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Actors on Scala.js

Check out a rough example project at https://github.com/CodeMettle/jsactor-spa-example

Disclaimer

Much of this code (the good parts) are straight from Typesafe/Akka and I don't claim authorship. Most of the code that isn't straight from Akka was at least written while looking at Akka code and uses their concepts and techniques. Anything broken is my fault, everything else is thanks to their work. My hope is that they create and maintain an official Scala.js version of Akka; until then jsactor seems to mostly work.

While this projects works, somewhat, it's not close to production-quality.

Why?

I'm used to working with the Actor model on the backend, with Scala.js it feels natural to use Actors and have hierarchy, a well-defined lifecycle, an EventStream, and so on.

Import

libraryDependencies ++= Seq(
  "com.codemettle.jsactor" %%% "jsactor" % "0.8.0",
  "com.codemettle.jsactor" %%% "jsactor-bridge-client" % "0.8.0" // if using jsactor-bridge
  "com.codemettle.jsactor" %%% "jsactor-loglevel" % "0.8.0" // to use the LogLevel logging adapter
)

Usage

(deleted JsActor section since it's now gone in favor of Akka.JS)

Server-client Interop

This is the part we care about, right? Why use Actors on the client if we're not interacting with a backend ActorSystem?

To that end, I'm including some extra projects that make interaction pretty seamless, at least in our usage. We have many µservices running on the backend, one of which consolidates large amounts of data from many different sources and transforms to model objects and pushes changes to the GUI. No pull, no REST, no client-side polling for changes - the backend reacts to changes it cares about, based on the current client view (what it cares about), and when the model objects change then the server pushes them.

Shared model

// project with model classes shared between client & server, cross-compiled to JVM & JS
 ...
libraryDependencies += "com.codemettle.jsactor" %%% "jsactor-bridge-shared" % "0.8.0"
 ...

Currently this project (so, by extension, client and server bridge projects) depends on uPickle, with no way to change that.

This whole readme is out of date, but nobody uses this library anyway, so NBD. But there now are *-upickle projects that are based on uPickle. I aim to add BooPickle and/or Circe implementations of client-server bridges.

uPickle was written, I assume, with autowire in mind - all types are known in advance. That presents an issue for jsactor since the Akka model is typeless and depends on pattern matching. I worked around this with inspiration from (or, really, by copying) scala-js-pickling's PicklerRegistry. The top-level bridge actor on client and server need an implicit BridgeProtocol in scope. The protocol has to register every top-level message that is going to be sent across the bridge.

object MyProjectProtocol extends BridgeProtocol {
  override def registerMessages(registry: MessageRegistry): Unit = {
    def add[A : Reader : Writer : ClassTag] = registry.add[A]
    def addObj[A <: Singleton : Reader : Writer : ClassTag](obj: A) = registry.addObj(obj)

    addObj(SessionApi.SubscribeToUserAndAlarmPrefs)
    add[SessionApi.UserUpdate]
    add[SessionApi.PreferenceUpdate]
    add[SessionApi.UpdatePreferences]
    add[SessionApi.PreferenceUpdateResult]
  }
}

Server-side

// server project; also needs to depend on the shared model project
 ...
libraryDependencies += "com.codemettle.jsactor" %%% "jsactor-bridge-server" % "0.8.0"
 ...

The server-side of the bridge takes a reference to an Actor which sends/receives Strings to/from the client, presumably over a WebSocket.

class ServerBridgeActor(clientWebSocket: ActorRef)(implicit bridgeProtocol: BridgeProtocol) extends Actor

Play 2.3 happens to give us an Actor representing a WebSocket, so this is pretty trivial in Play:

object MyController extends Controller {
  ...

  private def getSession(implicit request: RequestHeader): Future[Option[ClientSession]] = ???

  def ws = WebSocket.tryAcceptWithActor[String, String] { request ⇒
    getSession(request) map {
      case None ⇒ Left(Forbidden)

      case Some(session) ⇒
        implicit val bridgeProtocol = MyProjectProtocol
        Right((wsActor) ⇒ ServerBridgeActor.props(wsActor))
    }
  }

  ...
}

Client-side

// client project; also needs to depend on the shared model project
 ...
libraryDependencies += "com.codemettle.jsactor" %%% "jsactor-bridge-client" % "0.8.0"
 ...

jsactor-bridge-client provides building blocks for creating the client-side of a bridge, any/all of the provided classes can be replaced with your own, but it's easiest to use jsactor.bridge.client.{ClientBridgeActor, SocketManager, WebSocketActor}.

  val actorSystem: ActorSystem = ???

  private implicit val bridgeProtocal = MyProjectProtocol
  val wsManager = actorSystem.actorOf(SocketManager.props(SocketManager.Config("ws://localhost:9000/ws"), "socketManager")

  class MyActor extends Actor {
    override def preStart() = {
      super.preStart()

      wsManager ! SocketManager.Events.SubscribeToEvents
    }

    override def receive = {
      case SocketManager.Events.WebSocketConnected(socket) ⇒
        socket ! WebSocketActor.Messages.SendMessageToServer("/user/MyServerActor", MessageThatIHaveRegistered())
        socket ! WebSocketActor.Messages.SendMessageToServer("akka.tcp://ActorSystem@address:port/user/MyServerActor", MessageThatIHaveRegistered())

        // could receive a jsactor.bridge.protocol.ServerActorNotFound message,
        // otherwise the server-side actor will have a proxy representing this client-side actor, any messages the
        // server sends to that actor will come to this actor; the server can DeathWatch that proxy to be notified
        // that this actor has shutdown (or the WebSocket was interrupted)

        socket ! WebSocketActor.Messages.IdentifyServerActor("/user/MyServerActor")

        // this actor will receive a jsactor.bridge.protocol.ServerActorNotFound message, or will receive a
        // jsactor.bridge.protocol.ServerActorFound message (sent from the client-side proxy of that server-side actor).
        // Any message sent to this client-side proxy will be sent to the corresponding server-side actor, and the
        // client-side proxy can be DeathWatched to notify that the server actor has shutdown (or WebSocket was interrupted).
    }
  }

I've also included a rough utility trait, jsactor.bridge.client.util.RemoteActorListener, that can be used to automatically start trying to Identify a server-side actor when the websocket connects (or reconnects):

import jsactor.bridge.client.util.RemoteActorListener

class MyClientActor(val wsManager: ActorRef) extends RemoteActorListener {
  def actorPath = "/user/MyServerActor"

  def onConnect(serverActor: ActorRef) = {
    serverActor ! MyRegisteredMessage
  }

  def whenConnected(serverActor: ActorRef) = {
    case RegisteredMessageFromServer(param) => log.info(s"got $param from server")
    case msg@RegisteredMessageFromClient => serverActor forward msg
  }
}