Akka Typed - Version 2.5-SNAPSHOT

Akka Typed

Warning

This module is currently marked as may change in the sense of being the subject of active research. This means that API or semantics can change without warning or deprecation period and it is not recommended to use this module in production just yet—you have been warned.

As discussed in Actor Systems (and following chapters) Actors are about sending messages between independent units of computation, but how does that look like? In all of the following these imports are assumed:

import akka.typed._
import akka.typed.scaladsl.Actor._
import akka.typed.scaladsl.AskPattern._
import scala.concurrent.Future
import scala.concurrent.duration._
import scala.concurrent.Await

With these in place we can define our first Actor, and of course it will say hello!

object HelloWorld {
  final case class Greet(whom: String, replyTo: ActorRef[Greeted])
  final case class Greeted(whom: String)

  val greeter = Stateless[Greet] { (_, msg) 
    println(s"Hello ${msg.whom}!")
    msg.replyTo ! Greeted(msg.whom)
  }
}

This small piece of code defines two message types, one for commanding the Actor to greet someone and one that the Actor will use to confirm that it has done so. The Greet type contains not only the information of whom to greet, it also holds an ActorRef that the sender of the message supplies so that the HelloWorld Actor can send back the confirmation message.

The behavior of the Actor is defined as the greeter value with the help of the Stateless behavior constructor—there are many different ways of formulating behaviors as we shall see in the following. The “stateless” behavior is not capable of changing in response to a message, it will stay the same until the Actor is stopped by its parent.

The type of the messages handled by this behavior is declared to be of class Greet, which implies that the supplied function’s msg argument is also typed as such. This is why we can access the whom and replyTo members without needing to use a pattern match.

On the last line we see the HelloWorld Actor send a message to another Actor, which is done using the ! operator (pronounced “tell”). Since the replyTo address is declared to be of type ActorRef[Greeted] the compiler will only permit us to send messages of this type, other usage will not be accepted.

The accepted message types of an Actor together with all reply types defines the protocol spoken by this Actor; in this case it is a simple request–reply protocol but Actors can model arbitrarily complex protocols when needed. The protocol is bundled together with the behavior that implements it in a nicely wrapped scope—the HelloWorld object.

Now we want to try out this Actor, so we must start an ActorSystem to host it:

import HelloWorld._
// using global pool since we want to run tasks after system.terminate
import scala.concurrent.ExecutionContext.Implicits.global

val system: ActorSystem[Greet] = ActorSystem("hello", greeter)

val future: Future[Greeted] = system ? (Greet("world", _))

for {
  greeting  future.recover { case ex  ex.getMessage }
  done  { println(s"result: $greeting"); system.terminate() }
} println("system terminated")

After importing the Actor’s protocol definition we start an Actor system from the defined behavior.

As Carl Hewitt said, one Actor is no Actor—it would be quite lonely with nobody to talk to. In this sense the example is a little cruel because we only give the HelloWorld Actor a fake person to talk to—the “ask” pattern (represented by the ? operator) can be used to send a message such that the reply fulfills a Promise to which we get back the corresponding Future.

Note that the Future that is returned by the “ask” operation is properly typed already, no type checks or casts needed. This is possible due to the type information that is part of the message protocol: the ? operator takes as argument a function that accepts an ActorRef[U] (which explains the _ hole in the expression on line 7 above) and the replyTo parameter which we fill in is of type ActorRef[Greeted], which means that the value that fulfills the Promise can only be of type Greeted.

We use this here to send the Greet command to the Actor and when the reply comes back we will print it out and tell the actor system to shut down. Once that is done as well we print the "system terminated" messages and the program ends. The recovery combinator on the original Future is needed in order to ensure proper system shutdown even in case something went wrong; the flatMap and map combinators that the for expression gets turned into care only about the “happy path” and if the future failed with a timeout then no greeting would be extracted and nothing would happen.

This shows that there are aspects of Actor messaging that can be type-checked by the compiler, but this ability is not unlimited, there are bounds to what we can statically express. Before we go on with a more complex (and realistic) example we make a small detour to highlight some of the theory behind this.

A Little Bit of Theory

The Actor Model as defined by Hewitt, Bishop and Steiger in 1973 is a computational model that expresses exactly what it means for computation to be distributed. The processing units—Actors—can only communicate by exchanging messages and upon reception of a message an Actor can do the following three fundamental actions:

  1. send a finite number of messages to Actors it knows
  2. create a finite number of new Actors
  3. designate the behavior to be applied to the next message

The Akka Typed project expresses these actions using behaviors and addresses. Messages can be sent to an address and behind this façade there is a behavior that receives the message and acts upon it. The binding between address and behavior can change over time as per the third point above, but that is not visible on the outside.

