=type Cluster and local Receptionist, #23634

* a Receptionists extension

It's basically an improved copy of the former receptionist pattern which is
removed here as well.

* Cluster implementation using Distributed Data

* =typ make ActorRef.apply work for adapted actor systems

akka-actor/src/main/scala/akka/util/TypedMultiMap.scala

@@ -88,6 +88,17 @@ class TypedMultiMap[T <: AnyRef, K[_ <: T]] private (private val map: Map[T, Set
     }
   }
 
+  def setAll(key: T)(values: Set[K[key.type]]): TypedMultiMap[T, K] =
+    new TypedMultiMap[T, K](map.updated(key, values.asInstanceOf[Set[Any]]))
+
+  /**
+   * Add all entries from the other map, overwriting existing entries.
+   *
+   * FIXME: should it merge, instead?
+   */
+  def ++(other: TypedMultiMap[T, K]): TypedMultiMap[T, K] =
+    new TypedMultiMap[T, K](map ++ other.map)
+
   override def toString: String = s"TypedMultiMap($map)"
   override def equals(other: Any) = other match {
     case o: TypedMultiMap[_, _] ⇒ map == o.map

akka-typed-tests/src/test/scala/akka/typed/cluster/receptionist/ClusterReceptionistSpec.scala

@@ -0,0 +1,155 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.typed.cluster.receptionist
+
+import java.nio.charset.StandardCharsets
+
+import akka.actor.ExtendedActorSystem
+import akka.cluster.Cluster
+import akka.serialization.SerializerWithStringManifest
+import akka.typed.ActorRef
+import akka.typed.ActorSystem
+import akka.typed.TypedSpec
+import akka.typed.TypedSpec.Command
+import akka.typed.cluster.ActorRefResolver
+import akka.typed.internal.adapter.ActorRefAdapter
+import akka.typed.internal.adapter.ActorSystemAdapter
+import akka.typed.internal.receptionist.ReceptionistImpl
+import akka.typed.receptionist.Receptionist
+import akka.typed.scaladsl.Actor
+import akka.typed.scaladsl.adapter._
+import akka.typed.testkit.TestKitSettings
+import akka.typed.testkit.scaladsl.TestProbe
+import com.typesafe.config.ConfigFactory
+
+import scala.concurrent.Await
+import scala.concurrent.duration._
+
+object ClusterReceptionistSpec {
+  val config = ConfigFactory.parseString(
+    """
+      akka.log-level = DEBUG
+      akka.actor {
+        provider = cluster
+        serialize-messages = off
+        allow-java-serialization = off
+        serializers {
+          test = "akka.typed.cluster.receptionist.ClusterReceptionistSpec$PingSerializer"
+        }
+        serialization-bindings {
+          "akka.typed.cluster.receptionist.ClusterReceptionistSpec$Ping" = test
+          "akka.typed.cluster.receptionist.ClusterReceptionistSpec$Pong$" = test
+          "akka.typed.cluster.receptionist.ClusterReceptionistSpec$Perish$" = test
+          "akka.typed.internal.receptionist.ReceptionistImpl$DefaultServiceKey" = test
+          "akka.typed.internal.adapter.ActorRefAdapter" = test
+        }
+      }
+      akka.remote.artery.enabled = true
+      akka.remote.artery.canonical.port = 25552
+      akka.cluster.jmx.multi-mbeans-in-same-jvm = on
+    """)
+
+  trait PingProtocol
+  case object Pong
+  case class Ping(respondTo: ActorRef[Pong.type]) extends PingProtocol
+
+  case object Perish extends PingProtocol
+
+  val pingPong = Actor.immutable[PingProtocol] { (ctx, msg) ⇒
+
+    msg match {
+      case Ping(respondTo) ⇒
+        respondTo ! Pong
+        Actor.same
+
+      case Perish ⇒
+        Actor.stopped
+    }
+
+  }
+
+  class PingSerializer(system: ExtendedActorSystem) extends SerializerWithStringManifest {
+    def identifier: Int = 47
+    def manifest(o: AnyRef): String = o match {
+      case _: Ping                                ⇒ "a"
+      case Pong                                   ⇒ "b"
+      case Perish                                 ⇒ "c"
+      case ReceptionistImpl.DefaultServiceKey(id) ⇒ "d"
+      case a: ActorRefAdapter[_]                  ⇒ "e"
+    }
+
+    def toBinary(o: AnyRef): Array[Byte] = o match {
+      case p: Ping                                ⇒ ActorRefResolver(system.toTyped).toSerializationFormat(p.respondTo).getBytes(StandardCharsets.UTF_8)
+      case Pong                                   ⇒ Array.emptyByteArray
+      case Perish                                 ⇒ Array.emptyByteArray
+      case ReceptionistImpl.DefaultServiceKey(id) ⇒ id.getBytes(StandardCharsets.UTF_8)
+      case a: ActorRefAdapter[_]                  ⇒ ActorRefResolver(system.toTyped).toSerializationFormat(a).getBytes(StandardCharsets.UTF_8)
+    }
+
+    def fromBinary(bytes: Array[Byte], manifest: String): AnyRef = manifest match {
+      case "a" ⇒ Ping(ActorRefResolver(system.toTyped).resolveActorRef(new String(bytes, StandardCharsets.UTF_8)))
+      case "b" ⇒ Pong
+      case "c" ⇒ Perish
+      case "d" ⇒ ReceptionistImpl.DefaultServiceKey[Any](new String(bytes, StandardCharsets.UTF_8))
+      case "e" ⇒ ActorRefResolver(system.toTyped).resolveActorRef(new String(bytes, StandardCharsets.UTF_8))
+    }
+  }
+
+  val PingKey = Receptionist.ServiceKey[PingProtocol]("pingy")
+}
+
+class ClusterReceptionistSpec extends TypedSpec(ClusterReceptionistSpec.config) {
+  import ClusterReceptionistSpec._
+
+  implicit val testSettings = TestKitSettings(adaptedSystem)
+  val untypedSystem1 = ActorSystemAdapter.toUntyped(adaptedSystem)
+  val clusterNode1 = Cluster(untypedSystem1)
+
+  val system2 = akka.actor.ActorSystem(
+    adaptedSystem.name,
+    ConfigFactory.parseString(
+      """
+        akka.remote.artery.canonical.port = 0
+      """
+    ).withFallback(adaptedSystem.settings.config))
+  val adaptedSystem2 = system2.toTyped
+  val clusterNode2 = Cluster(system2)
+
+  clusterNode1.join(clusterNode1.selfAddress)
+  clusterNode2.join(clusterNode1.selfAddress)
+
+  object `The ClusterReceptionist` extends StartSupport {
+    def system: ActorSystem[Command] = adaptedSystem
+    import Receptionist._
+
+    def `must eventually replicate registrations to the other side`() = new TestSetup {
+      val regProbe = TestProbe[Any]()(adaptedSystem, testSettings)
+      val regProbe2 = TestProbe[Any]()(adaptedSystem2, testSettings)
+
+      adaptedSystem2.receptionist ! Subscribe(PingKey, regProbe2.ref)
+      regProbe2.expectMsg(Listing(PingKey, Set.empty[ActorRef[PingProtocol]]))
+
+      val service = start(pingPong)
+      system.receptionist ! Register(PingKey, service, regProbe.ref)
+      regProbe.expectMsg(Registered(PingKey, service))
+
+      val Listing(PingKey, remoteServiceRefs) = regProbe2.expectMsgType[Listing[PingProtocol]]
+      val theRef = remoteServiceRefs.head
+      theRef ! Ping(regProbe2.ref)
+      regProbe2.expectMsg(Pong)
+
+      service ! Perish
+      regProbe2.expectMsg(Listing(PingKey, Set.empty[ActorRef[PingProtocol]]))
+    }
+  }
+
+  trait TestSetup {
+  }
+
+  override def afterAll(): Unit = {
+    super.afterAll()
+    Await.result(adaptedSystem.terminate(), 3.seconds)
+    Await.result(system2.terminate(), 3.seconds)
+  }
+}

