Publish InstantMemberEvent immediately, see #2803

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

@@ -68,54 +68,140 @@ object ClusterEvent {
   }
 
   /**
-   * Marker interface for member related events.
+   * Marker interface for membership events.
+   * Only published after convergence, when all members have seen current
+   * state.
    */
   sealed trait MemberEvent extends ClusterDomainEvent {
     def member: Member
   }
 
   /**
-   * A new member joined the cluster. Only published after convergence.
+   * A new member joined the cluster.
+   * Only published after convergence, when all members have seen current
+   * state.
    */
   case class MemberJoined(member: Member) extends MemberEvent {
     if (member.status != Joining) throw new IllegalArgumentException("Expected Joining status, got: " + member)
   }
 
   /**
-   * Member status changed to Up. Only published after convergence.
+   * Member status changed to Up.
+   * Only published after convergence, when all members have seen current
+   * state.
    */
   case class MemberUp(member: Member) extends MemberEvent {
     if (member.status != Up) throw new IllegalArgumentException("Expected Up status, got: " + member)
   }
 
   /**
-   * Member status changed to Leaving. Only published after convergence.
+   * Member status changed to Leaving.
+   * Only published after convergence, when all members have seen current
+   * state.
    */
   case class MemberLeft(member: Member) extends MemberEvent {
     if (member.status != Leaving) throw new IllegalArgumentException("Expected Leaving status, got: " + member)
   }
 
   /**
-   * Member status changed to Exiting. Only published after convergence.
+   * Member status changed to Exiting.
+   * Only published after convergence, when all members have seen current
+   * state.
    */
   case class MemberExited(member: Member) extends MemberEvent {
     if (member.status != Exiting) throw new IllegalArgumentException("Expected Exiting status, got: " + member)
   }
 
   /**
-   * Member status changed to Down. Only published after convergence.
+   * Member status changed to Down.
+   * Only published after convergence, when all members have seen current
+   * state.
    */
   case class MemberDowned(member: Member) extends MemberEvent {
     if (member.status != Down) throw new IllegalArgumentException("Expected Down status, got: " + member)
   }
 
   /**
-   * Member completely removed from the cluster. Only published after convergence.
+   * Member completely removed from the cluster. Only published after convergence,
+   * when all other members have seen the state.
    */
   case class MemberRemoved(member: Member) extends MemberEvent {
     if (member.status != Removed) throw new IllegalArgumentException("Expected Removed status, got: " + member)
   }
 
+  /**
+   * Current snapshot state of the cluster. Sent to new subscriber of
+   * [akka.cluster.ClusterEvent.InstantMemberEvent].
+   */
+  case class InstantClusterState(members: immutable.SortedSet[Member] = immutable.SortedSet.empty)
+    extends ClusterDomainEvent {
+
+    /**
+     * Java API
+     * Read only
+     */
+    def getMembers: java.lang.Iterable[Member] = {
+      import scala.collection.JavaConverters._
+      members.asJava
+    }
+  }
+
+  /**
+   * Marker interface for membership events published immediately.
+   * All other members might not have seen the state.
+   */
+  sealed trait InstantMemberEvent extends ClusterDomainEvent {
+    def member: Member
+  }
+
+  /**
+   * A new member joined the cluster. Published immediately when it happened.
+   * All other members might not have seen the state.
+   */
+  case class InstantMemberJoined(member: Member) extends InstantMemberEvent {
+    if (member.status != Joining) throw new IllegalArgumentException("Expected Joining status, got: " + member)
+  }
+
+  /**
+   * Member status changed to Up. Published immediately when it happened.
+   * All other members might not have seen the state.
+   */
+  case class InstantMemberUp(member: Member) extends InstantMemberEvent {
+    if (member.status != Up) throw new IllegalArgumentException("Expected Up status, got: " + member)
+  }
+
+  /**
+   * Member status changed to Leaving. Published immediately when it happened.
+   * All other members might not have seen the state.
+   */
+  case class InstantMemberLeft(member: Member) extends InstantMemberEvent {
+    if (member.status != Leaving) throw new IllegalArgumentException("Expected Leaving status, got: " + member)
+  }
+
+  /**
+   * Member status changed to Exiting. Published immediately when it happened.
+   * All other members might not have seen the state.
+   */
+  case class InstantMemberExited(member: Member) extends InstantMemberEvent {
+    if (member.status != Exiting) throw new IllegalArgumentException("Expected Exiting status, got: " + member)
+  }
+
+  /**
+   * Member status changed to Down. Published immediately when it happened.
+   * All other members might not have seen the state.
+   */
+  case class InstantMemberDowned(member: Member) extends InstantMemberEvent {
+    if (member.status != Down) throw new IllegalArgumentException("Expected Down status, got: " + member)
+  }
+
+  /**
+   * Member completely removed from the cluster. Published immediately when it happened.
+   * All other members might not have seen the state.
+   */
+  case class InstantMemberRemoved(member: Member) extends InstantMemberEvent {
+    if (member.status != Removed) throw new IllegalArgumentException("Expected Removed status, got: " + member)
+  }
+
   /**
    * Leader of the cluster members changed. Only published after convergence.
    */
@@ -209,6 +295,21 @@ object ClusterEvent {
         ++= removedEvents).result()
     }
 
