fix some more comments and make Router dispatcher configurable

- assert locking balance when using Unsafe.instance.monitorExit
- add RouterConfig.routerDispatcher
- re-enable “busy” resizer test after switching to BalancingDispatcher
- document resizer asynchronicity and how to configure dispatchers

akka-actor-tests/src/test/scala/akka/routing/ResizerSpec.scala

@@ -24,6 +24,9 @@ object ResizerSpec {
         }
       }
     }
+    bal-disp {
+      type = BalancingDispatcher
+    }
     """
 
   class TestActor extends Actor {
@@ -123,15 +126,7 @@ class ResizerSpec extends AkkaSpec(ResizerSpec.config) with DefaultTimeout with
       current.routees.size must be(2)
     }
 
-    /*
-     * TODO RK This test seems invalid to me, because it relies on that no resize() event is lost;
-     * this currently fails because I made resize() asynchronous (by sending a message to the
-     * Router), but it could also fail for concurrent send operations, i.e. when one of thread
-     * fails the resizeInProgress.compareAndSet(false, true) check.
-     * 
-     * Either the test must be fixed/removed or resize() must be changed to be blocking.
-     */
-    "resize when busy" ignore {
+    "resize when busy" in {
 
       val busy = new TestLatch(1)
 
@@ -141,7 +136,7 @@ class ResizerSpec extends AkkaSpec(ResizerSpec.config) with DefaultTimeout with
         pressureThreshold = 0,
         messagesPerResize = 1)
 
-      val router = system.actorOf(Props[BusyActor].withRouter(RoundRobinRouter(resizer = Some(resizer))))
+      val router = system.actorOf(Props[BusyActor].withRouter(RoundRobinRouter(resizer = Some(resizer))).withDispatcher("bal-disp"))
 
       val latch1 = new TestLatch(1)
       router ! (latch1, busy)

akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala

@@ -15,6 +15,7 @@ import com.typesafe.config.ConfigFactory
 import akka.pattern.ask
 import java.util.concurrent.ConcurrentHashMap
 import com.typesafe.config.Config
+import akka.dispatch.Dispatchers
 
 object RoutingSpec {
 
@@ -51,6 +52,7 @@ object RoutingSpec {
         case (sender, message) ⇒ Nil
       }
     }
+    def routerDispatcher: String = Dispatchers.DefaultDispatcherId
   }
 
 }
@@ -539,6 +541,8 @@ class RoutingSpec extends AkkaSpec(RoutingSpec.config) with DefaultTimeout with
     //#crRouter
     case class VoteCountRouter() extends RouterConfig {
 
+      def routerDispatcher: String = Dispatchers.DefaultDispatcherId
+
       //#crRoute
       def createRoute(routeeProps: Props, routeeProvider: RouteeProvider): Route = {
         val democratActor = routeeProvider.context.actorOf(Props(new DemocratActor()), "d")

akka-actor/src/main/scala/akka/dispatch/Future.scala

@@ -796,7 +796,7 @@ class DefaultPromise[T](implicit val executor: ExecutionContext) extends Abstrac
 
   def result(atMost: Duration)(implicit permit: CanAwait): T =
     ready(atMost).value.get match {
-      case Left(e: AskTimeoutException) ⇒ throw new AskTimeoutException(e.getMessage, e)
+      case Left(e: AskTimeoutException) ⇒ throw new AskTimeoutException(e.getMessage, e) // to get meaningful stack trace
       case Left(e)                      ⇒ throw e
       case Right(r)                     ⇒ r
     }

akka-actor/src/main/scala/akka/routing/Routing.scala

@@ -16,6 +16,7 @@ import java.util.concurrent.TimeUnit
 import java.util.concurrent.locks.ReentrantLock
 import akka.jsr166y.ThreadLocalRandom
 import akka.util.Unsafe
+import akka.dispatch.Dispatchers
 
 /**
  * A RoutedActorRef is an ActorRef that has a set of connected ActorRef and it uses a Router to
@@ -24,7 +25,7 @@ import akka.util.Unsafe
 private[akka] class RoutedActorRef(_system: ActorSystemImpl, _props: Props, _supervisor: InternalActorRef, _path: ActorPath)
   extends LocalActorRef(
     _system,
-    _props.copy(creator = () ⇒ _props.routerConfig.createActor()),
+    _props.copy(creator = () ⇒ _props.routerConfig.createActor(), dispatcher = _props.routerConfig.routerDispatcher),
     _supervisor,
     _path) {
 
