akka.md

akka

Basic introduction into the akka framework.

SBT dependency

Declare akka dependency in build.sbt

"com.typesafe.akka" %% "akka-actor" % "2.3.12"

Restart SBT to apply changes.

A basic actor

Actors are living in an actor system

import akka.actor._

object HotelLife extends App {
  val system = ActorSystem("hotel-life")
  Thread.sleep(5000)
  system.shutdown()
}

Create a basic actor

class Concierge extends Actor {
  override def receive = {
    case _ => println(s"Good morning, Sir. I am $self")
  }
}

val jenkins = system.actorOf(Props[Concierge], "jenkins")

The ActorRef

An actor lives inside the actor system. It is not accessed directly, instead it is accessed via an ActorRef. An ActorRef provides the method tell to send the actor a message

jenkins.tell("Good morning", ActorRef.noSender)

The actor is identified by an URL. The URL identifies an actor even across machines. Actors can also be created inside another actor.

class Guest extends Actor {
  override def receive = {
    case _ => println(s"Nice to meet you. I am $self")
  }
}

val bob = context.actorOf(Props[Guest], "bob")

bob.tell("Good morning", self)

Bob is now a child of Jenkins and his URL is a sub path.

An actor can answer to the message sender by sending a message.

sender().tell("Do you have some caviar?", self)

The ! operator allows to send a message without a sender

jenkins.tell("Good morning", ActorRef.noSender) ⇔ jenkins ! "Good morning"
bob.tell("Good morning", self) ⇔ bob ! "Good morning"

Pass an ActorRef to another arctor

class Concierge extends Actor {
  override def receive = {
    case "Good morning" => {
      println("Someone said good morning to me")
      sender() ! "Good morning, Sir"
    }
  }
}

class Guest(concierge: ActorRef) extends Actor {
  override def receive = {
    case "Go to lobby" => concierge ! "Good morning"
    case message => println(s"Oh he said: $message")
  }
}

val bob = system.actorOf(Props(new Guest(jenkins)), "bob)
bob ! "Go to lobby"

Ask pattern

Sometimes we want to ask an actor for an answer

import akka.pattern.ask

class Guest(concierge: ActorRef) extends Actor {
  import ExecutionContext.Implicits.global
  implicit val timeout = Timeout(10 seconds)

  override def receive = {
    case "Go to lobby" => for {
      message <- concierge ? "Good morning"
    } yield {
      println(s"Oh he said: $message")
      message
    }
  }
}