akka-typed-tests/src/test/scala/akka/typed/internal/ActorSystemStub.scala

@@ -58,9 +58,6 @@ private[typed] class ActorSystemStub(val name: String)
 
   override def printTree: String = "no tree for ActorSystemStub"
 
-  val receptionistInbox = Inbox[patterns.Receptionist.Command]("receptionist")
-  override def receptionist: ActorRef[patterns.Receptionist.Command] = receptionistInbox.ref
-
   def systemActorOf[U](behavior: Behavior[U], name: String, props: Props)(implicit timeout: Timeout): Future[ActorRef[U]] = {
     Future.failed(new UnsupportedOperationException("ActorSystemStub cannot create system actors"))
   }

akka-typed-tests/src/test/scala/akka/typed/internal/FunctionRefSpec.scala

@@ -26,7 +26,7 @@ class FunctionRefSpec extends TypedSpecSetup {
       val ref = ActorRef(f)
       ref ! "42"
       ref ! "43"
-      target.receiveAll() should ===(Left(Watch(target, ref)) :: Right("42") :: Right("43") :: Nil)
+      target.receiveAll() should ===(Right("42") :: Right("43") :: Nil)
     }
 
     def `must forward messages that are received before getting the ActorRef`(): Unit = {

akka-typed-tests/src/test/scala/akka/typed/receptionist/LocalReceptionistSpec.scala

@@ -1,36 +1,54 @@
 /**
  * Copyright (C) 2014-2017 Lightbend Inc. <http://www.lightbend.com>
  */
-package akka.typed.patterns
+package akka.typed.receptionist
 
-import Receptionist._
+import akka.typed._
+import akka.typed.receptionist.Receptionist._
+import akka.typed.scaladsl.Actor
 import akka.typed.scaladsl.AskPattern._
+import akka.typed.testkit.EffectfulActorContext
+import akka.typed.testkit.Inbox
+import akka.typed.testkit.TestKitSettings
+import akka.typed.testkit.scaladsl.TestProbe
+import org.scalatest.concurrent.Eventually
 
 import scala.concurrent.duration._
-import akka.typed._
-import akka.typed.scaladsl.Actor
-import akka.typed.scaladsl.Actor._
-import akka.typed.testkit.{ Effect, EffectfulActorContext, Inbox }
 
-class ReceptionistSpec extends TypedSpec {
+class LocalReceptionistSpec extends TypedSpec with Eventually {
 
   trait ServiceA
-  case object ServiceKeyA extends ServiceKey[ServiceA]
+  val ServiceKeyA = Receptionist.ServiceKey[ServiceA]("service-a")
   val behaviorA = Actor.empty[ServiceA]
 
   trait ServiceB
-  case object ServiceKeyB extends ServiceKey[ServiceB]
+  val ServiceKeyB = Receptionist.ServiceKey[ServiceB]("service-b")
   val behaviorB = Actor.empty[ServiceB]
 
-  trait CommonTests {
+  case object Stop extends ServiceA with ServiceB
+  val stoppableBehavior = Actor.immutable[Any] { (ctx, msg) ⇒
+    msg match {
+      case Stop ⇒ Behavior.stopped
+      case _    ⇒ Behavior.same
+    }
+  }
+
+  import akka.typed.internal.receptionist.ReceptionistImpl.{ localOnlyBehavior ⇒ behavior }
+
+  trait CommonTests extends StartSupport {
     implicit def system: ActorSystem[TypedSpec.Command]
+    implicit val testSettings = TestKitSettings(system)
+
+    abstract class TestSetup {
+      val receptionist = start(behavior)
+    }
 
     def `must register a service`(): Unit = {
       val ctx = new EffectfulActorContext("register", behavior, 1000, system)
       val a = Inbox[ServiceA]("a")
       val r = Inbox[Registered[_]]("r")
       ctx.run(Register(ServiceKeyA, a.ref)(r.ref))
-      ctx.getAllEffects() should be(Effect.Watched(a.ref) :: Nil)
+      ctx.getEffect() // watching however that is implemented
       r.receiveMsg() should be(Registered(ServiceKeyA, a.ref))
       val q = Inbox[Listing[ServiceA]]("q")
       ctx.run(Find(ServiceKeyA)(q.ref))
@@ -73,38 +91,62 @@ class ReceptionistSpec extends TypedSpec {
       assertEmpty(a1, a2, r, q)
     }
 