@@ -76,7 +77,10 @@ private[akka] class RoutedActorRef(_system: ActorSystemImpl, _props: Props, _sup
           r.resize(routeeProps, routeeProvider)
       }
       r
-    } finally Unsafe.instance.monitorExit(actorContext) // unblock Router’s constructor
+    } finally {
+      assert(Thread.holdsLock(actorContext))
+      Unsafe.instance.monitorExit(actorContext) // unblock Router’s constructor
+    }
 
   if (routerConfig.resizer.isEmpty && _routees.isEmpty)
     throw new ActorInitializationException("router " + routerConfig + " did not register routees!")
@@ -169,6 +173,11 @@ trait RouterConfig {
 
   def createActor(): Router = new Router {}
 
+  /**
+   * Dispatcher ID to use for running the “head” actor, i.e. the [[akka.routing.Router]].
+   */
+  def routerDispatcher: String
+
   /**
    * Overridable merge strategy, by default completely prefers “this” (i.e. no merge).
    */
@@ -343,6 +352,7 @@ case class Destination(sender: ActorRef, recipient: ActorRef)
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
 case object NoRouter extends RouterConfig {
   def createRoute(props: Props, routeeProvider: RouteeProvider): Route = null
+  def routerDispatcher: String = ""
   override def withFallback(other: RouterConfig): RouterConfig = other
 }
 
@@ -352,13 +362,17 @@ case object NoRouter extends RouterConfig {
 case object FromConfig extends RouterConfig {
   def createRoute(props: Props, routeeProvider: RouteeProvider): Route =
     throw new ConfigurationException("router " + routeeProvider.context.self + " needs external configuration from file (e.g. application.conf)")
+  def routerDispatcher: String = Dispatchers.DefaultDispatcherId
 }
 
 /**
  * Java API: Router configuration which has no default, i.e. external configuration is required.
+ *
+ * This can be used when the dispatcher to be used for the head Router needs to be configured
+ * (defaults to default-dispatcher).
  */
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
-case class FromConfig() extends RouterConfig {
+case class FromConfig(val routerDispatcher: String = Dispatchers.DefaultDispatcherId) extends RouterConfig {
   def createRoute(props: Props, routeeProvider: RouteeProvider): Route =
     throw new ConfigurationException("router " + routeeProvider.context.self + " needs external configuration from file (e.g. application.conf)")
 }
@@ -389,7 +403,8 @@ object RoundRobinRouter {
  *   using `actorFor` in [[akka.actor.ActorRefProvider]]
  */
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
-case class RoundRobinRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None)
+case class RoundRobinRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None,
+                            val routerDispatcher: String = Dispatchers.DefaultDispatcherId)
   extends RouterConfig with RoundRobinLike {
 
   /**
@@ -415,6 +430,11 @@ case class RoundRobinRouter(nrOfInstances: Int = 0, routees: Iterable[String] =
    * Java API
    */
   def this(resizer: Resizer) = this(resizer = Some(resizer))
+
+  /**
+   * Java API for setting routerDispatcher
+   */
+  def withDispatcher(dispatcherId: String) = copy(routerDispatcher = dispatcherId)
 }
 
 trait RoundRobinLike { this: RouterConfig ⇒
@@ -469,7 +489,8 @@ object RandomRouter {
  *   using `actorFor` in [[akka.actor.ActorRefProvider]]
  */
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
-case class RandomRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None)
+case class RandomRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None,
+                        val routerDispatcher: String = Dispatchers.DefaultDispatcherId)
   extends RouterConfig with RandomLike {
 
   /**
@@ -495,6 +516,11 @@ case class RandomRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil,
    * Java API
    */
   def this(resizer: Resizer) = this(resizer = Some(resizer))
+
+  /**
+   * Java API for setting routerDispatcher
+   */
+  def withDispatcher(dispatcherId: String) = copy(routerDispatcher = dispatcherId)
 }
 
 trait RandomLike { this: RouterConfig ⇒
@@ -555,7 +581,8 @@ object SmallestMailboxRouter {
  *   using `actorFor` in [[akka.actor.ActorRefProvider]]
  */
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
-case class SmallestMailboxRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None)
+case class SmallestMailboxRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None,
+                                 val routerDispatcher: String = Dispatchers.DefaultDispatcherId)
   extends RouterConfig with SmallestMailboxLike {
 
   /**
@@ -581,6 +608,11 @@ case class SmallestMailboxRouter(nrOfInstances: Int = 0, routees: Iterable[Strin
    * Java API
    */
   def this(resizer: Resizer) = this(resizer = Some(resizer))
+
+  /**
+   * Java API for setting routerDispatcher
+   */
+  def withDispatcher(dispatcherId: String) = copy(routerDispatcher = dispatcherId)
 }
 
 trait SmallestMailboxLike { this: RouterConfig ⇒
@@ -700,7 +732,8 @@ object BroadcastRouter {
  *   using `actorFor` in [[akka.actor.ActorRefProvider]]
  */
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
-case class BroadcastRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None)
+case class BroadcastRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, override val resizer: Option[Resizer] = None,
+                           val routerDispatcher: String = Dispatchers.DefaultDispatcherId)
   extends RouterConfig with BroadcastLike {
 
