Project Gålbma:
Actors vs Types
Dr. Roland Kuhn
@rolandkuhn — Akka Tech Lead

Motivation

Motivation
3
case class Get
case class Got(contents: Map[String, ActorRef])
class Server extends Actor {
var map = Map.empty[String, ActorRef]
def receive = {
case Get =>
sender ! Got(map)
}
}

4
case class GetRef(name: String)
case class GetRefReply(ref: Option[ActorRef])
class Client(server: ActorRef) extends Actor {
def receive = {
case GetRef(name) =>
val worker = context.actorOf(Worker.props(name, sender()))
server.tell(Get, worker)
}
}
object Worker {
def props(name: String, replyTo: ActorRef) =
Props(new Worker(name, replyTo))
}
class Worker(name: String, replyTo: ActorRef) extends Actor {
def receive = {
case Got(map) =>
replyTo ! GetRefReply(map.get(name))
context.stop(self)
}
}

5
case class Get(id: Int)
case class Got(id: Int, contents: Map[String, ActorRef])
class Server extends Actor {
var map = Map.empty[String, ActorRef]
def receive = {
case Get(id) =>
sender ! Got(id, map)
}
}

6
case class GetRef(name: String)
case class GetRefReply(ref: Option[ActorRef])
class Client(server: ActorRef) extends Actor {
def receive = {
case GetRef(name) =>
val worker = context.actorOf(Worker.props(name, sender()))
server.tell(Get, worker)
}
}
object Worker {
def props(name: String, replyTo: ActorRef) =
Props(new Worker(name, replyTo))
}
class Worker(name: String, replyTo: ActorRef) extends Actor {
def receive = {
case Got(id, map) =>
replyTo ! GetRefReply(map.get(name))
context.stop(self)
}
}

7
class Asker(server: ActorRef) extends Actor {
implicit val timeout = Timeout(1.second)
import context.dispatcher
def receive = {
case GetRef(name) =>
(server ? Get(42))
.mapTo[Got]
.map(got => GetRefReply(got.contents get name))
.pipeTo(sender())
}
}

Failed Attempts

Akka 1.2: Channel[-T]
9
/**
* Abstraction for unification of sender and senderFuture for later reply.
* Can be stored away and used at a later point in time.
*
* The possible reply channel which can be passed into ! and tryTell is always
* untyped, as there is no way to utilize its real static type without
* requiring runtime-costly manifests.
*/
trait Channel[-T] extends japi.Channel[T] {
/**
* Scala API. <p/>
* Sends the specified message to the channel.
*/
def !(msg: T)(implicit sender: UntypedChannel): Unit
...
}

Akka 2.1: Typed Channels
10

Akka 2.1: Typed Channels
11

Akka 2.1: Typed Channels
12

The Failures Summarized
• first no clear vision of the goal
• then trying to go too far
• too complicated to declare
• white-box macros required
• not bold enough
• untyped Actors have features that are incompatible with
static typing
13

The Solution

What we want: Parameterized ActorRef
15
object Server {
case class Get(id: Int)(val replyTo: ActorRef[Got])
case class Got(id: Int, contents: Map[String, ActorRef[OtherCommand]])
}
object Client {
case class GetRef(name: String)(val replyTo: ActorRef[GetRefReply])
case class GetRefReply(ref: Option[ActorRef[OtherCommand]])
}
val server: ActorRef[Server.Get] = ???
val behavior: PartialFunction[Any, Unit] = {
case g @ GetRef(name) =>
(server ? Server.Get(42))
.map(got => g.replyTo ! GetRefReply(got.contents get name))
}

What we want: Parameterized ActorRef
16
object Server {
case class Get(id: Int)(val replyTo: ActorRef[Got])
case class Got(id: Int, contents: Map[String, ActorRef[OtherCommand]])
}
object Client {
case class GetRef(name: String)(val replyTo: ActorRef[GetRefReply])
case class GetRefReply(ref: Option[ActorRef[OtherCommand]])
}
val server: ActorRef[Server.Get] = ???
val behavior: PartialFunction[Any, Unit] = {
case g @ GetRef(name) =>
(server ? Server.Get(42))
.map(got => g.replyTo ! GetRefReply(got.contents get name))
}