-    def `must unregister services when they terminate`(): Unit = {
-      val ctx = new EffectfulActorContext("registertwosame", behavior, 1000, system)
-      val r = Inbox[Registered[_]]("r")
-      val a = Inbox[ServiceA]("a")
-      ctx.run(Register(ServiceKeyA, a.ref)(r.ref))
-      ctx.getEffect() should be(Effect.Watched(a.ref))
-      r.receiveMsg() should be(Registered(ServiceKeyA, a.ref))
+    def `must unregister services when they terminate`(): Unit = new TestSetup {
+      val regProbe = TestProbe[Any]("regProbe")
 
-      val b = Inbox[ServiceB]("b")
-      ctx.run(Register(ServiceKeyB, b.ref)(r.ref))
-      ctx.getEffect() should be(Effect.Watched(b.ref))
-      r.receiveMsg() should be(Registered(ServiceKeyB, b.ref))
+      val serviceA = start(stoppableBehavior.narrow[ServiceA])
+      receptionist ! Register(ServiceKeyA, serviceA, regProbe.ref)
+      regProbe.expectMsg(Registered(ServiceKeyA, serviceA))
 
-      val c = Inbox[Any]("c")
-      ctx.run(Register(ServiceKeyA, c.ref)(r.ref))
-      ctx.run(Register(ServiceKeyB, c.ref)(r.ref))
-      ctx.getAllEffects() should be(Seq(Effect.Watched(c.ref), Effect.Watched(c.ref)))
-      r.receiveMsg() should be(Registered(ServiceKeyA, c.ref))
-      r.receiveMsg() should be(Registered(ServiceKeyB, c.ref))
+      val serviceB = start(stoppableBehavior.narrow[ServiceB])
+      receptionist ! Register(ServiceKeyB, serviceB, regProbe.ref)
+      regProbe.expectMsg(Registered(ServiceKeyB, serviceB))
 
-      val q = Inbox[Listing[_]]("q")
-      ctx.run(Find(ServiceKeyA)(q.ref))
-      q.receiveMsg() should be(Listing(ServiceKeyA, Set(a.ref, c.ref)))
-      ctx.run(Find(ServiceKeyB)(q.ref))
-      q.receiveMsg() should be(Listing(ServiceKeyB, Set(b.ref, c.ref)))
+      val serviceC = start(stoppableBehavior)
+      receptionist ! Register(ServiceKeyA, serviceC, regProbe.ref)
+      receptionist ! Register(ServiceKeyB, serviceC, regProbe.ref)
+      regProbe.expectMsg(Registered(ServiceKeyA, serviceC))
+      regProbe.expectMsg(Registered(ServiceKeyB, serviceC))
 
-      ctx.signal(Terminated(c.ref)(null))
-      ctx.run(Find(ServiceKeyA)(q.ref))
-      q.receiveMsg() should be(Listing(ServiceKeyA, Set(a.ref)))
-      ctx.run(Find(ServiceKeyB)(q.ref))
-      q.receiveMsg() should be(Listing(ServiceKeyB, Set(b.ref)))
-      assertEmpty(a, b, c, r, q)
+      receptionist ! Find(ServiceKeyA, regProbe.ref)
+      regProbe.expectMsg(Listing(ServiceKeyA, Set(serviceA, serviceC)))
+      receptionist ! Find(ServiceKeyB, regProbe.ref)
+      regProbe.expectMsg(Listing(ServiceKeyB, Set(serviceB, serviceC)))
+
+      serviceC ! Stop
+
+      eventually {
+        receptionist ! Find(ServiceKeyA, regProbe.ref)
+        regProbe.expectMsg(Listing(ServiceKeyA, Set(serviceA)))
+        receptionist ! Find(ServiceKeyB, regProbe.ref)
+        regProbe.expectMsg(Listing(ServiceKeyB, Set(serviceB)))
+      }
+    }
+
+    def `must support subscribing to service changes`(): Unit = new TestSetup {
+      val regProbe = TestProbe[Registered[_]]("regProbe")
+
+      val aSubscriber = TestProbe[Listing[ServiceA]]("aUser")
+      receptionist ! Subscribe(ServiceKeyA, aSubscriber.ref)
+
+      aSubscriber.expectMsg(Listing(ServiceKeyA, Set.empty[ActorRef[ServiceA]]))
+
+      val serviceA: ActorRef[ServiceA] = start(stoppableBehavior)
+      receptionist ! Register(ServiceKeyA, serviceA, regProbe.ref)
+      regProbe.expectMsg(Registered(ServiceKeyA, serviceA))
+
+      aSubscriber.expectMsg(Listing(ServiceKeyA, Set(serviceA)))
+
+      val serviceA2: ActorRef[ServiceA] = start(stoppableBehavior)
+      receptionist ! Register(ServiceKeyA, serviceA2, regProbe.ref)
+      regProbe.expectMsg(Registered(ServiceKeyA, serviceA2))
+
+      aSubscriber.expectMsg(Listing(ServiceKeyA, Set(serviceA, serviceA2)))
+
+      serviceA ! Stop
+      aSubscriber.expectMsg(Listing(ServiceKeyA, Set(serviceA2)))
+      serviceA2 ! Stop
+      aSubscriber.expectMsg(Listing(ServiceKeyA, Set.empty[ActorRef[ServiceA]]))
     }
 
     def `must work with ask`(): Unit = sync(runTest("Receptionist") {
@@ -132,9 +174,9 @@ class ReceptionistSpec extends TypedSpec {
       StepWise[Listing[ServiceA]] { (ctx, startWith) ⇒
         val self = ctx.self
         startWith.withKeepTraces(true) {
-          ctx.system.receptionist ! Find(ServiceKeyA)(self)
+          system.receptionist ! Find(ServiceKeyA)(self)
         }.expectMessage(1.second) { (msg, _) ⇒
-          msg.addresses should ===(Set())
+          msg.serviceInstances should ===(Set())
         }
       }
     })

akka-typed/src/main/scala/akka/typed/ActorRef.scala

@@ -4,9 +4,11 @@
 package akka.typed
 
 import akka.{ actor ⇒ a }
+
 import scala.annotation.unchecked.uncheckedVariance
 import language.implicitConversions
 import scala.concurrent.Future
+import scala.util.Success
 
