pekko/akka-cluster/src/main/scala/akka/cluster/ClusterDaemon.scala

/**
 * Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
 */
package akka.cluster

import language.existentials
import language.postfixOps
import scala.collection.immutable
import scala.concurrent.duration._
import scala.concurrent.forkjoin.ThreadLocalRandom
import scala.util.control.NonFatal
import akka.actor.{ Actor, ActorLogging, ActorRef, Address, Cancellable, Props, PoisonPill, ReceiveTimeout, RootActorPath, Scheduler }
import akka.actor.OneForOneStrategy
import akka.actor.SupervisorStrategy.Stop
import akka.cluster.MemberStatus._
import akka.cluster.ClusterEvent._
import akka.actor.ActorSelection
import akka.dispatch.{ UnboundedMessageQueueSemantics, RequiresMessageQueue }
import akka.actor.Deploy

/**
 * Base trait for all cluster messages. All ClusterMessage's are serializable.
 */
trait ClusterMessage extends Serializable

/**
 * INTERNAL API
 * Cluster commands sent by the USER via
 * [[akka.cluster.Cluster]] extension
 * or JMX.
 */
private[cluster] object ClusterUserAction {

  /**
   * Command to initiate join another node (represented by `address`).
   * Join will be sent to the other node.
   */
  @SerialVersionUID(1L)
  case class JoinTo(address: Address)

  /**
   * Command to leave the cluster.
   */
  @SerialVersionUID(1L)
  case class Leave(address: Address) extends ClusterMessage

  /**
   * Command to mark node as temporary down.
   */
  @SerialVersionUID(1L)
  case class Down(address: Address) extends ClusterMessage

}

/**
 * INTERNAL API
 */
private[cluster] object InternalClusterAction {

  /**
   * Command to join the cluster. Sent when a node wants to join another node (the receiver).
   * @param node the node that wants to join the cluster
   */
  @SerialVersionUID(1L)
  case class Join(node: UniqueAddress, roles: Set[String]) extends ClusterMessage

  /**
   * Reply to Join
   * @param from the sender node in the cluster, i.e. the node that received the Join command
   */
  @SerialVersionUID(1L)
  case class Welcome(from: UniqueAddress, gossip: Gossip) extends ClusterMessage

  /**
   * Command to initiate the process to join the specified
   * seed nodes.
   */
  case class JoinSeedNodes(seedNodes: immutable.IndexedSeq[Address])

  /**
   * Start message of the process to join one of the seed nodes.
   * The node sends `InitJoin` to all seed nodes, which replies
   * with `InitJoinAck`. The first reply is used others are discarded.
   * The node sends `Join` command to the seed node that replied first.
   * If a node is uninitialized it will reply to `InitJoin` with
   * `InitJoinNack`.
   */
  case object JoinSeedNode

  /**
   * @see JoinSeedNode
   */
  @SerialVersionUID(1L)
  case object InitJoin extends ClusterMessage

  /**
   * @see JoinSeedNode
   */
  @SerialVersionUID(1L)
  case class InitJoinAck(address: Address) extends ClusterMessage

  /**
   * @see JoinSeedNode
   */
  @SerialVersionUID(1L)
  case class InitJoinNack(address: Address) extends ClusterMessage

  /**
   * Marker interface for periodic tick messages
   */
  sealed trait Tick

  case object GossipTick extends Tick

  case object HeartbeatTick extends Tick

  case object ReapUnreachableTick extends Tick

  case object MetricsTick extends Tick

  case object LeaderActionsTick extends Tick

  case object PublishStatsTick extends Tick

  case class SendGossipTo(address: Address)

  case object GetClusterCoreRef

  case class PublisherCreated(publisher: ActorRef)

  /**
   * Comand to [[akka.cluster.ClusterDaemon]] to create a
   * [[akka.cluster.OnMemberUpListener]].
   */
  case class AddOnMemberUpListener(callback: Runnable)

  sealed trait SubscriptionMessage
  case class Subscribe(subscriber: ActorRef, to: Class[_]) extends SubscriptionMessage
  case class Unsubscribe(subscriber: ActorRef, to: Option[Class[_]]) extends SubscriptionMessage
  /**
   * @param receiver if `receiver` is defined the event will only be sent to that
   *   actor, otherwise it will be sent to all subscribers via the `eventStream`.
   */
  case class PublishCurrentClusterState(receiver: Option[ActorRef]) extends SubscriptionMessage

  sealed trait PublishMessage
  case class PublishChanges(newGossip: Gossip) extends PublishMessage
  case class PublishEvent(event: ClusterDomainEvent) extends PublishMessage
}

