How to compile a sequence of results using ask pattern - scala

Question

Say I have a worker actor that receives a message, does a bit of processing and returns a result. And I have a sequence of messages that need to be converted into a sequence of results:
object Test {
case class Message(str: String)
case class Result(str: String)
class Worker extends Actor {
def receive = {
case Message(data) =>
println("Sleeping: " + data)
Thread.sleep(10000)
val result = Result(data + " - result")
println("Sending result: " + result)
sender ! result
}
}
def test(messages: Seq[Message]): Future[Seq[Result]] = {
val worker = ActorSystem().actorOf(Props(new Worker))
val results = messages.map { m =>
implicit val timeout = Timeout(20 seconds)
println("Sending: " + m)
val result = worker ? m
result.asInstanceOf[Future[Result]]
}
Future.sequence(results)
}
def main(args: Array[String]): Unit = {
val messages: Seq[Message] = args.map(Message(_))
test(messages).foreach { r =>
println("Result: " + r)
}
}
}
If I run the above with just "message-1" as an argument it runs fine giving the the output is below:
Sending: Message(message-1)
Sleeping: message-1
Sending result: Result(message-1 - result)
Result: ArraySeq(Result(message-1 - result))
However say I do it with: "message-1" "message-2" "message-3" then the last message ends up being sent to deadLetters:
Sending: Message(message-1) Sending: Message(message-2) Sleeping:
message-1 Sending: Message(message-3)
Sending result: Result(message-1 - result)
Sleeping: message-2
Sending result: Result(message-2 - result)
Sleeping: message-3
Sending result: Result(message-3 - result)
[INFO] [07/15/2016 09:07:49.832]
[default-akka.actor.default-dispatcher-2] [akka://default/deadLetters]
Message [util.Tester$Result] from
Actor[akka://default/user/$a#1776546850] to
Actor[akka://default/deadLetters] was not delivered. [1] dead letters
encountered. This logging can be turned off or adjusted with
configuration settings 'akka.log-dead-letters' and
'akka.log-dead-letters-during-shutdown'.
I am guessing this is because my calling thread has gone out of scope by the time the last message is sent. How can correctly collect all results into a sequence?
Note that changing my test method to below gives the same results:
def test(messages: Seq[Message]): Future[Seq[Result]] = {
val worker = ActorSystem().actorOf(Props(new Worker))
Future.traverse(messages) { m =>
implicit val timeout = Timeout(20 seconds)
println("Sending: " + m)
val result = worker ? m
result.asInstanceOf[Future[Result]]
}
}

Answer 1

The dumb answer is:
Future.traverse(messages)(m => actor ? m).map(_.asInstanceOf[Result])
But it might be better to send data all at once:
class Worker extends Actor {
def receive = {
case Message(data) =>
// Convert data into result
...
sender ! result
case seq: Seq[Message] =>
...
sender ! results
}
}

Answer 2

Seems to be because my timeout was set too low. Should have been large enough to cover all the work - for example 40 seconds.