 /**
  * An ActorRef is the identity or address of an Actor instance. It is valid
@@ -65,7 +67,13 @@ object ActorRef {
    * messages in while the Future is not fulfilled.
    */
   def apply[T](f: Future[ActorRef[T]], bufferSize: Int = 1000): ActorRef[T] =
-    new internal.FutureRef(FuturePath, bufferSize, f)
+    f.value match {
+      // an AdaptedActorSystem will always create refs eagerly, so it will take this path
+      case Some(Success(ref)) ⇒ ref
+      // for other ActorSystem implementations, this might work, it currently doesn't work
+      // for the adapted system, because the typed FutureRef cannot be watched from untyped
+      case x                  ⇒ new internal.FutureRef(FuturePath, bufferSize, f)
+    }
 
   /**
    * Create an ActorRef by providing a function that is invoked for sending

akka-typed/src/main/scala/akka/typed/ActorSystem.scala

@@ -17,6 +17,8 @@ import akka.annotation.DoNotInherit
 import akka.annotation.ApiMayChange
 import java.util.Optional
 
+import akka.typed.receptionist.Receptionist
+
 /**
  * An ActorSystem is home to a hierarchy of Actors. It is created using
  * [[ActorSystem#apply]] from a [[Behavior]] object that describes the root
@@ -146,9 +148,10 @@ abstract class ActorSystem[-T] extends ActorRef[T] with Extensions { this: inter
   def systemActorOf[U](behavior: Behavior[U], name: String, props: Props = Props.empty)(implicit timeout: Timeout): Future[ActorRef[U]]
 
   /**
-   * Return a reference to this system’s [[akka.typed.patterns.Receptionist$]].
+   * Return a reference to this system’s [[akka.typed.receptionist.Receptionist]].
    */
-  def receptionist: ActorRef[patterns.Receptionist.Command]
+  def receptionist: ActorRef[Receptionist.Command] =
+    Receptionist(this).ref
 }
 
 object ActorSystem {

akka-typed/src/main/scala/akka/typed/cluster/internal/receptionist/ClusterReceptionist.scala

@@ -0,0 +1,119 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.typed.cluster.internal.receptionist
+
+import akka.annotation.InternalApi
+import akka.cluster.Cluster
+import akka.cluster.ddata.DistributedData
+import akka.cluster.ddata.ORMultiMap
+import akka.cluster.ddata.ORMultiMapKey
+import akka.cluster.ddata.Replicator
+import akka.cluster.ddata.Replicator.WriteConsistency
+import akka.typed.ActorRef
+import akka.typed.Behavior
+import akka.typed.internal.receptionist.ReceptionistBehaviorProvider
+import akka.typed.internal.receptionist.ReceptionistImpl
+import akka.typed.internal.receptionist.ReceptionistImpl._
+import akka.typed.receptionist.Receptionist.AbstractServiceKey
+import akka.typed.receptionist.Receptionist.AllCommands
+import akka.typed.receptionist.Receptionist.Command
+import akka.typed.receptionist.Receptionist.ServiceKey
+import akka.typed.scaladsl.ActorContext
+
+import scala.language.existentials
+import scala.language.higherKinds
+
+/** Internal API */
+@InternalApi
+private[typed] object ClusterReceptionist extends ReceptionistBehaviorProvider {
+  private final val ReceptionistKey = ORMultiMapKey[ServiceKey[_], ActorRef[_]]("ReceptionistKey")
+  private final val EmptyORMultiMap = ORMultiMap.empty[ServiceKey[_], ActorRef[_]]
+
+  case class TypedORMultiMap[K[_], V[_]](val map: ORMultiMap[K[_], V[_]]) extends AnyVal {
+    def getOrElse[T](key: K[T], default: ⇒ Set[V[T]]): Set[V[T]] =
+      map.getOrElse(key, default.asInstanceOf[Set[V[_]]]).asInstanceOf[Set[V[T]]]
+
+    def getOrEmpty[T](key: K[T]): Set[V[T]] = getOrElse(key, Set.empty)
+
+    def addBinding[T](key: K[T], value: V[T])(implicit cluster: Cluster): TypedORMultiMap[K, V] =
+      TypedORMultiMap[K, V](map.addBinding(key, value))
+
+    def removeBinding[T](key: K[T], value: V[T])(implicit cluster: Cluster): TypedORMultiMap[K, V] =
+      TypedORMultiMap[K, V](map.removeBinding(key, value))
+
+    def toORMultiMap: ORMultiMap[K[_], V[_]] = map
+  }
+  object TypedORMultiMap {
+    def empty[K[_], V[_]] = TypedORMultiMap[K, V](ORMultiMap.empty[K[_], V[_]])
+  }
+  type ServiceRegistry = TypedORMultiMap[ServiceKey, ActorRef]
+  object ServiceRegistry {
+    def empty: ServiceRegistry = TypedORMultiMap.empty
+    def apply(map: ORMultiMap[ServiceKey[_], ActorRef[_]]): ServiceRegistry = TypedORMultiMap[ServiceKey, ActorRef](map)
+  }
+
+  def behavior: Behavior[Command] = clusterBehavior
+  val clusterBehavior: Behavior[Command] = ReceptionistImpl.init(clusteredReceptionist())
+
+  case class ClusterReceptionistSettings(
+    writeConsistency: WriteConsistency = Replicator.WriteLocal
+  )
+
+  /**
+   * Returns an ReceptionistImpl.ExternalInterface that synchronizes registered services with
+   */
+  def clusteredReceptionist(settings: ClusterReceptionistSettings = ClusterReceptionistSettings())(ctx: ActorContext[AllCommands]): ReceptionistImpl.ExternalInterface = {
+    import akka.typed.scaladsl.adapter._
+    val untypedSystem = ctx.system.toUntyped
+
+    val replicator = DistributedData(untypedSystem).replicator
+    implicit val cluster = Cluster(untypedSystem)
+
+    var state = ServiceRegistry.empty
+
+    def diff(lastState: ServiceRegistry, newState: ServiceRegistry): Map[AbstractServiceKey, Set[ActorRef[_]]] = {
+      def changesForKey[T](registry: Map[AbstractServiceKey, Set[ActorRef[_]]], key: ServiceKey[T]): Map[AbstractServiceKey, Set[ActorRef[_]]] = {
+        val oldValues = lastState.getOrEmpty(key)
+        val newValues = newState.getOrEmpty(key)
+        if (oldValues != newValues)
+          registry + (key → newValues.asInstanceOf[Set[ActorRef[_]]])
+        else
+          registry
+      }
+
+      val allKeys = lastState.toORMultiMap.entries.keySet ++ newState.toORMultiMap.entries.keySet
+      allKeys
+        .foldLeft(Map.empty[AbstractServiceKey, Set[ActorRef[_]]])(changesForKey(_, _))
+    }
+
+    val adapter: ActorRef[Replicator.ReplicatorMessage] =
+      ctx.spawnAdapter[Replicator.ReplicatorMessage] { (x: Replicator.ReplicatorMessage) ⇒
+        x match {
+          case changed @ Replicator.Changed(ReceptionistKey) ⇒
+            val value = changed.get(ReceptionistKey)
+            val oldState = state
+            state = ServiceRegistry(value) // is that thread-safe?
+            val changes = diff(oldState, state)
+            RegistrationsChangedExternally(changes)
+        }
+      }
+
+    replicator ! Replicator.Subscribe(ReceptionistKey, adapter.toUntyped)
+
+    new ExternalInterface {
+      private def updateRegistry(update: ServiceRegistry ⇒ ServiceRegistry): Unit = {
+        state = update(state)
+        replicator ! Replicator.Update(ReceptionistKey, EmptyORMultiMap, settings.writeConsistency) { registry ⇒
+          update(ServiceRegistry(registry)).toORMultiMap
+        }
+      }
+
+      def onRegister[T](key: ServiceKey[T], address: ActorRef[T]): Unit =
+        updateRegistry(_.addBinding(key, address))
+
+      def onUnregister[T](key: ServiceKey[T], address: ActorRef[T]): Unit =
+        updateRegistry(_.removeBinding(key, address))
+    }
+  }
+}

