Don't gossip to unreachable, see #2263

* Also, ignore gossip from unreachable, see #2264
* Update gossip protocol in cluster doc

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

@@ -197,6 +197,9 @@ case class GossipOverview(
   seen: Map[Address, VectorClock] = Map.empty,
   unreachable: Set[Member] = Set.empty) {
 
+  def isNonDownUnreachable(address: Address): Boolean =
+    unreachable.exists { m ⇒ m.address == address && m.status != Down }
+
   override def toString =
     "GossipOverview(seen = [" + seen.mkString(", ") +
       "], unreachable = [" + unreachable.mkString(", ") +
@@ -751,7 +754,7 @@ class Cluster(system: ExtendedActorSystem, val failureDetector: FailureDetector)
     val localUnreachable = localGossip.overview.unreachable
 
     val alreadyMember = localMembers.exists(_.address == node)
-    val isUnreachable = localUnreachable.exists { m ⇒ m.address == node && m.status != Down }
+    val isUnreachable = localGossip.overview.isNonDownUnreachable(node)
 
     if (!alreadyMember && !isUnreachable) {
 
@@ -898,6 +901,8 @@ class Cluster(system: ExtendedActorSystem, val failureDetector: FailureDetector)
     val localState = state.get
     val localGossip = localState.latestGossip
 
+    if (!localGossip.overview.isNonDownUnreachable(from)) {
+
       val winningGossip =
         if (isSingletonCluster(localState) && localGossip.overview.unreachable.isEmpty && remoteGossip.members.contains(self)) {
           // a fresh singleton cluster that is joining, no need to merge, use received gossip
@@ -940,6 +945,7 @@ class Cluster(system: ExtendedActorSystem, val failureDetector: FailureDetector)
         notifyMembershipChangeListeners(localState, newState)
       }
     }
+  }
 
   /**
    * INTERNAL API.
@@ -975,15 +981,6 @@ class Cluster(system: ExtendedActorSystem, val failureDetector: FailureDetector)
     peer
   }
 
-  /**
-   * INTERNAL API.
-   */
-  private[cluster] def gossipToUnreachableProbablity(membersSize: Int, unreachableSize: Int): Double =
-    (membersSize + unreachableSize) match {
-      case 0   ⇒ 0.0
-      case sum ⇒ unreachableSize.toDouble / sum
-    }
-
   /**
    * INTERNAL API.
    */
@@ -1019,13 +1016,6 @@ class Cluster(system: ExtendedActorSystem, val failureDetector: FailureDetector)
       // 1. gossip to alive members
       val gossipedToAlive = gossipToRandomNodeOf(localMemberAddresses)
 
-      // 2. gossip to unreachable members
-      if (localUnreachableSize > 0) {
-        val probability = gossipToUnreachableProbablity(localMembersSize, localUnreachableSize)
-        if (ThreadLocalRandom.current.nextDouble() < probability)
-          gossipToRandomNodeOf(localUnreachableMembers.map(_.address))
-      }
-
       // 3. gossip to a deputy nodes for facilitating partition healing
       val deputies = deputyNodes(localMemberAddresses)
       val alreadyGossipedToDeputy = gossipedToAlive.map(deputies.contains(_)).getOrElse(false)

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

@@ -50,14 +50,6 @@ class ClusterSpec extends AkkaSpec(ClusterSpec.config) with BeforeAndAfter {
       testActor ! GossipTo(address)
     }
 
-    @volatile
-    var _gossipToUnreachableProbablity = 0.0
-
-    override def gossipToUnreachableProbablity(membersSize: Int, unreachableSize: Int): Double = {
-      if (_gossipToUnreachableProbablity < 0.0) super.gossipToUnreachableProbablity(membersSize, unreachableSize)
-      else _gossipToUnreachableProbablity
-    }
-
     @volatile
     var _gossipToDeputyProbablity = 0.0
 