With this preamble we can get to the unique property of this project, namely that it introduces static type checking to Actor interactions: addresses are parameterized and only messages that are of the specified type can be sent to them. The association between an address and its type parameter must be made when the address (and its Actor) is created. For this purpose each behavior is also parameterized with the type of messages it is able to process. Since the behavior can change behind the address façade, designating the next behavior is a constrained operation: the successor must handle the same type of messages as its predecessor. This is necessary in order to not invalidate the addresses that refer to this Actor.

What this enables is that whenever a message is sent to an Actor we can statically ensure that the type of the message is one that the Actor declares to handle—we can avoid the mistake of sending completely pointless messages. What we cannot statically ensure, though, is that the behavior behind the address will be in a given state when our message is received. The fundamental reason is that the association between address and behavior is a dynamic runtime property, the compiler cannot know it while it translates the source code.

This is the same as for normal Java objects with internal variables: when compiling the program we cannot know what their value will be, and if the result of a method call depends on those variables then the outcome is uncertain to a degree—we can only be certain that the returned value is of a given type.

We have seen above that the return type of an Actor command is described by the type of reply-to address that is contained within the message. This allows a conversation to be described in terms of its types: the reply will be of type A, but it might also contain an address of type B, which then allows the other Actor to continue the conversation by sending a message of type B to this new address. While we cannot statically express the “current” state of an Actor, we can express the current state of a protocol between two Actors, since that is just given by the last message type that was received or sent.

In the next section we demonstrate this on a more realistic example.

A More Complex Example

Consider an Actor that runs a chat room: client Actors may connect by sending a message that contains their screen name and then they can post messages. The chat room Actor will disseminate all posted messages to all currently connected client Actors. The protocol definition could look like the following:

sealed trait Command
final case class GetSession(screenName: String, replyTo: ActorRef[SessionEvent])
  extends Command

sealed trait SessionEvent
final case class SessionGranted(handle: ActorRef[PostMessage]) extends SessionEvent
final case class SessionDenied(reason: String) extends SessionEvent
final case class MessagePosted(screenName: String, message: String) extends SessionEvent

final case class PostMessage(message: String)

Initially the client Actors only get access to an ActorRef[GetSession] which allows them to make the first step. Once a client’s session has been established it gets a SessionGranted message that contains a handle to unlock the next protocol step, posting messages. The PostMessage command will need to be sent to this particular address that represents the session that has been added to the chat room. The other aspect of a session is that the client has revealed its own address, via the replyTo argument, so that subsequent MessagePosted events can be sent to it.

This illustrates how Actors can express more than just the equivalent of method calls on Java objects. The declared message types and their contents describe a full protocol that can involve multiple Actors and that can evolve over multiple steps. The implementation of the chat room protocol would be as simple as the following:

private final case class PostSessionMessage(screenName: String, message: String)
  extends Command

def chatRoom(sessions: List[ActorRef[SessionEvent]] = List.empty): Behavior[Command] =
  Stateful[Command] { (ctx, msg) 
    msg match {
      case GetSession(screenName, client) 
        val wrapper = ctx.spawnAdapter {
          p: PostMessage  PostSessionMessage(screenName, p.message)
        }
        client ! SessionGranted(wrapper)
        chatRoom(client :: sessions)
      case PostSessionMessage(screenName, message) 
        val mp = MessagePosted(screenName, message)
        sessions foreach (_ ! mp)
        Same
    }
  }

The core of this behavior is stateful, the chat room itself does not change into something else when sessions are established, but we introduce a variable that tracks the opened sessions. Note that by using a method parameter a var is not needed. When a new GetSession command comes in we add that client to the list that is in the returned behavior. Then we also need to create the session’s ActorRef that will be used to post messages. In this case we want to create a very simple Actor that just repackages the PostMessage command into a PostSessionMessage command which also includes the screen name. Such a wrapper Actor can be created by using the spawnAdapter method on the ActorContext, so that we can then go on to reply to the client with the SessionGranted result.

The behavior that we declare here can handle both subtypes of Command. GetSession has been explained already and the PostSessionMessage commands coming from the wrapper Actors will trigger the dissemination of the contained chat room message to all connected clients. But we do not want to give the ability to send PostSessionMessage commands to arbitrary clients, we reserve that right to the wrappers we create—otherwise clients could pose as completely different screen names (imagine the GetSession protocol to include authentication information to further secure this). Therefore PostSessionMessage has private visibility and can't be created outside the actor.

If we did not care about securing the correspondence between a session and a screen name then we could change the protocol such that PostMessage is removed and all clients just get an ActorRef[PostSessionMessage] to send to. In this case no wrapper would be needed and we could just use ctx.self. The type-checks work out in that case because ActorRef[-T] is contravariant in its type parameter, meaning that we can use a ActorRef[Command] wherever an ActorRef[PostSessionMessage] is needed—this makes sense because the former simply speaks more languages than the latter. The opposite would be problematic, so passing an ActorRef[PostSessionMessage] where ActorRef[Command] is required will lead to a type error.

