Ask pattern not working using akka streams - scala

Question

I try to implement a sample slightly different from the tutorial:
object Test extends App {
class A extends Actor {
override def receive: Receive = {
case 10 => context.system.terminate()
case x =>
println(s"Received: $x")
sender() ! x
}
}
implicit val system = ActorSystem()
implicit val materializer = ActorMaterializer()
implicit val dispatcher = system.dispatcher
implicit val askTimeout = Timeout(1, TimeUnit.SECONDS)
val a = system.actorOf(Props[A])
val graph = RunnableGraph.fromGraph(GraphDSL.create() {
{
implicit builder: GraphDSL.Builder[NotUsed] => {
import GraphDSL.Implicits._
val source: Source[Int, NotUsed] = Source(1 to 10)
source ~> Flow[Int].ask(2)(a) ~> Sink.foreach[String](println)
ClosedShape
}
}
})
val x = graph.run()
}
In result i get this:
Received: 1
Received: 2
But i expect this:
Received: 1
1
Received: 2
2
...
Received: 9
9
If i send messages to actor via Source.actorRef then all is ok and actor receive all messages. It seems like materialized actor wait a response from referenced actor.
How to fix it?

Answer 1

If you replace the Sink.foreach[String](println) with Sink.onComplete(println)
you will see that the flow terminates with an error:
Received: 1
Received: 2
Failure(java.lang.ClassCastException: Cannot cast java.lang.Integer to scala.runtime.Nothing$)
The problem is that Source.ask needs to know what class to map the actor's reply to see API docs. Otherwise it will try to cast the reply to Nothing and internally throw an exception.
To fix your original code replace ask with ask[Int]:
val graph = RunnableGraph.fromGraph(GraphDSL.create() {
{
implicit builder: GraphDSL.Builder[NotUsed] => {
import GraphDSL.Implicits._
val source: Source[Int, NotUsed] = Source(1 to 10)
source ~> Flow[Int].ask[Int](2)(a) ~> Sink.foreach(println)
ClosedShape
}
}
})