@@ -81,7 +73,6 @@ class ClusterSpec extends AkkaSpec(ClusterSpec.config) with BeforeAndAfter {
     cluster.latestGossip.members.collectFirst { case m if m.address == address ⇒ m.status }
 
   before {
-    cluster._gossipToUnreachableProbablity = 0.0
     cluster._gossipToDeputyProbablity = 0.0
     addresses foreach failureDetector.remove
     deterministicRandom.set(0)
@@ -133,17 +124,6 @@ class ClusterSpec extends AkkaSpec(ClusterSpec.config) with BeforeAndAfter {
       expectNoMsg(1 second)
     }
 
-    "use certain probability for gossiping to unreachable node depending on the number of unreachable and live nodes" in {
-      cluster._gossipToUnreachableProbablity = -1.0 // use real impl
-      cluster.gossipToUnreachableProbablity(10, 1) must be < (cluster.gossipToUnreachableProbablity(9, 1))
-      cluster.gossipToUnreachableProbablity(10, 1) must be < (cluster.gossipToUnreachableProbablity(10, 2))
-      cluster.gossipToUnreachableProbablity(10, 5) must be < (cluster.gossipToUnreachableProbablity(10, 9))
-      cluster.gossipToUnreachableProbablity(0, 10) must be <= (1.0)
-      cluster.gossipToUnreachableProbablity(1, 10) must be <= (1.0)
-      cluster.gossipToUnreachableProbablity(10, 0) must be(0.0 plusOrMinus (0.0001))
-      cluster.gossipToUnreachableProbablity(0, 0) must be(0.0 plusOrMinus (0.0001))
-    }
-
     "use certain probability for gossiping to deputy node depending on the number of unreachable and live nodes" in {
       cluster._gossipToDeputyProbablity = -1.0 // use real impl
       cluster.gossipToDeputyProbablity(10, 1, 2) must be < (cluster.gossipToDeputyProbablity(9, 1, 2))
@@ -178,22 +158,6 @@ class ClusterSpec extends AkkaSpec(ClusterSpec.config) with BeforeAndAfter {
 
     }
 
-    "gossip to random unreachable node" in {
-      val dead = Set(addresses(1))
-      dead foreach failureDetector.markNodeAsUnavailable
-      cluster._gossipToUnreachableProbablity = 1.0 // always
-
-      cluster.reapUnreachableMembers()
-      cluster.latestGossip.overview.unreachable.map(_.address) must be(dead)
-
-      cluster.gossip()
-
-      expectMsg(GossipTo(addresses(2))) // first available
-      expectMsg(GossipTo(addresses(1))) // the unavailable
-
-      expectNoMsg(1 second)
-    }
-
     "gossip to random deputy node if number of live nodes is less than number of deputy nodes" in {
       cluster._gossipToDeputyProbablity = -1.0 // real impl
       // 0 and 2 still alive

akka-docs/cluster/cluster.rst

@@ -213,7 +213,7 @@ nodes involved in a gossip exchange.
 
 Periodically, the default is every 1 second, each node chooses another random
 node to initiate a round of gossip with. The choice of node is random but can
-also include extra gossiping for unreachable nodes, ``deputy`` nodes, and nodes with
+also include extra gossiping for ``deputy`` nodes, and nodes with
 either newer or older state versions.
 
 The gossip overview contains the current state version for all nodes and also a
@@ -228,14 +228,11 @@ During each round of gossip exchange the following process is used:
 
 1. Gossip to random live node (if any)
 
-2. Gossip to random unreachable node with certain probability depending on the
+2. If the node gossiped to at (1) was not a ``deputy`` node, or the number of live
-   number of unreachable and live nodes
-
-3. If the node gossiped to at (1) was not a ``deputy`` node, or the number of live
    nodes is less than number of ``deputy`` nodes, gossip to random ``deputy`` node with
    certain probability depending on number of unreachable, ``deputy``, and live nodes.
 
-4. Gossip to random node with newer or older state information, based on the
+3. Gossip to random node with newer or older state information, based on the
    current gossip overview, with some probability (?)
 
 The gossiper only sends the gossip overview to the chosen node. The recipient of