   /**
@@ -727,6 +760,10 @@ case class BroadcastRouter(nrOfInstances: Int = 0, routees: Iterable[String] = N
    */
   def this(resizer: Resizer) = this(resizer = Some(resizer))
 
+  /**
+   * Java API for setting routerDispatcher
+   */
+  def withDispatcher(dispatcherId: String) = copy(routerDispatcher = dispatcherId)
 }
 
 trait BroadcastLike { this: RouterConfig ⇒
@@ -773,7 +810,8 @@ object ScatterGatherFirstCompletedRouter {
  */
 //TODO add @SerialVersionUID(1L) when SI-4804 is fixed
 case class ScatterGatherFirstCompletedRouter(nrOfInstances: Int = 0, routees: Iterable[String] = Nil, within: Duration,
-                                             override val resizer: Option[Resizer] = None)
+                                             override val resizer: Option[Resizer] = None,
+                                             val routerDispatcher: String = Dispatchers.DefaultDispatcherId)
   extends RouterConfig with ScatterGatherFirstCompletedLike {
 
   if (within <= Duration.Zero) throw new IllegalArgumentException(
@@ -802,6 +840,11 @@ case class ScatterGatherFirstCompletedRouter(nrOfInstances: Int = 0, routees: It
    * Java API
    */
   def this(resizer: Resizer, w: Duration) = this(resizer = Some(resizer), within = w)
+
+  /**
+   * Java API for setting routerDispatcher
+   */
+  def withDispatcher(dispatcherId: String) = copy(routerDispatcher = dispatcherId)
 }
 
 trait ScatterGatherFirstCompletedLike { this: RouterConfig ⇒

akka-docs/java/code/akka/docs/jrouting/CustomRouterDocTestBase.java

@@ -17,6 +17,7 @@ import akka.util.Duration;
 import akka.util.Timeout;
 import akka.dispatch.Await;
 import akka.dispatch.Future;
+import akka.dispatch.Dispatchers;
 import akka.testkit.AkkaSpec;
 import com.typesafe.config.ConfigFactory;
 import static akka.pattern.Patterns.ask;
@@ -39,6 +40,19 @@ public class CustomRouterDocTestBase {
     system.shutdown();
   }
   
+  public static class MyActor extends UntypedActor {
+    @Override public void onReceive(Object o) {}
+  }
+  
+  @Test
+  public void demonstrateDispatchers() {
+    //#dispatchers
+    final ActorRef router = system.actorOf(new Props(MyActor.class)
+      .withRouter(new RoundRobinRouter(5).withDispatcher("head")) // “head” router runs on "head" dispatcher
+      .withDispatcher("workers")); // MyActor “workers” run on "workers" dispatcher
+    //#dispatchers
+  }
+
   //#crTest
   @Test
   public void countVotesAsIntendedNotAsInFlorida() {
@@ -106,6 +120,10 @@ public class CustomRouterDocTestBase {
   //#crRouter
   public static class VoteCountRouter extends CustomRouterConfig {
     
+    @Override public String routerDispatcher() {
+      return Dispatchers.DefaultDispatcherId();
+    }
+
     //#crRoute
     @Override
     public CustomRoute createCustomRoute(Props props, RouteeProvider routeeProvider) {

akka-docs/java/routing.rst

@@ -8,11 +8,6 @@ Routing (Java)
 
    .. contents:: :local:
 
-Akka-core includes some building blocks to build more complex message flow handlers, they are listed and explained below:
-
-Router
-------
-
 A Router is an actor that routes incoming messages to outbound actors.
 The router routes the messages sent to it to its underlying actors called 'routees'.
 
@@ -249,6 +244,16 @@ This is an example of how to programatically create a resizable router:
 *It is also worth pointing out that if you define the ``router`` in the configuration file then this value
 will be used instead of any programmatically sent parameters.*
 
+.. note::
+
+  Resizing is triggered by sending messages to the actor pool, but it is not
+  completed synchronously; instead a message is sent to the “head”
+  :class:`Router` to perform the size change. Thus you cannot rely on resizing
+  to instantaneously create new workers when all others are busy, because the
+  message just sent will be queued to the mailbox of a busy actor. To remedy
+  this, configure the pool to use a balancing dispatcher, see `Configuring
+  Dispatchers`_ for more information.
+
 Custom Router
 ^^^^^^^^^^^^^
 
@@ -312,3 +317,23 @@ A router with dynamically resizable number of routees is implemented by providin
 in ``resizer`` method of the ``RouterConfig``. See ``akka.routing.DefaultResizer`` for inspiration
 of how to write your own resize strategy.
 