What we want: Parameterized ActorRef
17
object Server {
case class Get(id: Int)(val replyTo: ActorRef[Got])
case class Got(id: Int, contents: Map[String, ActorRef[OtherCommand]])
}
object Client {
case class GetRef(name: String)(val replyTo: ActorRef[GetRefReply])
case class GetRefReply(ref: Option[ActorRef[OtherCommand]])
}
val server: ActorRef[Server.Get] = ???
val behavior: PartialFunction[Any, Unit] = {
case g @ GetRef(name) =>
(server ? Server.Get(42))
.map(got => g.replyTo ! GetRefReply(got.contents get name))
}

The Guiding Principle
• build everything around ActorRef[-T]
• do not use macros or type calculations that Java
cannot do (i.e. “keep it simple”)
• remove all features that are incompatible with this
• in particular the automatic “sender” capture must go
18

Possible Plan
• add type parameter to ActorRef, Actor, …
• remove sender()
• type Receive = PartialFunction[T, Unit]
• restrict context.become to this type
• type-safety achieved—everyone happy!
19

But why stop here?

« … and determine the behavior to be
applied to the next message.»
— Carl Hewitt, 1973

gålbma (sami) — kolme (finnish): THREE
We have one chance to rectify some things

Project Gålbma
• distill an Actor to its essence: the Behavior
• everything is a message—for real this time
• remove the danger to close over Actor environment
• behavior composition
• allow completely pure formulation of Actors
23

Behavior is King, no more Actor trait
24
object Server {
sealed trait Command
case class Get(id: Int)(val replyTo: ActorRef[Got]) extends Command
case class Put(name: String, ref: ActorRef[OtherCommand]) extends Command
case class Got(id: Int, contents: Map[String, ActorRef[OtherCommand]])
val initial: Behavior[Command] = withMap(Map.empty)
private def withMap(map: Map[String, ActorRef[OtherCommand]]) =
Total[Command] {
case g @ Get(id) =>
g.replyTo ! Got(id, Map.empty)
Same
case Put(name, ref) =>
withMap(map.updated(name, ref))
}
}

No More Closing over ActorContext
• ActorContext is passed in for every message
• processing a message returns the next behavior
• lifecycle hooks, Terminated and ReceiveTimeout
are management “signals”
25
final case class Total[T](behavior: T => Behavior[T]) extends Behavior[T] {
override def management(ctx: ActorContext[T], msg: Signal): Behavior[T] = Unhandled
override def message(ctx: ActorContext[T], msg: T): Behavior[T] = behavior(msg)
override def toString = s"Total(${LineNumbers(behavior)})"
}

Everything behaves like a Message
• ActorContext remains the system interface:
• spawn, stop, watch, unwatch, setReceiveTimeout, schedule,
executionContext, spawnAdapter, props, system, self
• actorOf — for interoperability with untyped Actors
26
Full[Command] {
case Msg(ctx, cmd) => // def receive
case Sig(ctx, PreStart) => // def preStart()
case Sig(ctx, PreRestart(ex)) => // def preRestart(...)
case Sig(ctx, PostRestart(ex)) => // def postRestart(...)
case Sig(ctx, PostStop) => // def postStop()
case Sig(ctx, Failed(ex, child)) => // val supervisorStrategy
case Sig(ctx, ReceiveTimeout) => // case ReceiveTimeout
case Sig(ctx, Terminated(ref)) => // case Terminated(...)
}

