Load Cluster extension from ClusterActorRefProvider, see #2704

akka-cluster/src/main/scala/akka/cluster/ClusterActorRefProvider.scala

@@ -37,6 +37,10 @@ import akka.cluster.routing.MetricsSelector
 
 /**
  * INTERNAL API
+ *
+ * The `ClusterActorRefProvider` will load the [[akka.cluster.Cluster]]
+ * extension, i.e. the cluster will automatically be started when
+ * the `ClusterActorRefProvider` is used.
  */
 class ClusterActorRefProvider(
   _systemName: String,
@@ -51,6 +55,9 @@ class ClusterActorRefProvider(
   override def init(system: ActorSystemImpl): Unit = {
     super.init(system)
 
+    // initialize/load the Cluster extension
+    Cluster(system)
+
     remoteDeploymentWatcher = system.systemActorOf(Props[RemoteDeploymentWatcher], "RemoteDeploymentWatcher")
   }
 

akka-cluster/src/test/scala/akka/cluster/ClusterDomainEventPublisherSpec.scala

@@ -18,21 +18,9 @@ import akka.testkit.ImplicitSender
 import akka.actor.ActorRef
 import akka.testkit.TestProbe
 
-object ClusterDomainEventPublisherSpec {
-  val config = """
-    akka.cluster.auto-join = off
-    akka.actor.provider = "akka.cluster.ClusterActorRefProvider"
-    akka.remote.log-remote-lifecycle-events = off
-    akka.remote.netty.port = 0
-    """
-
-  case class GossipTo(address: Address)
-}
-
 @org.junit.runner.RunWith(classOf[org.scalatest.junit.JUnitRunner])
-class ClusterDomainEventPublisherSpec extends AkkaSpec(ClusterDomainEventPublisherSpec.config)
+class ClusterDomainEventPublisherSpec extends AkkaSpec
   with BeforeAndAfterEach with ImplicitSender {
-  import ClusterDomainEventPublisherSpec._
 
   var publisher: ActorRef = _
   val a1 = Member(Address("akka", "sys", "a", 2552), Up)

akka-docs/rst/cluster/cluster-usage-java.rst

@@ -31,7 +31,7 @@ A Simple Cluster Example
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 The following small program together with its configuration starts an ``ActorSystem``
-with the Cluster extension enabled. It joins the cluster and logs some membership events.
+with the Cluster enabled. It joins the cluster and logs some membership events.
 
 Try it out:
 
@@ -42,8 +42,7 @@ Try it out:
 
 To enable cluster capabilities in your Akka project you should, at a minimum, add the :ref:`remoting-java`
 settings, but with ``akka.cluster.ClusterActorRefProvider``.
-The ``akka.cluster.seed-nodes`` and cluster extension should normally also be added to your
-``application.conf`` file.
+The ``akka.cluster.seed-nodes`` should normally also be added to your ``application.conf`` file.
 
 The seed nodes are configured contact points for initial, automatic, join of the cluster.
 

akka-docs/rst/cluster/cluster-usage-scala.rst

@@ -25,7 +25,7 @@ A Simple Cluster Example
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 The following small program together with its configuration starts an ``ActorSystem``
-with the Cluster extension enabled. It joins the cluster and logs some membership events.
+with the Cluster enabled. It joins the cluster and logs some membership events.
 
 Try it out:
 
@@ -36,8 +36,7 @@ Try it out:
 
 To enable cluster capabilities in your Akka project you should, at a minimum, add the :ref:`remoting-scala`
 settings, but with ``akka.cluster.ClusterActorRefProvider``.
-The ``akka.cluster.seed-nodes`` and cluster extension should normally also be added to your
-``application.conf`` file.
+The ``akka.cluster.seed-nodes`` should normally also be added to your ``application.conf`` file.
 
 The seed nodes are configured contact points for initial, automatic, join of the cluster.
 

akka-samples/akka-sample-cluster/src/main/resources/application.conf

@@ -12,8 +12,6 @@ akka {
     }
   }
 
-  extensions = ["akka.cluster.Cluster"]
-
   cluster {
     seed-nodes = [
       "akka://ClusterSystem@127.0.0.1:2551",