/**
 * INTERNAL API.
 *
 * Supervisor managing the different Cluster daemons.
 */
private[cluster] final class ClusterDaemon(settings: ClusterSettings) extends Actor with ActorLogging
  with RequiresMessageQueue[UnboundedMessageQueueSemantics] {
  import InternalClusterAction._
  // Important - don't use Cluster(context.system) here because that would
  // cause deadlock. The Cluster extension is currently being created and is waiting
  // for response from GetClusterCoreRef in its constructor.
  val coreSupervisor = context.actorOf(Props[ClusterCoreSupervisor].
    withDispatcher(context.props.dispatcher), name = "core")
  context.actorOf(Props[ClusterHeartbeatReceiver].
    withDispatcher(context.props.dispatcher), name = "heartbeatReceiver")

  def receive = {
    case msg @ GetClusterCoreRef ⇒ coreSupervisor forward msg
    case AddOnMemberUpListener(code) ⇒
      context.actorOf(Props(classOf[OnMemberUpListener], code).withDeploy(Deploy.local))
    case PublisherCreated(publisher) ⇒
      if (settings.MetricsEnabled) {
        // metrics must be started after core/publisher to be able
        // to inject the publisher ref to the ClusterMetricsCollector
        context.actorOf(Props(classOf[ClusterMetricsCollector], publisher).
          withDispatcher(context.props.dispatcher), name = "metrics")
      }
  }

}

/**
 * INTERNAL API.
 *
 * ClusterCoreDaemon and ClusterDomainEventPublisher can't be restarted because the state
 * would be obsolete. Shutdown the member if any those actors crashed.
 */
private[cluster] final class ClusterCoreSupervisor extends Actor with ActorLogging
  with RequiresMessageQueue[UnboundedMessageQueueSemantics] {
  import InternalClusterAction._

  val publisher = context.actorOf(Props[ClusterDomainEventPublisher].
    withDispatcher(context.props.dispatcher), name = "publisher")
  val coreDaemon = context.watch(context.actorOf(Props(classOf[ClusterCoreDaemon], publisher).
    withDispatcher(context.props.dispatcher), name = "daemon"))

  context.parent ! PublisherCreated(publisher)

  override val supervisorStrategy =
    OneForOneStrategy() {
      case NonFatal(e) ⇒
        log.error(e, "Cluster node [{}] crashed, [{}] - shutting down...", Cluster(context.system).selfAddress, e.getMessage)
        self ! PoisonPill
        Stop
    }

  override def postStop(): Unit = Cluster(context.system).shutdown()

  def receive = {
    case InternalClusterAction.GetClusterCoreRef ⇒ sender ! coreDaemon
  }
}

/**
 * INTERNAL API.
 */
private[cluster] class ClusterCoreDaemon(publisher: ActorRef) extends Actor with ActorLogging
  with RequiresMessageQueue[UnboundedMessageQueueSemantics] {
  import InternalClusterAction._

  val cluster = Cluster(context.system)
  import cluster.{ selfAddress, scheduler, failureDetector }
  import cluster.settings._
  import cluster.InfoLogger._

  protected def selfUniqueAddress = cluster.selfUniqueAddress

  val NumberOfGossipsBeforeShutdownWhenLeaderExits = 3

  def vclockName(node: UniqueAddress): String = node.address + "-" + node.uid
  val vclockNode = VectorClock.Node(vclockName(selfUniqueAddress))

  // note that self is not initially member,
  // and the Gossip is not versioned for this 'Node' yet
  var latestGossip: Gossip = Gossip.empty

  val statsEnabled = PublishStatsInterval.isFinite
  var gossipStats = GossipStats()

  var seedNodeProcess: Option[ActorRef] = None

  /**
   * Looks up and returns the remote cluster command connection for the specific address.
   */
  private def clusterCore(address: Address): ActorSelection =
    context.actorSelection(RootActorPath(address) / "system" / "cluster" / "core" / "daemon")

  import context.dispatcher

  // start periodic gossip to random nodes in cluster
  val gossipTask = scheduler.schedule(PeriodicTasksInitialDelay.max(GossipInterval),
    GossipInterval, self, GossipTick)

  // start periodic cluster failure detector reaping (moving nodes condemned by the failure detector to unreachable list)
  val failureDetectorReaperTask = scheduler.schedule(PeriodicTasksInitialDelay.max(UnreachableNodesReaperInterval),
    UnreachableNodesReaperInterval, self, ReapUnreachableTick)