Trying it out

In order to see this chat room in action we need to write a client Actor that can use it:

import ChatRoom._

val gabbler =
  Stateful[SessionEvent] { (_, msg) 
    msg match {
      case SessionDenied(reason) 
        println(s"cannot start chat room session: $reason")
        Stopped
      case SessionGranted(handle) 
        handle ! PostMessage("Hello World!")
        Same
      case MessagePosted(screenName, message) 
        println(s"message has been posted by '$screenName': $message")
        Stopped
    }
  }

From this behavior we can create an Actor that will accept a chat room session, post a message, wait to see it published, and then terminate. The last step requires the ability to change behavior, we need to transition from the normal running behavior into the terminated state. This is why this Actor uses a different behavior constructor named Total. This constructor takes as argument a function from the handled message type, in this case SessionEvent, to the next behavior. That next behavior must again be of the same type as we discussed in the theory section above. Here we either stay in the very same behavior or we terminate, and both of these cases are so common that there are special values Same and Stopped that can be used. The behavior is named “total” (as opposed to “partial”) because the declared function must handle all values of its input type. Since SessionEvent is a sealed trait the Scala compiler will warn us if we forget to handle one of the subtypes; in this case it reminded us that alternatively to SessionGranted we may also receive a SessionDenied event.

Now to try things out we must start both a chat room and a gabbler and of course we do this inside an Actor system. Since there can be only one guardian supervisor we could either start the chat room from the gabbler (which we don’t want—it complicates its logic) or the gabbler from the chat room (which is nonsensical) or we start both of them from a third Actor—our only sensible choice:

val main: Behavior[akka.NotUsed] =
  Stateful(
    behavior = (_, _)  Unhandled,
    signal = { (ctx, sig) 
    sig match {
      case PreStart 
        val chatRoom = ctx.spawn(ChatRoom.chatRoom(), "chatroom")
        val gabblerRef = ctx.spawn(gabbler, "gabbler")
        ctx.watch(gabblerRef)
        chatRoom ! GetSession("ol’ Gabbler", gabblerRef)
        Same
      case Terminated(ref) 
        Stopped
      case _ 
        Unhandled
    }
  })

val system = ActorSystem("ChatRoomDemo", main)
Await.result(system.whenTerminated, 1.second)

In good tradition we call the main Actor what it is, it directly corresponds to the main method in a traditional Java application. This Actor will perform its job on its own accord, we do not need to send messages from the outside, so we declare it to be of type NotUsed. Actors receive not only external messages, they also are notified of certain system events, so-called Signals. In order to get access to those we choose to implement this particular one using the Stateful behavior decorator. The provided signal function will be invoked for signals (subclasses of Signal) or the mesg function for user messages.

This particular main Actor reacts to two signals: when it is started it will first receive the PreStart signal, upon which the chat room and the gabbler are created and the session between them is initiated, and when the gabbler is finished we will receive the Terminated event due to having called ctx.watch for it. This allows us to shut down the Actor system: when the main Actor terminates there is nothing more to do.

Therefore after creating the Actor system with the main Actor’s Props we just await its termination.

Status of this Project and Relation to Akka Actors

Akka Typed is the result of many years of research and previous attempts (including Typed Channels in the 2.2.x series) and it is on its way to stabilization, but maturing such a profound change to the core concept of Akka will take a long time. We expect that this module will stay marked may change for multiple major releases of Akka and the plain akka.actor.Actor will not be deprecated or go away anytime soon.

Being a research project also entails that the reference documentation is not as detailed as it will be for a final version, please refer to the API documentation for greater depth and finer detail.

Main Differences

The most prominent difference is the removal of the sender() functionality. This turned out to be the Achilles heel of the Typed Channels project, it is the feature that makes its type signatures and macros too complex to be viable. The solution chosen in Akka Typed is to explicitly include the properly typed reply-to address in the message, which both burdens the user with this task but also places this aspect of protocol design where it belongs.

The other prominent difference is the removal of the Actor trait. In order to avoid closing over unstable references from different execution contexts (e.g. Future transformations) we turned all remaining methods that were on this trait into messages: the behavior receives the ActorContext as an argument during processing and the lifecycle hooks have been converted into Signals.

A side-effect of this is that behaviors can now be tested in isolation without having to be packaged into an Actor, tests can run fully synchronously without having to worry about timeouts and spurious failures. Another side-effect is that behaviors can nicely be composed and decorated, see the And, Or, Widened, ContextAware combinators; nothing about these is special or internal, new combinators can be written as external libraries or tailor-made for each project.

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