Akka and cake pattern

Question

I'm confused how to ensure that my Actors have the appropriate dependencies using the cake pattern. I'm still getting to grips with this, and I can't find any examples anywhere. I'm basically just looking for a tutorial/resource to follow.

Cheers, Chris.

Answer 1

Actors as dependency:

trait DBComponent {
   def db: ActorRef // no compile time guarantees

   type K
   type V

   object DBActor {
      case class Put(key: K, value: V)
      case class Get(key: K)
   }

   class DBActor {
      import DBActor._
      val db = scala.collection.mutable.Map.empty[K, V]
      def receive = {
         case Put(k, v) => db.put(k, v)
         case Get(k) => sender ! db.get(k)
      }
   }
}

trait ServiceComponent {
   this: DBComponent =>

   import DBActor._

   // you could be talking to deadLetters for all you know
   def put(k: K, v: V): Unit = db ! Put(k, v)
   def get(k: K): Option[V] = {
      implicit val timeout = Timeout(5 seconds)
      val future = ask(actor, Get(k)).mapTo[Option[V]]
      Await.result(future, timeout.duration)
   }
}

Actors having dependencies (where there is nothing special to it):

trait DBComponent {
   def db: DB

   type K
   type V

   trait DB {
      def put(key: K, value: V): Unit
      def get(key: K): Option[V]
   }
}

trait ServiceComponent {
   this: DBComponent =>

   object ServiceActor {
      case class Put(key: K, value: V)
      case class Get(key: K)
   }

   class ServiceActor {
      import ServiceActor._
      def receive = {
         case Put(k, v) => db.put(k, v) // db is in scope
         case Get(k) => sender ! db.get(k)
      }
   }
}

Answer 2

One additional gotcha, as pointed out by Owen, is that creating actors using actorOf(Props[MyClass]) won't work for inner classes. Ie: the following will fail:

trait MyComponent {
  class MyActor {
    def receive = ...
  }
}

new MyComponent {
  val myActorRef = system.actorOf( Props[MyActor] )
}

According to the documentation at http://doc.akka.io/docs/akka/snapshot/scala/actors.html,

if they are not declared within a top-level object then the enclosing instance’s this reference needs to be passed as the first argument

However, this does not seem to be supported by the scaladoc Props method signatures. The only way around this I found was to use the Props(actor: Actor) constructor, instantiating the actor mysql and passing it to Props.

val myActorRef = system.actorOf( Props( new MyActor(arg1, arg2) ) )

Would be interested to know if there's a better way..