AWS Flow Framework Basic Concepts: Task Lists and Task Execution

Amazon SWF manages workflow and activity tasks by posting them to named lists. Amazon SWF maintains at least two task lists, one for workflow workers and one for activity workers.

The following excerpt from the HelloWorldWorkflow host application creates a new activity worker and assigns it to the HelloWorldList activities task list.

public class GreeterWorker  {
    public static void main(String[] args) throws Exception {
    ...
    String domain = " helloWorldExamples";
    String taskListToPoll = "HelloWorldList";

    ActivityWorker aw = new ActivityWorker(service, domain, taskListToPoll);
    aw.addActivitiesImplementation(new GreeterActivitiesImpl());
    aw.start();
    ...
  }
}

By default, Amazon SWF schedules the worker's tasks on the HelloWorldList list. Then the worker polls that list for tasks. You can assign any name to a task list. You can even use the same name for both workflow and activity lists. Internally, Amazon SWF puts workflow and activity task list names in different namespaces, so the two lists will be distinct.

If you don't specify a task list, the AWS Flow Framework specifies a default list when the worker registers the type with Amazon SWF. For more information, see Workflow and Activity Type Registration.

Sometimes it's useful to have a specific worker or group of workers perform certain tasks. For example, an image processing workflow might use one activity to download an image and another activity to process the image. It's more efficient to perform both tasks on the same system, and avoid the overhead of transferring large files over the network.

To support such scenarios, you can explicitly specify a task list when you call an activity client method by using an overload that includes a schedulingOptions parameter. You specify the task list by passing the method an appropriately configured ActivitySchedulingOptions object.

For example, suppose that the say activity of the HelloWorldWorkflow application is hosted by an activity worker different from getName and getGreeting. The following example shows how to ensure that say uses the same task list as getName and getGreeting, even if they were originally assigned to different lists.

public class GreeterWorkflowImpl implements GreeterWorkflow {
  private GreeterActivitiesClient operations1 = new GreeterActivitiesClientImpl1(); //getGreeting and getName
  private GreeterActivitiesClient operations2 = new GreeterActivitiesClientImpl2(); //say
  @Override
  public void greet() {
    Promise<String> name = operations1.getName();
    Promise<String> greeting = operations1.getGreeting(name);
    runSay(greeting);
  }
  @Asynchronous
  private void runSay(Promise<String> greeting){
    String taskList = operations1.getSchedulingOptions().getTaskList();
    ActivitySchedulingOptions schedulingOptions = new ActivitySchedulingOptions();
    schedulingOptions.setTaskList(taskList);
    operations2.say(greeting, schedulingOptions);
  }
}

The asynchronous runSay method gets the getGreeting task list from its client object. Then it creates and configures an ActivitySchedulingOptions object that ensures that say polls the same task list as getGreeting.