akka-typed/src/main/scala/akka/typed/internal/ActorSystemImpl.scala

@@ -5,22 +5,29 @@ package akka.typed
 package internal
 
 import com.typesafe.config.Config
+
 import scala.concurrent.ExecutionContext
 import java.util.concurrent.ThreadFactory
+
 import scala.concurrent.{ ExecutionContextExecutor, Future }
 import akka.{ actor ⇒ a, dispatch ⇒ d, event ⇒ e }
+
 import scala.util.control.NonFatal
 import scala.util.control.ControlThrowable
 import scala.collection.immutable
 import akka.typed.Dispatchers
+
 import scala.concurrent.Promise
 import java.util.concurrent.ConcurrentSkipListSet
 import java.util.concurrent.atomic.AtomicBoolean
+
 import scala.collection.JavaConverters._
 import scala.util.Success
 import akka.util.Timeout
 import java.io.Closeable
 import java.util.concurrent.atomic.AtomicInteger
+
+import akka.typed.receptionist.Receptionist
 import akka.typed.scaladsl.AskPattern
 
 object ActorSystemImpl {
@@ -227,9 +234,6 @@ private[typed] class ActorSystemImpl[-T](
 
   private val systemGuardian: ActorRefImpl[SystemCommand] = createTopLevel(systemGuardianBehavior, "system", EmptyProps)
 
-  override val receptionist: ActorRef[patterns.Receptionist.Command] =
-    ActorRef(systemActorOf(patterns.Receptionist.behavior, "receptionist")(settings.untyped.CreationTimeout))
-
   private val userGuardian: ActorRefImpl[T] = createTopLevel(_userGuardianBehavior, "user", _userGuardianProps)
 
   // now we can start up the loggers

akka-typed/src/main/scala/akka/typed/internal/ExtensionsImpl.scala

@@ -25,7 +25,7 @@ trait ExtensionsImpl extends Extensions { self: ActorSystem[_] ⇒
   /**
    * Hook for ActorSystem to load extensions on startup
    */
-  protected final def loadExtensions() {
+  protected final def loadExtensions(): Unit = {
     /**
      * @param throwOnLoadFail Throw exception when an extension fails to load (needed for backwards compatibility)
      */

akka-typed/src/main/scala/akka/typed/internal/adapter/ActorSystemAdapter.scala

@@ -5,17 +5,13 @@ package akka.typed
 package internal
 package adapter
 
-import akka.{ actor ⇒ a, dispatch ⇒ d }
-import akka.dispatch.sysmsg
+import akka.{ actor ⇒ a }
 
 import scala.concurrent.ExecutionContextExecutor
 import akka.util.Timeout
 
 import scala.concurrent.Future
 import akka.annotation.InternalApi
-import akka.typed.scaladsl.adapter.AdapterExtension
-
-import scala.annotation.unchecked.uncheckedVariance
 
 /**
  * INTERNAL API. Lightweight wrapper for presenting an untyped ActorSystem to a Behavior (via the context).
@@ -66,9 +62,6 @@ import scala.annotation.unchecked.uncheckedVariance
   override def uptime: Long = untyped.uptime
   override def printTree: String = untyped.printTree
 
