Akka BindException when trying to connect to remote actor: Address already in use

Question

I am experimenting with Akka remote actors, trying to establish a simple 2 player game over the network. This answer to an earlier question gave me a really good starting point but now I'm having a hard time understanding how to adapt it to my situation. ISTM that the original is connecting twice from the same client (see commented section below). What I want to do is run it twice from separate clients, but when I do I get a BindException, Address already in use. I guess that's because each time I run the code it attempts to start the server? I need a situation where I can start and stop the master actor independently of clients connecting and disconnecting. The (minimal) Akka config and exception are after the code:

import akka.actor._
//example from answer to https://stackoverflow.com/questions/15527193/keeping-references-to-two-actors
// by Patrick Nordwall
case object JoinMsg
case class Msg(s: String)

class Server extends Actor {

  def receive = {
    case JoinMsg =>
      println("got player 1")
      sender ! Msg("Waiting for player 2")
      context.become(waitingForPlayer2(sender))
  }

  def waitingForPlayer2(client1: ActorRef): Actor.Receive = {
    case JoinMsg =>
      println("got player 2")
      sender ! Msg("hi")
      client1 ! Msg("hi")
      context.become(ready(client1, sender))
  }

  def ready(client1: ActorRef, client2: ActorRef): Actor.Receive = {
    case m: Msg if sender == client1 => client2 ! m
    case m: Msg if sender == client2 => client1 ! m
  }
}

/* I want to run this once for each "player" */
object Demo extends App {
  val system = ActorSystem("Game")
  val server = system.actorOf(Props[Server], "server")

  system.actorOf(Props(new Actor {
    server ! JoinMsg
    def receive = {
      case Msg(s) => println(s)
    }
  }))

  /* Rather than connecting twice from the same node, I want to run this
     program twice from different nodes 
    system.actorOf(Props(new Actor {
    server ! JoinMsg
    def receive = {
      case Msg(s) => println(s)
    }
  }))*/
}

Config:

akka {
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  }
  remote {
    transport = "akka.remote.netty.NettyRemoteTransport"
    netty {
      hostname = "localhost"
      port = 9000
    }
 }
}

Exception:

Exception in thread "main" java.lang.ExceptionInInitializerError
    at akkademo.main(akkademo.scala)
Caused by: org.jboss.netty.channel.ChannelException: Failed to bind to: localhost/127.0.0.1:9000
    at org.jboss.netty.bootstrap.ServerBootstrap.bind(ServerBootstrap.java:298)
    at akka.remote.netty.NettyRemoteServer.start(Server.scala:54)
    at akka.remote.netty.NettyRemoteTransport.start(NettyRemoteSupport.scala:90)
    at akka.remote.RemoteActorRefProvider.init(RemoteActorRefProvider.scala:94)
    at akka.actor.ActorSystemImpl._start(ActorSystem.scala:588)
    at akka.actor.ActorSystemImpl.start(ActorSystem.scala:595)
    at akka.actor.ActorSystem$.apply(ActorSystem.scala:111)
    at akka.actor.ActorSystem$.apply(ActorSystem.scala:93)
    at akkademo$.<init>(akkademo.scala:4)
    at akkademo$.<clinit>(akkademo.scala)
    ... 1 more
Caused by: java.net.BindException: Address already in use

TIA.

Answer 1

When running several instances on same machine you need to configure different ports for them. In this example it is only the server that needs a know port (9000). For the clients you can use 0 for a random available port.

Define another configuration file for the clients. client.conf:

akka {
  actor {
    provider = "akka.remote.RemoteActorRefProvider"
  }
  remote {
    transport = "akka.remote.netty.NettyRemoteTransport"
    netty {
      hostname = "localhost"
      port = 0
    }
 }
}

Start the ActorSystem in the clients with this configuration:

import com.typesafe.config.ConfigFactory
val system = ActorSystem("Game", ConfigFactory.load("client"))

In the clients you will lookup the server with:

val server = system.actorFor("akka://Game@localhost:9000/user/server")