tasks

The AWS Flow Framework for Java allows the marking of certain tasks as daemon. This allows you to create tasks that do some background work that should get canceled when all other work is done. For example, a health monitoring task should be canceled when the rest of the workflow is complete. You can accomplish this by setting the daemon flag on an asynchronous method or an instance of TryCatchFinally. In the following example, the asynchronous method monitorHealth() is marked as daemon.

public class MyWorkflowImpl implements MyWorkflow {
  MyActivitiesClient activitiesClient = new MyActivitiesClientImpl();

  @Override
  public void startMyWF(int a, String b) {
    activitiesClient.doUsefulWorkActivity();
    monitorHealth();
  }

  @Asynchronous(daemon=true)
  void monitorHealth(Promise<?>... waitFor) {
    activitiesClient.monitoringActivity();
  }
}

In the above example, when doUsefulWorkActivity completes, monitoringHealth will be automatically canceled. This will in turn cancel the whole execution branch rooted at this asynchronous method. The semantics of cancellation are the same as in TryCatchFinally. Similarly, you can mark a TryCatchFinally daemon by passing a Boolean flag to the constructor.

public class MyWorkflowImpl implements MyWorkflow {
    MyActivitiesClient activitiesClient = new MyActivitiesClientImpl();

    @Override
    public void startMyWF(int a, String b) {
        activitiesClient.doUsefulWorkActivity();
        new TryFinally(true) {
            @Override
            protected void doTry() throws Throwable {
                activitiesClient.monitoringActivity();
            }

            @Override
            protected void doFinally() throws Throwable {
                // clean up
            }
        };
    }
}

A daemon task started within a TryCatchFinally is scoped to the context it is created in—that is, it will be scoped to either the doTry(), doCatch(), or doFinally() methods. For example, in the following example the startMonitoring asynchronous method is marked daemon and called from doTry(). The task created for it will be canceled as soon as the other tasks (doUsefulWorkActivity in this case) started within doTry() are complete.

public class MyWorkflowImpl implements MyWorkflow {
    MyActivitiesClient activitiesClient = new MyActivitiesClientImpl();

    @Override
    public void startMyWF(int a, String b) {
        new TryFinally() {
            @Override
            protected void doTry() throws Throwable {
                activitiesClient.doUsefulWorkActivity();
                startMonitoring();
            }

            @Override
            protected void doFinally() throws Throwable {
                // Clean up
            }
        };
    }

    @Asynchronous(daemon = true)
    void startMonitoring(){
      activitiesClient.monitoringActivity();
    }