SWF: End workflow based on Promise

Question

I have a decider that calls an activity. The activity returns a list of validation errors. If the decider receives that list and it's not empty, I want to exit out of the workflow.

To exit the workflow I can just return in the decider, and it exits the workflow. The problem is that I'm not able to get the result from the activity. I'd have expected this to work, but it doesn't.. violations.isReady() always returns false:

public class WorkflowImpl implements Workflow {
    ActivitiesClient activities;

    public Workflow(/*...*/) {
        // ...
    }

    @Override
    public void do(WorkflowInput input) {
        Promise<List<String>> violations = activities.validate(input);

        if (!violations.isReady()) { // "do()" will be called when violations is ready.. right?
            return;
        } else if (!CollectionUtils.isEmpty(violations.get())) {
            return; // or throw ValidationException
        }

        // do other stuff
    }
}

I don't want to do the alternative which is to poll for violations.isReady() to be true, because this will tie up the decider thread for who knows how long. I'm not even sure if that would work.

Help?

Maxim Fateev · Accepted Answer · 2016-08-07T06:53:53.513

The workflow code is asynchronous. So a Promise returned from an activity is never ready in the same callback. So in your code violations.isReady() always returning false is correct behavior. You have to use a method annotated as @Asynchronous or Task to associate callback to a Promise. So your code should look like:

public class WorkflowImpl implements Workflow {
  ActivitiesClient activities;

  public Workflow(/*...*/) {
      // ...
  }

  @Override
  public void do(WorkflowInput input) {
    Promise<List<String>> violations = activities.validate(input);
    processValidationResult(violations);
  }

  @Asynchronous
  private void processValidationResult(Promise<List<String>> violations) {
    // As method is @Asynchronous framework guarantees violations is ready 
    // when its body is executed.
    if (!CollectionUtils.isEmpty(violations.get())) {
        return; // or throw ValidationException
    }
    // do other stuff
  }
}

Note that AspectJ should be configured correctly with Flow Framework Aspects for @Asynchronous annotation to take effect.

Another option is to use Task directly:

public class WorkflowImpl implements Workflow {
  ActivitiesClient activities;

  public Workflow(/*...*/) {
      // ...
  }

  @Override
  public void do(WorkflowInput input) {
    Promise<List<String>> violations = activities.validate(input);
    new Task(violations) {
       @Override
       public void  doExecute() {
         // Framework guarantees violations is ready 
         // (as it is Task constructor parameter)
         // when execute method is executed.
         if (!CollectionUtils.isEmpty(violations.get())) {
             return; // or throw ValidationException
         }
         // do other stuff
       }
    };
  }
}

I would recommend reading through SWF Recipes and samples to get better idea about patterns used when writing SWF workflows.

Also make sure that you go through TryCatchFinally javadoc as AWS Flow Framework error handling while very powerful is quite different from what most people are used to.

Added to show example of returning value from processValidationResult(...):

  @Override
  public void do(WorkflowInput input) {
    Promise<List<String>> violations = activities.validate(input);
    Promise<String> whatever = processValidationResult(violations);
    processNextStep(whatever);
  }

  @Asynchronous
  private Promise<String> processValidationResult(Promise<List<String>> violations) {
    // As method is @Asynchronous framework guarantees violations is ready 
    // when its body is executed.
    if (!CollectionUtils.isEmpty(violations.get())) {
        throw new ValidationException(...);
    }
    // do other stuff
    return Promise.asPromise("result string");
  }

  @Asynchronous
  private void processNextStep(Promise<String> whatever) {
     ...
  }

In your example, returning from the async method would not cause a return in the workflow method. I read through the recipes and examples, but couldn't find one where the async method returned something and that value was used in the workflow. — Kias, Sep 03 '15 at 03:37
From async method you can return either void or Promise. I've added an example to my post. — Maxim Fateev, Sep 03 '15 at 04:09
It all makes sense now. I've gotten it working by using an @Asynchronous method. It seems that nesting \@Asynchronous methods is the appropriate technique for handling the callbacks. — Kias, Sep 04 '15 at 04:12

SWF: End workflow based on Promise

1 Answers1