Introduction

After posting the first article on Using Dotty Union types with Akka Typed, I received valuable feedback via Twitter which made me decide to write this follow-up article. I will clean up a few loose ends and add some new insights.

Let’s take a bird’s eye view of this follow-on content.

Loose End – The Protocol Problem!

Both the Scala 2 and the Dotty version of the Pinger actor suffer from the same problem: the actor’s protocol has been unnecessarily widened. It should be narrowed back to the absolute minimum shown in figure 1.

  • In the Scala 2 version of the code, a message adapter is used that wraps the response from the PingPong actor. As part of the implementation, a WrappedPongResponse message widens the actor protocol.
  • In the Dotty version of the code, the protocol is widened with the PingPong.Response type of messages (of which there is only one specific instance: the PingPong.Pong message).
PingerProtocol

Figure 1. The Pinger actor protocol

Solutions and insights

As a fix for the protocol problem, we use a method in the Akka 2.6 library, narrow, and we’ll look at how Dotty has the potential to simplify this approach.

Further Insights & Summary

I’ll finish up with summarising a few of the best practices for coding actors that are to a large extent based on the Akka Typed Style guide.

Restoring the Pinger actor’s protocol

Let’s have a look at the final version of the protocol of the Pinger actor using Scala 2.13 and Akka 2.6:

object Pinger {

  sealed trait Command
  case object SendPing extends Command
  case object StopPingPong extends Command
  final case class WrappedPongResponse(pong: PingPong.Response)
    extends Command

  ...
}

The problem should be obvious: we shouldn’t expose the WrappedPongResponse wrapper message as part of the Pinger actor’s protocol. In other words, we have unnecessarily widened the actor’s protocol by adding the publicly accessible WrappedPongResponse message to it.

Obviously, this is undesirable, and the fix in this case is quite simple: we can mark this message as private and we’re done:

object Pinger {

  sealed trait Command
  case object SendPing extends Command
  case object StopPingPong extends Command
  private final case class WrappedPongResponse(pong: PingPong.Response)
    extends Command

  ...
}

Restoring the Pinger actor’s protocol – Dotty version

The Dotty version of the Pinger actor suffers from the same problem. The protocol of the actor is widened with the PingPong.Pong message. This means that any actor can send a Pong message to the Pinger actor. The solution in this case is different from previous one:

  • The apply method in the Pinger object, will return a Behavior[Command] thus restoring the original protocol
  • The Pinger actor defines an internal behavior of type Behavior[CommandsAndResponses]
  • The Behavior[Command] is derived from the Behavior[CommandsAndResponses] by applying the narrowmethod on the latter

This leads to the following code:

object Pinger {

  // My protocol
  sealed trait Command
  case object SendPing extends Command
  case object StopPingPong extends Command

  // My protocol + the responses I need to understand...
  private type CommandsAndResponses = Command | PingPong.Response

  def apply(pingPong: ActorRef[PingPong.Ping]): Behavior[Command] = {
    val internalBehavior: Behavior[CommandsAndResponses] =
    Behaviors.setup { context =>
      Behaviors.receiveMessage {
        case StopPingPong =>
          context.log.info(s"End of the ping-pong game - calling it a day!")
          context.system.terminate()
          Behaviors.stopped
        case SendPing =>
          pingPong ! PingPong.Ping(replyTo = context.self)
          Behaviors.same
        case response : PingPong.Response =>
          context.log.info(s"Hey: I just received a $response !!!")
          Behaviors.same
      }
    }
    internalBehavior.narrow
  }
}

Admittedly, some magic has happened here and the question to ask is, What is this narrow method doing? Let’s look at that in the next section.

Clearing up some magic

The Behavior abstract class in Akka 2.6 defines the narrow method, and here is a part of the relevant source code:

abstract class Behavior[T](private[akka] val _tag: Int) { behavior =>

  /**
   * Narrow the type of this Behavior, which is always a safe operation. This
   * method is necessary to implement the contravariant nature of Behavior
   * (which cannot be expressed directly due to type inference problems).
   */
  final def narrow[U <: T]: Behavior[U] = this.asInstanceOf[Behavior[U]]

  ...
}

abstract class ExtensibleBehavior[T] extends Behavior[T](BehaviorTags.ExtensibleBehavior) {

  ...

  def receive(ctx: TypedActorContext[T], msg: T): Behavior[T]

  ...

There’s quite a bit going on here.

First, we note that the Behavior class is generic: it has a type parameter T, which, because of no specific variance annotation on T, implies that Behavior is invariant in its type parameter T. Also note the comment on the narrowmethod, stating:

This method is necessary to implement the contravariant nature of Behavior (which cannot be expressed directly due to type inference problems).

Second, we see that the class ExtensibleBehavior, which is a subclass of Behavior, has a receive method which takes a parameter msg of type T. Because functions (or methods), are contravariant in their argument types, the only possible variance case for the type parameter is invariant (T) or contravariant (-T). Because of type inference problems in Scala 2, the former was chosen.

NoteVariance manifests itself in specific contexts and is a big topic in itself with contravariance being the least intuitive. We’ll see however that, in the case of Behavior, it is actually quite easy to understand. I’ll get back to that later. For a comprehensive explanation of variance in Scala read this article.

Finally, we see from the definition of the narrow method, that it returns a behavior which is more restrictive in its type than the behavior on which it is called. The implementation of narrow uses asInstanceOf to apply this restriction.

Clearing up some magic in the context of Dotty

All the above is nice, but it will leave some readers with questions. So, let’s look at this from a practical point of view by looking at the Dotty version of the Pinger which uses Union types.