-  override def receptionist: ActorRef[patterns.Receptionist.Command] =
-    ReceptionistExtension(untyped).receptionist
-
   import akka.dispatch.ExecutionContexts.sameThreadExecutionContext
 
   override def terminate(): scala.concurrent.Future[akka.typed.Terminated] =
@@ -103,19 +96,5 @@ private[typed] object ActorSystemAdapter {
       case _ ⇒ throw new UnsupportedOperationException("only adapted untyped ActorSystem permissible " +
         s"($sys of class ${sys.getClass.getName})")
     }
-
-  object ReceptionistExtension extends a.ExtensionId[ReceptionistExtension] with a.ExtensionIdProvider {
-    override def get(system: a.ActorSystem): ReceptionistExtension = super.get(system)
-    override def lookup = ReceptionistExtension
-    override def createExtension(system: a.ExtendedActorSystem): ReceptionistExtension =
-      new ReceptionistExtension(system)
-  }
-
-  class ReceptionistExtension(system: a.ExtendedActorSystem) extends a.Extension {
-    val receptionist: ActorRef[patterns.Receptionist.Command] =
-      ActorRefAdapter(system.systemActorOf(
-        PropsAdapter(() ⇒ patterns.Receptionist.behavior, EmptyProps),
-        "receptionist"))
-  }
 }
 

akka-typed/src/main/scala/akka/typed/internal/receptionist/ReceptionistImpl.scala

@@ -0,0 +1,160 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.typed.internal.receptionist
+
+import akka.annotation.InternalApi
+import akka.typed.ActorRef
+import akka.typed.Behavior
+import akka.typed.Terminated
+import akka.typed.receptionist.Receptionist._
+import akka.typed.scaladsl.Actor
+import akka.typed.scaladsl.Actor.immutable
+import akka.typed.scaladsl.Actor.same
+import akka.typed.scaladsl.ActorContext
+import akka.util.TypedMultiMap
+
+import scala.reflect.ClassTag
+
+/**
+ * Marker interface to use with dynamic access
+ *
+ * Internal API
+ */
+@InternalApi
+private[typed] trait ReceptionistBehaviorProvider {
+  def behavior: Behavior[Command]
+}
+
+/** Internal API */
+@InternalApi
+private[typed] object ReceptionistImpl extends ReceptionistBehaviorProvider {
+  // FIXME: make sure to provide serializer
+  case class DefaultServiceKey[T](id: String)(implicit tTag: ClassTag[T]) extends ServiceKey[T] {
+    override def toString: String = s"ServiceKey[$tTag]($id)"
+  }
+
+  /**
+   * Interface to allow plugging of external service discovery infrastructure in to the existing receptionist API.
+   */
+  trait ExternalInterface {
+    def onRegister[T](key: ServiceKey[T], address: ActorRef[T]): Unit
+    def onUnregister[T](key: ServiceKey[T], address: ActorRef[T]): Unit
+  }
+  object LocalExternalInterface extends ExternalInterface {
+    def onRegister[T](key: ServiceKey[T], address: ActorRef[T]): Unit = ()
+    def onUnregister[T](key: ServiceKey[T], address: ActorRef[T]): Unit = ()
+  }
+
+  override def behavior: Behavior[Command] = localOnlyBehavior
+  val localOnlyBehavior: Behavior[Command] = init(_ ⇒ LocalExternalInterface)
+
+  type KV[K <: AbstractServiceKey] = ActorRef[K#Protocol]
+  type LocalServiceRegistry = TypedMultiMap[AbstractServiceKey, KV]
+  object LocalServiceRegistry {
+    val empty: LocalServiceRegistry = TypedMultiMap.empty[AbstractServiceKey, KV]
+  }
+
+  sealed abstract class ReceptionistInternalCommand extends InternalCommand
+  final case class RegisteredActorTerminated[T](key: ServiceKey[T], address: ActorRef[T]) extends ReceptionistInternalCommand
+  final case class SubscriberTerminated[T](key: ServiceKey[T], address: ActorRef[Listing[T]]) extends ReceptionistInternalCommand
+  final case class RegistrationsChangedExternally(changes: Map[AbstractServiceKey, Set[ActorRef[_]]]) extends ReceptionistInternalCommand
+
+  type SubscriptionsKV[K <: AbstractServiceKey] = ActorRef[Listing[K#Protocol]]
+  type SubscriptionRegistry = TypedMultiMap[AbstractServiceKey, SubscriptionsKV]
+
+  private[typed] def init(externalInterfaceFactory: ActorContext[AllCommands] ⇒ ExternalInterface): Behavior[Command] =
+    Actor.deferred[AllCommands] { ctx ⇒
+      val externalInterface = externalInterfaceFactory(ctx)
+      behavior(
+        TypedMultiMap.empty[AbstractServiceKey, KV],
+        TypedMultiMap.empty[AbstractServiceKey, SubscriptionsKV],
+        externalInterface)
+    }.narrow[Command]
+
+  private def behavior(
+    serviceRegistry:   LocalServiceRegistry,
+    subscriptions:     SubscriptionRegistry,
+    externalInterface: ExternalInterface): Behavior[AllCommands] = {
+
+    /** Helper to create new state */
+    def next(newRegistry: LocalServiceRegistry = serviceRegistry, newSubscriptions: SubscriptionRegistry = subscriptions) =
+      behavior(newRegistry, newSubscriptions, externalInterface)
+
+    /**
+     * Hack to allow multiple termination notifications per target
+     * FIXME: replace by simple map in our state
+     */
+    def watchWith(ctx: ActorContext[AllCommands], target: ActorRef[_], msg: AllCommands): Unit =
+      ctx.spawnAnonymous[Nothing](Actor.deferred[Nothing] { innerCtx ⇒
+        innerCtx.watch(target)
+        Actor.immutable[Nothing]((_, _) ⇒ Actor.same)
+          .onSignal {
+            case (_, Terminated(`target`)) ⇒
+              ctx.self ! msg
+              Actor.stopped
+          }
+      })
+
+    /** Helper that makes sure that subscribers are notified when an entry is changed */
+    def updateRegistry(changedKeysHint: Set[AbstractServiceKey], f: LocalServiceRegistry ⇒ LocalServiceRegistry): Behavior[AllCommands] = {
+      val newRegistry = f(serviceRegistry)
+
+      def notifySubscribersFor[T](key: AbstractServiceKey): Unit = {
+        val newListing = newRegistry.get(key)
+        subscriptions.get(key).foreach(_ ! Listing(key.asServiceKey, newListing))
+      }
+
+      changedKeysHint foreach notifySubscribersFor
+      next(newRegistry = newRegistry)
+    }
+
+    def replyWithListing[T](key: ServiceKey[T], replyTo: ActorRef[Listing[T]]): Unit =
+      replyTo ! Listing(key, serviceRegistry get key)
+
+    immutable[AllCommands] { (ctx, msg) ⇒
+      msg match {
+        case Register(key, serviceInstance, replyTo) ⇒
+          println(s"[${ctx.self}] Actor was registered: $key $serviceInstance")
+          watchWith(ctx, serviceInstance, RegisteredActorTerminated(key, serviceInstance))
+          replyTo ! Registered(key, serviceInstance)
+          externalInterface.onRegister(key, serviceInstance)
+
+          updateRegistry(Set(key), _.inserted(key)(serviceInstance))
+
+        case Find(key, replyTo) ⇒
+          replyWithListing(key, replyTo)
+
+          same
+
+        case RegistrationsChangedExternally(changes) ⇒
+          println(s"[${ctx.self}] Registration changed: $changes")
+
+          // FIXME: get rid of casts
+          def makeChanges(registry: LocalServiceRegistry): LocalServiceRegistry =
+            changes.foldLeft(registry) {
+              case (reg, (key, values)) ⇒
+                reg.setAll(key)(values.asInstanceOf[Set[ActorRef[key.Protocol]]])
+            }
+
+          updateRegistry(changes.keySet, makeChanges) // overwrite all changed keys
+
+        case RegisteredActorTerminated(key, serviceInstance) ⇒
+          println(s"[${ctx.self}] Registered actor terminated: $key $serviceInstance")
+          externalInterface.onUnregister(key, serviceInstance)
+          updateRegistry(Set(key), _.removed(key)(serviceInstance))
+
+        case Subscribe(key, subscriber) ⇒
+          watchWith(ctx, subscriber, SubscriberTerminated(key, subscriber))
+
+          // immediately reply with initial listings to the new subscriber
+          replyWithListing(key, subscriber)
+
+          next(newSubscriptions = subscriptions.inserted(key)(subscriber))
+
+        case SubscriberTerminated(key, subscriber) ⇒
+          next(newSubscriptions = subscriptions.removed(key)(subscriber))
+      }
+    }
+  }
+}

