Dead Letters using Akka to create a message ring

Question

I'm trying to create an example Akka application using remote actors. The goal is to create for example 16 actors that exchange messages in a sequential manner (actor 16 talks to actor 15, 15 to 14, etc, and 1 talks to actor 16). However, I'm having trouble with the communication, since I continuosly have this error.

[INFO] [05/04/2017 15:45:58.248] [ActorFlasks-akka.actor.default-dispatcher-4] [akka://ActorFlasks/deadLetters] Message [java.lang.String] from Actor[akka://ActorFlasks/user/16#-2022012132] to Actor[akka://ActorFlasks/deadLetters] was not delivered. [1] dead letters encountered.

To do this, I run 16 terminal instances of the application, always with a different configuration file. I create the actorsystem in each instance like so:

object Main extends App {

    val localId = args(0)

    val configFile = getClass.getClassLoader.getResource(s"application$localId.conf").getFile
    val config = ConfigFactory.parseFile(new File(configFile))
    val system = ActorSystem("ActorFlasks" , config)
    val remote = system.actorOf(Props[CyclonManager], name=localId)

    remote ! "START"
}

An example of a configuration file is this:

akka {
  actor {
    provider = remote
  }
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = "localhost"
      port = 50001
    }
 }
}

And the actor is defined like this:

class CyclonManager extends Actor {

  def propagateMessage(): Unit = {
    val localId = self.path.name.toInt
    val currentPort = 50000 + localId
    val nextHopPort = if (currentPort == 50001) 50016 else currentPort - 1
    val nextHopId = localId-1

    val nextHopRef = context.actorSelection(s"akka.tcp://ActorFlasks@localhost:$nextHopPort/user/$nextHopId")

    nextHopRef ! "NEXT"
  }

  override def receive: Receive = {
    case "START" =>
      if (self.path.name == "16") {
        propagateMessage()
      }
    case "NEXT" =>
      propagateMessage()
    case _ =>
      println("Unrecognized message")
  }
}

It is a simple example to get me started, but I can't get it working no matter what I try. Does someone know where I'm failing?

Thank you in advance,

EDIT:

akka {
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  }
  remote {
    enabled-transports = ["akka.remote.netty.tcp"]
    netty.tcp {
      hostname = "localhost"
      port = 50015
    }
 }
}

Unless I've a wrong understanding of how to create actors and actorsystems, yes, I'm sure. — PablodeAcero, May 04 '17 at 16:20

score 1 · Accepted Answer · answered May 04 '17 at 21:47

1

After reconstructing and running your example I found one mistake in the propagateMessage function.

val nextHopId = localId-1

Should be

val nextHopId = if (currentPort == 50001) 16 else localId-1

If it doesn't solve your issue, try running my quick and dirty but working code and see how is it different from yours: https://gist.github.com/grantzvolsky/4a53ce78610038a9d44788d7151dc416

In my code I only used actors 14, 15, and 16. You can run each using sbt "run 16", etc.

answered May 04 '17 at 21:47

Yes, I saw that mistake only after submitting the question, but fixing it didn't help. I changed my code to yours, but it still doesn't work... Can the problem be with my localhost configuration maybe? Or with the version of akka I'm using (2.5.0) – PablodeAcero May 05 '17 at 13:19
I did add the `akka-remote` dependency, it is at the last line of my build.sbt. In theory your firewall could be blocking the communication, but it seems unlikely. Would you provide the full output of all three nodes? Here's mine: (note that I first start node 14, then 15 and then 16): http://imgur.com/a/l53pV – May 05 '17 at 13:21
Hi! I tried you code and configuration. The nodes 14 and 15 have a normal output. However, the node 16 gives me the following: http://imgur.com/a/tZi6T – PablodeAcero May 05 '17 at 13:44
I tried using Akka 2.5.0 and it still works. Are you on Windows? Try disabling your firewall / antivirus. Something is blocking the connection. – May 05 '17 at 14:13
Also, what are the contents of your application15.conf? – May 05 '17 at 14:31
I'm able to run your code, and it works! However, when I start my peers with your code, they don't show the "Starting remoting" INFO messages. It still shows the deadletters message when I start peer 16... I updated my question with the contents of application15.conf. – PablodeAcero May 08 '17 at 10:45
Since the only difference in our code is this line `val configFile = getClass.getClassLoader.getResource(s"application$localId.conf").getFile `, if you print the content of `configFile`, is it what it should be? – May 08 '17 at 13:24
Yes! Thank, I had the conf files in the resources directory, and they were being read from the root of the project... Since it never gave any error or warning, I didn't assume anything about it. Thank you so much for your help! – PablodeAcero May 08 '17 at 13:38

Dead Letters using Akka to create a message ring

1 Answers1