Don't stop dynamic router when all routees terminated, see #2734

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

@@ -100,6 +100,26 @@ class RoutingSpec extends AkkaSpec(RoutingSpec.config) with DefaultTimeout with
       }
     }
 
+    "not terminate when resizer is used" in {
+      val latch = TestLatch(1)
+      val resizer = new Resizer {
+        def isTimeForResize(messageCounter: Long): Boolean = messageCounter == 0
+        def resize(routeeProvider: RouteeProvider): Unit = {
+          routeeProvider.createRoutees(nrOfInstances = 2)
+          latch.countDown()
+        }
+      }
+      val router = system.actorOf(Props[TestActor].withRouter(RoundRobinRouter(resizer = Some(resizer))))
+      watch(router)
+      Await.ready(latch, remaining)
+      router ! CurrentRoutees
+      val routees = expectMsgType[RouterRoutees].routees
+      routees.size must be(2)
+      routees foreach system.stop
+      // expect no Terminated
+      expectNoMsg(2.seconds)
+    }
+
     "be able to send their routees" in {
       case class TestRun(id: String, names: immutable.Iterable[String], actors: Int)
       val actor = system.actorOf(Props(new Actor {

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

@@ -215,6 +215,12 @@ trait RouterConfig {
    */
   def verifyConfig(): Unit = ()
 
+  /*
+   * Specify that this router should stop itself when all routees have terminated (been removed).
+   * By Default it is `true`, unless a `resizer` is used.
+   */
+  def stopRouterWhenAllRouteesRemoved: Boolean = resizer.isEmpty
+
 }
 
 /**
@@ -373,7 +379,8 @@ trait Router extends Actor {
 
     case Terminated(child) ⇒
       ref.removeRoutees(child :: Nil)
-      if (ref.routees.isEmpty) context.stop(self)
+      if (ref.routees.isEmpty && ref.routerConfig.stopRouterWhenAllRouteesRemoved)
+        context.stop(self)
 
   }: Receive) orElse routerReceive
 

akka-cluster/src/main/scala/akka/cluster/routing/ClusterRouterConfig.scala

@@ -60,6 +60,8 @@ final case class ClusterRouterConfig(local: RouterConfig, settings: ClusterRoute
 
   override def resizer: Option[Resizer] = local.resizer
 
+  override def stopRouterWhenAllRouteesRemoved: Boolean = false
+
   override def withFallback(other: RouterConfig): RouterConfig = other match {
     case ClusterRouterConfig(_: RemoteRouterConfig, _) ⇒ throw new IllegalStateException(
       "ClusterRouterConfig is not allowed to wrap a RemoteRouterConfig")

akka-docs/rst/java/routing.rst

@@ -149,7 +149,8 @@ direct children of their grand-parent instead.
 
   If the child of a router terminates, the router will not automatically spawn
   a new child. In the event that all children of a router have terminated the
-  router will terminate itself.
+  router will terminate itself, unless it is a dynamic router, e.g. using
+  a resizer.
 
 Router usage
 ^^^^^^^^^^^^
@@ -378,7 +379,7 @@ documentation for details.
 
 .. includecode:: code/docs/jrouting/RouterViaProgramDocTestBase.java#poisonPill
 
-For a router, which normally passes on messages to routees, it is important to realised that
+For a router, which normally passes on messages to routees, it is important to realise that
 ``PoisonPill`` messages are processed by the router only. ``PoisonPill`` messages sent to a router
 will *not* be sent on to routees.
 
@@ -399,7 +400,8 @@ routees aren't children of the router, i.e. even routees programmatically provid
 With the code shown above, each routee will receive a ``PoisonPill`` message. Each routee will
 continue to process its messages as normal, eventually processing the ``PoisonPill``. This will
 cause the routee to stop. After all routees have stopped the router will itself be :ref:`stopped
-automatically <note-router-terminated-children-java>`.
+automatically <note-router-terminated-children-java>` unless it is a dynamic router, e.g. using
+a resizer.
 
 .. note::
 

akka-docs/rst/scala/routing.rst

@@ -151,7 +151,8 @@ direct children of their grand-parent instead.
 
   If the child of a router terminates, the router will not automatically spawn
   a new child. In the event that all children of a router have terminated the
-  router will terminate itself.
+  router will terminate itself unless it is a dynamic router, e.g. using
+  a resizer.
 
 Router usage
 ^^^^^^^^^^^^
@@ -396,7 +397,8 @@ routees aren't children of the router, i.e. even routees programmatically provid
 With the code shown above, each routee will receive a ``PoisonPill`` message. Each routee will
 continue to process its messages as normal, eventually processing the ``PoisonPill``. This will
 cause the routee to stop. After all routees have stopped the router will itself be :ref:`stopped
-automatically <note-router-terminated-children-scala>`.
+automatically <note-router-terminated-children-scala>` unless it is a dynamic router, e.g. using
+a resizer.
 
 .. note::