akka-typed/src/main/scala/akka/typed/patterns/Receptionist.scala

@@ -1,93 +0,0 @@
-/**
- * Copyright (C) 2014-2017 Lightbend Inc. <http://www.lightbend.com>
- */
-package akka.typed.patterns
-
-import akka.typed.ActorRef
-import akka.typed.Behavior
-import akka.typed.Terminated
-import akka.util.TypedMultiMap
-import akka.typed.scaladsl.Actor._
-
-/**
- * A Receptionist is an entry point into an Actor hierarchy where select Actors
- * publish their identity together with the protocols that they implement. Other
- * Actors need only know the Receptionist’s identity in order to be able to use
- * the services of the registered Actors.
- */
-object Receptionist {
-
-  /**
-   * Internal representation of [[Receptionist.ServiceKey]] which is needed
-   * in order to use a TypedMultiMap (using keys with a type parameter does not
-   * work in Scala 2.x).
-   */
-  trait AbstractServiceKey {
-    type Type
-  }
-
-  /**
-   * A service key is an object that implements this trait for a given protocol
-   * T, meaning that it signifies that the type T is the entry point into the
-   * protocol spoken by that service (think of it as the set of first messages
-   * that a client could send).
-   */
-  trait ServiceKey[T] extends AbstractServiceKey {
-    final override type Type = T
-  }
-
-  /**
-   * The set of commands accepted by a Receptionist.
-   */
-  sealed trait Command
-  /**
-   * Associate the given [[akka.typed.ActorRef]] with the given [[ServiceKey]]. Multiple
-   * registrations can be made for the same key. Unregistration is implied by
-   * the end of the referenced Actor’s lifecycle.
-   */
-  final case class Register[T](key: ServiceKey[T], address: ActorRef[T])(val replyTo: ActorRef[Registered[T]]) extends Command
-  /**
-   * Query the Receptionist for a list of all Actors implementing the given
-   * protocol.
-   */
-  final case class Find[T](key: ServiceKey[T])(val replyTo: ActorRef[Listing[T]]) extends Command
-
-  /**
-   * Confirmation that the given [[akka.typed.ActorRef]] has been associated with the [[ServiceKey]].
-   */
-  final case class Registered[T](key: ServiceKey[T], address: ActorRef[T])
-  /**
-   * Current listing of all Actors that implement the protocol given by the [[ServiceKey]].
-   */
-  final case class Listing[T](key: ServiceKey[T], addresses: Set[ActorRef[T]])
-
-  /**
-   * Initial behavior of a receptionist, used to create a new receptionist like in the following:
-   *
-   * {{{
-   * val receptionist: ActorRef[Receptionist.Command] = ctx.spawn(Props(Receptionist.behavior), "receptionist")
-   * }}}
-   */
-  val behavior: Behavior[Command] = behavior(TypedMultiMap.empty[AbstractServiceKey, KV])
-
-  private type KV[K <: AbstractServiceKey] = ActorRef[K#Type]
-
-  private def behavior(map: TypedMultiMap[AbstractServiceKey, KV]): Behavior[Command] = immutable[Command] { (ctx, msg) ⇒
-    msg match {
-      case r: Register[t] ⇒
-        ctx.watch(r.address)
-        r.replyTo ! Registered(r.key, r.address)
-        behavior(map.inserted(r.key)(r.address))
-      case f: Find[t] ⇒
-        val set = map get f.key
-        f.replyTo ! Listing(f.key, set)
-        same
-    }
-  } onSignal {
-    case (ctx, Terminated(ref)) ⇒
-      behavior(map valueRemoved ref)
-    case x ⇒ unhandled
-  }
-}
-
-abstract class Receptionist