27
object Client {
sealed trait Command
case class GetRef(name: String)(val replyTo: ActorRef[GetRefReply]) extends Command
case class GotWrapper(id: Int, contents: Map[String, ActorRef[OtherCommand]])
extends Command
case class GetRefReply(ref: Option[ActorRef[OtherCommand]])
def initial(server: ActorRef[Server.Command]) =
ContextAware[Command] { ctx =>
val adapter =
ctx.spawnAdapter((got: Server.Got) => GotWrapper(got.id, got.contents))
behv(0, Map.empty)(adapter, server)
}
def behv(nextId: Int,
replies: Map[Int, (String, ActorRef[GetRefReply])])(
implicit adapter: ActorRef[Server.Got],
server: ActorRef[Server.Command]): Behavior[Command] =
Total {
case g @ GetRef(name) =>
server ! Server.Get(nextId)(adapter)
behv(nextId + 1, replies.updated(nextId, name -> g.replyTo))
case GotWrapper(id, contents) =>
replies get id map (p => p._2 ! GetRefReply(contents get p._1))
behv(nextId, replies - id)
}
}

28
object Client {
sealed trait Command
case class GetRef(name: String)(val replyTo: ActorRef[GetRefReply]) extends Command
case class GotWrapper(id: Int, contents: Map[String, ActorRef[OtherCommand]])
extends Command
case class GetRefReply(ref: Option[ActorRef[OtherCommand]])
def initial(server: ActorRef[Server.Command]) =
ContextAware[Command] { ctx =>
val adapter: ActorRef[Server.Got] =
ctx.spawnAdapter((got: Server.Got) => GotWrapper(got.id, got.contents))
behv(0, Map.empty)(adapter, server)
}
def behv(nextId: Int,
replies: Map[Int, (String, ActorRef[GetRefReply])])(
implicit adapter: ActorRef[Server.Got],
server: ActorRef[Server.Command]): Behavior[Command] =
Total {
case g @ GetRef(name) =>
server ! Server.Get(nextId)(adapter)
behv(nextId + 1, replies.updated(nextId, name -> g.replyTo))
case GotWrapper(id, contents) =>
replies get id map (p => p._2 ! GetRefReply(contents get p._1))
behv(nextId, replies - id)
}
}

29
object Client {
sealed trait Command
case class GetRef(name: String)(val replyTo: ActorRef[GetRefReply]) extends Command
case class GotWrapper(id: Int, contents: Map[String, ActorRef[OtherCommand]])
extends Command
case class GetRefReply(ref: Option[ActorRef[OtherCommand]])
def initial(server: ActorRef[Server.Command]) =
ContextAware[Command] { ctx =>
val adapter =
ctx.spawnAdapter((got: Server.Got) => GotWrapper(got.id, got.contents))
behv(0, Map.empty)(adapter, server)
}
def behv(nextId: Int,
replies: Map[Int, (String, ActorRef[GetRefReply])]
)(implicit adapter: ActorRef[Server.Got],
server: ActorRef[Server.Command]): Behavior[Command] =
Total {
case g @ GetRef(name) =>
server ! Server.Get(nextId)(adapter)
behv(nextId + 1, replies.updated(nextId, name -> g.replyTo))
case GotWrapper(id, contents) =>
replies get id map (p => p._2 ! GetRefReply(contents get p._1))
behv(nextId, replies - id)
}
}

Under the Hood

The Implementation
• independent add-on library
• layered completely on top of untyped Actors
• currently 2kLOC main + 1.7kLOC tests
• fully interoperable
31

The most important interface: Behavior[T]
• Behaviors:
• Full, FullTotal, Total, Partial, Static
• Decorators:
• ContextAware, SelfAware, SynchronousSelf, Tap
• Combinators:
• And, Or, Widened
32
abstract class Behavior[T] {
def management(ctx: ActorContext[T], msg: Signal): Behavior[T]
def message(ctx: ActorContext[T], msg: T): Behavior[T]
def narrow[U <: T]: Behavior[U] = this.asInstanceOf[Behavior[U]]
}