Starting from the (internal) protocol definition:

object Pinger {
  sealed trait Command
  case object SendPing extends Command
  case object StopPingPong extends Command

  // My protocol + the responses I need to understand...
  type CommandsAndResponses = Command | PingPong.Response
}

object PingPong {
  sealed trait Command
  final case class Ping(replyTo: ActorRef[Response]) extends Command

  sealed trait Response
  case object Pong extends Response
}

We can run the following experiment (in dotr, the Dotty REPL):

scala> import akka.actor.typed.ActorRef

scala> object Pinger {
     |   sealed trait Command
     |   case object SendPing extends Command
     |   case object StopPingPong extends Command
     |
     |   // My protocol + the responses I need to understand...
     |   type CommandsAndResponses = Command | PingPong.Response
     | }
     |
     | object PingPong {
     |   sealed trait Command
     |   final case class Ping(replyTo: ActorRef[Response]) extends Command
     |
     |   sealed trait Response
     |   case object Pong extends Response
     | }
// defined object Pinger
// defined object PingPong

scala> summon[Pinger.Command <:< Pinger.CommandsAndResponses]
val res0: Pinger.Command =:= Pinger.Command = generalized constraint

The fact that the last command returned a generalized constraint means that Pinger.Command is a subtype of Pinger.CommandsAndResponses or, differently expressed: an instance of Pinger.Command can be considered as being an instance of Pinger.CommandAndResponses.

Imagine now that Behavior is defined as contravariant in its type parameter T (and define it as a trait instead of an abstract class in the Akka source code so that for this demo, we can easily create an instance of it).

scala> trait Behavior[-A] {}

scala> summon[Behavior[Pinger.CommandsAndResponses] <:< Behavior[Pinger.Command]]
val res1: Behavior[Pinger.CommandsAndResponses] =:=
  Behavior[Pinger.CommandsAndResponses] = generalized constraint

The last line in the dotr session tells us that an instance of Behavior[Pinger.CommandsAndResponses] can be considered to be an instance of Behavior[Pinger.Command]. This allows us to do the following:

// We can mark the following variable as private, but this doesn't work in the REPL
scala> val internalBehavior = new Behavior[Pinger.CommandsAndResponses] {}
val internalBehavior: Behavior[Pinger.CommandsAndResponses] = anon$1@8f221a7

scala> val externalBehavior: Behavior[Pinger.Command] = internalBehavior
val externalBehavior: Behavior[Pinger.Command] = anon$1@8f221a7

Let this sink in for a second…​ We have achieved something important: we derived our externalBehavior from the more specific internalBehavior by using the type system and appropriate variance definitions and this without having to apply the narrow method!

Does this make sense intuitively? It does: externalBehavior is declared as a behavior that is able to ‘process’ all messages that are part of the Pinger actor’s Command protocol. The behavior that is actually handling these messages is the internalBehavior which, on top of the messages of type Command, is able to process the PingPong.Pong message.

NoteOne could say that from the outside, the internal behavior is not utilised to its full extent.

The combination of Dotty Union types combined with the generic Behavior of having a contravariant type parameter leads to a very simple implementation of the Pinger actor. The future will tell if, with Dotty, Akka will be able to exploit this in a future version.

Returning to best practices for coding up actors in Akka Typed

The coding style I’m using is drawn from the Akka Typed Style guide. This guide leaves some choices to the programmer such as choosing between an object oriented style or a functional style. For the functional style, I prefer to put the core behavior of a typed actor in a companion class. An advantage of this approach is that the method that will return the initial behavior doesn’t have to take extra contextual parameters that need to be passed in: these can be added as class parameters. In simple cases, that may be considered overkill, but as a counter argument, I think that applying the same practice in a consistent manner helps to maintain a recurring and easily recognisable way of coding actors.

The Dotty version of the Pinger actor will then look as follows:

import akka.actor.typed.{ActorRef, Behavior}
import akka.actor.typed.scaladsl.{ActorContext, Behaviors}

object Pinger {

  // My protocol
  sealed trait Command
  case object SendPing extends Command
  case object StopPingPong extends Command

  // My protocol + the responses I need to understand...
  private type CommandsAndResponses = Command | PingPong.Response

  def apply(pingPong: ActorRef[PingPong.Ping]): Behavior[Command] = {
    val internalBehavior = Behaviors.setup[CommandsAndResponses] { context =>
      (new Pinger(context, pingPong)).run()
    }
    internalBehavior.narrow
  }
}

class Pinger private (context: ActorContext[Pinger.CommandsAndResponses], pingPong: ActorRef[PingPong.Ping]) {

  import Pinger._

  def run(): Behavior[CommandsAndResponses] =
    Behaviors.receiveMessage {
      case StopPingPong =>
        context.log.info(s"End of the ping-pong game - calling it a day!")
        context.system.terminate()
        Behaviors.stopped
      case SendPing =>
        pingPong ! PingPong.Ping(replyTo = context.self)
        Behaviors.same
      case response : PingPong.Response =>
        context.log.info(s"Hey: I just received a $response !!!")
        Behaviors.same
    }
}

Note that we prevent the direct creation of instances of the Pinger actor by marking the constructor of the Pingerclass private.

Conclusions

In this article:

  • I have shown in both Scala 2 and Dotty how to handle responses sent to other actors without unnecessarily widening the message protocol:
    • In Scala 2, we can use message adapters where the message wrapper is marked private.
    • In Dotty, we can use the narrow method on Behavior.
  • we looked at a potential alternative to Behavior.narrow which may become reality sometime in the future.
  • we concluded with a look at best practices to code an Actor using Akka Typed.

A big thanks to all people who reviewed this article: my colleagues at Lunatech, Leonor BogaChris KippPedro Ferreiraand my former colleague Kiki Carter!