+Configuring Dispatchers
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The dispatcher for created children of the router will be taken from
+:class:`Props` as described in :ref:`dispatchers-java`. For a dynamic pool it
+makes sense to configure the :class:`BalancingDispatcher` if the precise
+routing is not so important (i.e. no consistent hashing or round-robin is
+required); this enables newly created routees to pick up work immediately by
+stealing it from their siblings.
+
+The “head” router, of couse, cannot run on the same balancing dispatcher,
+because it does not process the same messages, hence this special actor does
+not use the dispatcher configured in :class:`Props`, but takes the
+``routerDispatcher`` from the :class:`RouterConfig` instead, which defaults to
+the actor system’s default dispatcher. All standard routers allow setting this
+property in their constructor or factory method, custom routers have to
+implement the method in a suitable way.
+
+.. includecode:: code/akka/docs/jrouting/CustomRouterDocTestBase.java#dispatchers
+

akka-docs/scala/code/akka/docs/routing/RouterDocSpec.scala

@@ -0,0 +1,29 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.docs.routing
+
+import RouterDocSpec.MyActor
+import akka.actor.{ Props, Actor }
+import akka.testkit.AkkaSpec
+import akka.routing.RoundRobinRouter
+
+object RouterDocSpec {
+  class MyActor extends Actor {
+    def receive = {
+      case _ ⇒
+    }
+  }
+}
+
+class RouterDocSpec extends AkkaSpec {
+
+  import RouterDocSpec._
+
+  //#dispatchers
+  val router = system.actorOf(Props[MyActor]
+    .withRouter(RoundRobinRouter(5, routerDispatcher = "router")) // “head” will run on "router" dispatcher
+    .withDispatcher("workers")) // MyActor workers will run on "workers" dispatcher
+  //#dispatchers
+
+}

akka-docs/scala/routing.rst

@@ -8,11 +8,6 @@ Routing (Scala)
 
    .. contents:: :local:
 
-Akka-core includes some building blocks to build more complex message flow handlers, they are listed and explained below:
-
-Router
-------
-
 A Router is an actor that routes incoming messages to outbound actors.
 The router routes the messages sent to it to its underlying actors called 'routees'.
 
@@ -250,6 +245,16 @@ This is an example of how to programatically create a resizable router:
 *It is also worth pointing out that if you define the ``router`` in the configuration file then this value
 will be used instead of any programmatically sent parameters.*
 
+.. note::
+
+  Resizing is triggered by sending messages to the actor pool, but it is not
+  completed synchronously; instead a message is sent to the “head”
+  :class:`Router` to perform the size change. Thus you cannot rely on resizing
+  to instantaneously create new workers when all others are busy, because the
+  message just sent will be queued to the mailbox of a busy actor. To remedy
+  this, configure the pool to use a balancing dispatcher, see `Configuring
+  Dispatchers`_ for more information.
+
 Custom Router
 ^^^^^^^^^^^^^
 
@@ -311,3 +316,23 @@ A router with dynamically resizable number of routees is implemented by providin
 in ``resizer`` method of the ``RouterConfig``. See ``akka.routing.DefaultResizer`` for inspiration
 of how to write your own resize strategy.
 
+Configuring Dispatchers
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The dispatcher for created children of the router will be taken from
+:class:`Props` as described in :ref:`dispatchers-scala`. For a dynamic pool it
+makes sense to configure the :class:`BalancingDispatcher` if the precise
+routing is not so important (i.e. no consistent hashing or round-robin is
+required); this enables newly created routees to pick up work immediately by
+stealing it from their siblings.
+
+The “head” router, of couse, cannot run on the same balancing dispatcher,
+because it does not process the same messages, hence this special actor does
+not use the dispatcher configured in :class:`Props`, but takes the
+``routerDispatcher`` from the :class:`RouterConfig` instead, which defaults to
+the actor system’s default dispatcher. All standard routers allow setting this
+property in their constructor or factory method, custom routers have to
+implement the method in a suitable way.
+
+.. includecode:: code/akka/docs/routing/RouterDocSpec.scala#dispatchers
+

akka-remote/src/main/scala/akka/routing/RemoteRouterConfig.scala

@@ -30,6 +30,8 @@ case class RemoteRouterConfig(local: RouterConfig, nodes: Iterable[String]) exte
 
   override def createActor(): Router = local.createActor()
 
+  override def routerDispatcher: String = local.routerDispatcher
+
   override def resizer: Option[Resizer] = local.resizer
 
   override def withFallback(other: RouterConfig): RouterConfig = other match {