Question

This question is probably relatively basic to one familiar with akka Futures, so please bear with me.

Assuming I have a hierarchy of akka actors per the following structure:

BigBoss                 (one)
|___ExecutiveActor      (one)
    |___ManagerActor    (many)
        |___WorkerActor (many)

And assuming that each non-BigBoss has internal state in a Status wrapper.

If the BigBoss were to want a List<Status> from all descendants, I can see this being the method:

// inside BigBoss getAllStatuses
Future<Object> futureStatuses = executive.ask("status", 5000);
List<Status> = (List<Status>)Await.(futureStatuses, timeout);

And after the initial message is propagated to all the way down to Worker, I can see Worker replying to Manager with:

getSender().tell(myStatus);

Manager would have a lot of these coming back and want to put them in a list for the Executive as a list of Futures – the above reply being in response to:

// inside Manager getAllStatuses
List<Future<Object>> statuses =...
for(Worker w : workers) {
   Future<Object> status = w.ask("status", 5000);
   statuses.add(status);
}

Future<List<Object>> futureStatuses = sequence(statuses, ...);
getSender().tell(futureStatuses);

The above sequence first converts the Futures into a Future<List<Object>> (which really has statuses in it)

So this is where it starts to get a bit hairy to me.

ExecutiveActor had done this:

for(Manager m : managers) {
   Future<Object> status = m.ask("status", 5000);
   // the above Future would be a Future<Future<List<Object>>
}

So, finally – my questions –

1. How do I reduce the above Future<Future<List<Object>> to a Future<List<Object>> without blocking until BigBoss finally calls Await?

2. Is there a way to retain the Status type throughout this rather than using Object? I know an UntypedActor cannot send a explicit typed reply, but perhaps there is a cleaner way to express this?

3. Is there any more elegant approach to this entirely that I’m not thinking of? Did I use Sequence too early, or should I have not used it at all?

I am (obviously) using Java – and would prefer not to use TypedActors, but would love all feedback!

Thank you very much for your time-

Answer 1

Something like this?

final ActorRef cacheSender = getSender();
Future<List<Object>> futureStatuses = sequence(statuses, ...);
futureStatuses.onComplete( new Procedure2<Throwable, List<Object>>() {
  public void apply(Throwable t, Object r) {
    if(t != null) cacheSender.tell("ohnoes");
    else cacheSender.tell(r);
  }
});