+  /**
+   * INTERNAL API
+   */
+  private[cluster] def convertToInstantMemberEvents(memberEvents: immutable.Seq[MemberEvent]): immutable.Seq[InstantMemberEvent] =
+    memberEvents map { event ⇒
+      event match {
+        case MemberJoined(m)  ⇒ InstantMemberJoined(m)
+        case MemberUp(m)      ⇒ InstantMemberUp(m)
+        case MemberDowned(m)  ⇒ InstantMemberDowned(m)
+        case MemberLeft(m)    ⇒ InstantMemberLeft(m)
+        case MemberExited(m)  ⇒ InstantMemberExited(m)
+        case MemberRemoved(m) ⇒ InstantMemberRemoved(m)
+      }
+    }
+
   /**
    * INTERNAL API
    */
@@ -269,8 +370,21 @@ private[cluster] final class ClusterDomainEventPublisher extends Actor with Acto
     }
   }
 
+  def publishInstantClusterState(receiver: ActorRef): Unit = {
+    // The state is based on latest gossip to mimic what you
+    // would have seen if you where listening to the InstantMemberEvent stream.
+    receiver ! InstantClusterState(members = latestGossip.members)
+  }
+
   def subscribe(subscriber: ActorRef, to: Class[_]): Unit = {
-    publishCurrentClusterState(Some(subscriber))
+    if (classOf[ClusterDomainEvent] == to) {
+      publishInstantClusterState(subscriber)
+      publishCurrentClusterState(Some(subscriber))
+    } else if (classOf[InstantMemberEvent].isAssignableFrom(to))
+      publishInstantClusterState(subscriber)
+    else
+      publishCurrentClusterState(Some(subscriber))
+
     eventStream.subscribe(subscriber, to)
   }
 
@@ -285,8 +399,10 @@ private[cluster] final class ClusterDomainEventPublisher extends Actor with Acto
     latestGossip = newGossip
     // first publish the diffUnreachable between the last two gossips
     diffUnreachable(oldGossip, newGossip) foreach publish
+    val newMemberEvents = diffMemberEvents(oldGossip, newGossip)
+    convertToInstantMemberEvents(newMemberEvents) foreach publish
     // buffer up the MemberEvents waiting for convergence
-    memberEvents ++= diffMemberEvents(oldGossip, newGossip)
+    memberEvents ++= newMemberEvents
     // if we have convergence then publish the MemberEvents and possibly a LeaderChanged
     if (newGossip.convergence) {
       val previousConvergedGossip = latestConvergedGossip