akka-typed/src/main/scala/akka/typed/receptionist/Receptionist.scala

@@ -0,0 +1,150 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.typed.receptionist
+
+import akka.annotation.InternalApi
+import akka.typed.ActorRef
+import akka.typed.ActorSystem
+import akka.typed.Extension
+import akka.typed.ExtensionId
+import akka.typed.internal.receptionist.ReceptionistBehaviorProvider
+import akka.typed.internal.receptionist.ReceptionistImpl
+
+import scala.collection.JavaConverters._
+import scala.concurrent.duration._
+import scala.reflect.ClassTag
+
+class Receptionist(system: ActorSystem[_]) extends Extension {
+  private def hasCluster: Boolean =
+    // FIXME: replace with better indicator that cluster is enabled
+    system.settings.config.getString("akka.actor.provider") == "cluster"
+
+  val ref: ActorRef[Receptionist.Command] = {
+    val behavior =
+      if (hasCluster)
+        system.dynamicAccess
+          .createInstanceFor[ReceptionistBehaviorProvider]("akka.typed.cluster.internal.receptionist.ClusterReceptionist$", Nil)
+          .recover {
+            case ex ⇒
+              system.log.error(
+                ex,
+                "ClusterReceptionist could not be loaded dynamically. Make sure you have all required binaries on the classpath.")
+              ReceptionistImpl
+          }.get.behavior
+
+      else ReceptionistImpl.localOnlyBehavior
+
+    ActorRef(
+      system.systemActorOf(behavior, "receptionist")(
+        // FIXME: where should that timeout be configured? Shouldn't there be a better `Extension`
+        //        implementation that does this dance for us?
+
+        10.seconds
+      ))
+  }
+}
+
+/**
+ * A Receptionist is an entry point into an Actor hierarchy where select Actors
+ * publish their identity together with the protocols that they implement. Other
+ * Actors need only know the Receptionist’s identity in order to be able to use
+ * the services of the registered Actors.
+ */
+object Receptionist extends ExtensionId[Receptionist] {
+  def createExtension(system: ActorSystem[_]): Receptionist = new Receptionist(system)
+  def get(system: ActorSystem[_]): Receptionist = apply(system)
+
+  /**
+   * Internal representation of [[ServiceKey]] which is needed
+   * in order to use a TypedMultiMap (using keys with a type parameter does not
+   * work in Scala 2.x).
+   *
+   * Internal API
+   */
+  @InternalApi
+  private[typed] sealed abstract class AbstractServiceKey {
+    type Protocol
+
+    /** Type-safe down-cast */
+    def asServiceKey: ServiceKey[Protocol]
+  }
+
+  /**
+   * A service key is an object that implements this trait for a given protocol
+   * T, meaning that it signifies that the type T is the entry point into the
+   * protocol spoken by that service (think of it as the set of first messages
+   * that a client could send).
+   */
+  abstract class ServiceKey[T] extends AbstractServiceKey {
+    final type Protocol = T
+    def id: String
+    def asServiceKey: ServiceKey[T] = this
+  }
+
+  object ServiceKey {
+    /**
+     * Creates a service key. The given ID should uniquely define a service with a given protocol.
+     */
+    // FIXME: not sure if the ClassTag pulls its weight. It's only used in toString currently.
+    def apply[T](id: String)(implicit tTag: ClassTag[T]): ServiceKey[T] = ReceptionistImpl.DefaultServiceKey(id)
+  }
+
+  /** Internal superclass for external and internal commands */
+  @InternalApi
+  sealed private[typed] abstract class AllCommands
+
+  /**
+   * The set of commands accepted by a Receptionist.
+   */
+  sealed abstract class Command extends AllCommands
+  @InternalApi
+  private[typed] abstract class InternalCommand extends AllCommands
+
+  /**
+   * Associate the given [[akka.typed.ActorRef]] with the given [[ServiceKey]]. Multiple
+   * registrations can be made for the same key. Unregistration is implied by
+   * the end of the referenced Actor’s lifecycle.
+   *
+   * Registration will be acknowledged with the [[Registered]] message to the given replyTo actor.
+   */
+  final case class Register[T](key: ServiceKey[T], serviceInstance: ActorRef[T], replyTo: ActorRef[Registered[T]]) extends Command
+  object Register {
+    /** Auxiliary constructor to be used with the ask pattern */
+    def apply[T](key: ServiceKey[T], service: ActorRef[T]): ActorRef[Registered[T]] ⇒ Register[T] =
+      replyTo ⇒ Register(key, service, replyTo)
+  }
+
+  /**
+   * Confirmation that the given [[akka.typed.ActorRef]] has been associated with the [[ServiceKey]].
+   */
+  final case class Registered[T](key: ServiceKey[T], serviceInstance: ActorRef[T])
+
+  /**
+   * Subscribe the given actor to service updates. When new instances are registered or unregistered to the given key
+   * the given subscriber will be sent a [[Listing]] with the new set of instances for that service.
+   *
+   * The subscription will be acknowledged by sending out a first [[Listing]]. The subscription automatically ends
+   * with the termination of the subscriber.
+   */
+  final case class Subscribe[T](key: ServiceKey[T], subscriber: ActorRef[Listing[T]]) extends Command
+
+  /**
+   * Query the Receptionist for a list of all Actors implementing the given
+   * protocol.
+   */
+  final case class Find[T](key: ServiceKey[T], replyTo: ActorRef[Listing[T]]) extends Command
+  object Find {
+    /** Auxiliary constructor to use with the ask pattern */
+    def apply[T](key: ServiceKey[T]): ActorRef[Listing[T]] ⇒ Find[T] =
+      replyTo ⇒ Find(key, replyTo)
+  }
+
+  /**
+   * Current listing of all Actors that implement the protocol given by the [[ServiceKey]].
+   */
+  final case class Listing[T](key: ServiceKey[T], serviceInstances: Set[ActorRef[T]]) {
+    /** Java API */
+    def getServiceInstances: java.util.Set[ActorRef[T]] = serviceInstances.asJava
+  }
+}