ActorSystem ≈ ActorRef
33
object Demo extends App {
implicit val t = Timeout(1.second)
val guardian = ContextAware[Client.Command] { ctx =>
val server = ctx.spawn(Props(Server.initial), "server")
val client = ctx.spawn(Props(Client.initial(server)), "client")
Static {
case msg => client ! msg
}
}
val system = ActorSystem("Demo", Props(guardian))
import system.executionContext
system ? Client.GetRef("X") map println foreach (_ => system.terminate())
}

Testing

Behavior Rulez!
• decoupling of logic from execution mechanism
• synchronous behavioral tests of individual Actors
• mock ActorContext allows inspection of effects
35

36
object `A Receptionist` {
def `must register a service`(): Unit = {
val ctx = new EffectfulActorContext("register", Props(behavior), system)
val a = Inbox.sync[ServiceA]("a")
val r = Inbox.sync[Registered[_]]("r")
ctx.run(Register(ServiceKeyA, a.ref)(r.ref))
ctx.getAllEffects() should be(Effect.Watched(a.ref) :: Nil)
r.receiveMsg() should be(Registered(ServiceKeyA, a.ref))
val q = Inbox.sync[Listing[ServiceA]]("q")
ctx.run(Find(ServiceKeyA)(q.ref))
ctx.getAllEffects() should be(Nil)
q.receiveMsg() should be(Listing(ServiceKeyA, Set(a.ref)))
assertEmpty(a, r, q)
}
...
}

What can we do with it?

Encoding Types with Members
38
class MyClass {
def myMethod(id: Int): String
def otherMethod(name: String): Unit
protected def helper(arg: Double): Unit
}

Encoding Types with Members
• Typed Actors provide complete modules with members
• Typed Actors can encode more flexible access privileges
• more verbose due to syntax being optimized for classes
39
object MyClass {
sealed trait AllCommand
sealed trait Command extends AllCommand
case class MyMethod(id: Int)(replyTo: ActorRef[String]) extends Command
case class OtherMethod(name: String) extends Command
case class Helper(arg: Double) extends AllCommand
val behavior: Behavior[Command] = behavior(42).narrow
private def behavior(x: Int): Behavior[AllCommand] = ???
}

Calling Methods
40
object MyClassDemo {
import MyClass._
val myClass: MyClass = ???
val myActor: ActorRef[Command] = ???
implicit val t: Timeout = ???
myClass.otherMethod("John")
myActor!OtherMethod("John")
val result = myClass.myMethod(42)
val future = myActor?MyMethod(42)
}

But Actors can do more: Protocols
41
object Protocol {
case class GetSession(replyTo: ActorRef[GetSessionResult])
sealed trait GetSessionResult
case class ActiveSession(service: ActorRef[SessionCommand])
extends GetSessionResult with AuthenticateResult
case class NewSession(auth: ActorRef[Authenticate])
extends GetSessionResult
case class Authenticate(username: String,
password: String,
replyTo: ActorRef[AuthenticateResult])
sealed trait AuthenticateResult
case object FailedSession extends AuthenticateResult
trait SessionCommand
}

But Actors can do more: Protocols
42

What can we express?
• everything a classical module with methods can
• pass object references as inputs and outputs
• patterns beyond request–response
• dynamic proxying / delegation
43

What can we NOT express?
• any dynamic behavior (e.g. internal state changes)
• session invalidation
44

Summary and Outlook

Current Status
• part of Akka 2.4-M1
• http://doc.akka.io/docs/akka/2.4-M1/scala/typed.html
• only bare Actors
• no persistence
• no stash
• no at-least-once delivery
• no Java API yet (but taken into account already)
46

Next Steps
• proper Java API (probably in 2.4-M2)
• Receptionist plus akka-distributed-data for Cluster
• port Actor-based APIs to typed ones (e.g. Akka IO)
• add FSM support with transition triggers
• completely pure Actor implementation,
«Actor Action Monad» (inspired by JoinCalculus)
• listen to community feedback
47

… and in the far future:
• reap internal benefits by inverting implementation:
• remove sender field (and thus Envelope)
• make untyped Actor a DSL layer on top of Akka Typed
• declare it non-experimental
48

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