Question

I have followed this tutorial: http://doc.akka.io/docs/akka/2.0/intro/getting-started-first-scala.html

Basically there is a Master actor which responds to these two messages:

def receive = {
    case Calculate => {
      //....
    }
    case Result(value) => {
      //....
    }
}

We send the message “Calculate” and after several “Result”s (from the slave actors) we have the correct calculation

Now I am inside an action of a play2 controller and I am using this:

val promise = (master ? Calculate)(10.seconds).mapTo[String].asPromise

Unfortunately (obviously) I get nothing because Calculate message does reply with a message to the sender.

I would like to somehow make the Akka Actor wait…. and when the computation is complete to send a message back to the sender.

But how?… Unless I am modelling it in the wrong way!

Answer 1

You should either forward the message to the slaves (which keeps the same sender), or include the sender in the message to the slaves. For example:

def receive = {
  case Calculate => slave ! CalculateWithSender(sender)

  case res @ Result(value, originalSender) =>        
    val result = resultMap(originalSender) + res   // assuming results are just added
    resultMap += (originalSender -> result) 
    if (resultsAreFinished(originalSender))    
      originalSender ! result
}

or

def receive = {
  case Calculate => slave.forward(Calculate)

  case res @ Result(value, originalSender) => 
    val result = resultMap(originalSender) + res   // assuming results are just added 
    resultMap += (originalSender -> result) 
    if (resultsAreFinished(originalSender))    
      originalSender ! result
}

or

def receive = {
  case Calculate => slave.tell(Calculate, sender)

  case res @ Result(value, originalSender) => 
    val result = resultMap(originalSender) + res   // assuming results are just added 
    resultMap += (originalSender -> result) 
    if (resultsAreFinished(originalSender))    
      originalSender ! result
}

I’m not familiar with Play promises, but ? (ask) returns a Akka Future. If .asPromise converts an Akka Future to a Play Promise then you’re set.