  // start periodic leader action management (only applies for the current leader)
  val leaderActionsTask = scheduler.schedule(PeriodicTasksInitialDelay.max(LeaderActionsInterval),
    LeaderActionsInterval, self, LeaderActionsTick)

  // start periodic publish of current stats
  val publishStatsTask: Option[Cancellable] = PublishStatsInterval match {
    case Duration.Zero | _: Duration.Infinite ⇒ None
    case d: FiniteDuration ⇒
      Some(scheduler.schedule(PeriodicTasksInitialDelay.max(d), d, self, PublishStatsTick))
  }

  override def preStart(): Unit =
    if (SeedNodes.isEmpty)
      logInfo("No seed-nodes configured, manual cluster join required")
    else
      self ! JoinSeedNodes(SeedNodes)

  override def postStop(): Unit = {
    gossipTask.cancel()
    failureDetectorReaperTask.cancel()
    leaderActionsTask.cancel()
    publishStatsTask foreach { _.cancel() }
  }

  def uninitialized: Actor.Receive = {
    case InitJoin                          ⇒ sender ! InitJoinNack(selfAddress)
    case ClusterUserAction.JoinTo(address) ⇒ join(address)
    case JoinSeedNodes(seedNodes)          ⇒ joinSeedNodes(seedNodes)
    case msg: SubscriptionMessage          ⇒ publisher forward msg
  }

  def tryingToJoin(joinWith: Address, deadline: Option[Deadline]): Actor.Receive = {
    case Welcome(from, gossip) ⇒ welcome(joinWith, from, gossip)
    case InitJoin              ⇒ sender ! InitJoinNack(selfAddress)
    case ClusterUserAction.JoinTo(address) ⇒
      context.become(uninitialized)
      join(address)
    case JoinSeedNodes(seedNodes) ⇒
      context.become(uninitialized)
      joinSeedNodes(seedNodes)
    case msg: SubscriptionMessage ⇒ publisher forward msg
    case _: Tick ⇒
      if (deadline.exists(_.isOverdue)) {
        context.become(uninitialized)
        if (SeedNodes.nonEmpty) joinSeedNodes(SeedNodes)
        else join(joinWith)
      }
  }

  def becomeInitialized(): Unit = {
    // start heartbeatSender here, and not in constructor to make sure that
    // heartbeating doesn't start before Welcome is received
    context.actorOf(Props[ClusterHeartbeatSender].
      withDispatcher(UseDispatcher), name = "heartbeatSender")
    context.become(initialized)
  }

  def initialized: Actor.Receive = {
    case msg: GossipEnvelope              ⇒ receiveGossip(msg)
    case msg: GossipStatus                ⇒ receiveGossipStatus(msg)
    case GossipTick                       ⇒ gossip()
    case ReapUnreachableTick              ⇒ reapUnreachableMembers()
    case LeaderActionsTick                ⇒ leaderActions()
    case PublishStatsTick                 ⇒ publishInternalStats()
    case InitJoin                         ⇒ initJoin()
    case Join(node, roles)                ⇒ joining(node, roles)
    case ClusterUserAction.Down(address)  ⇒ downing(address)
    case ClusterUserAction.Leave(address) ⇒ leaving(address)
    case SendGossipTo(address)            ⇒ sendGossipTo(address)
    case msg: SubscriptionMessage         ⇒ publisher forward msg
    case ClusterUserAction.JoinTo(address) ⇒
      logInfo("Trying to join [{}] when already part of a cluster, ignoring", address)
    case JoinSeedNodes(seedNodes) ⇒
      logInfo("Trying to join seed nodes [{}] when already part of a cluster, ignoring",
        seedNodes.mkString(", "))
  }

  def removed: Actor.Receive = {
    case msg: SubscriptionMessage ⇒ publisher forward msg
  }

  def receive = uninitialized

  override def unhandled(message: Any): Unit = message match {
    case _: Tick           ⇒
    case _: GossipEnvelope ⇒
    case _: GossipStatus   ⇒
    case other             ⇒ super.unhandled(other)
  }

  def initJoin(): Unit = sender ! InitJoinAck(selfAddress)

  def joinSeedNodes(seedNodes: immutable.IndexedSeq[Address]): Unit = {
    require(seedNodeProcess.isEmpty, "Join seed nodes is already in progress")
    if (seedNodes.nonEmpty) {
      seedNodeProcess =
        if (seedNodes == immutable.IndexedSeq(selfAddress)) {
          self ! ClusterUserAction.JoinTo(selfAddress)
          None
        } else if (seedNodes.head == selfAddress) {
          Some(context.actorOf(Props(classOf[FirstSeedNodeProcess], seedNodes).
            withDispatcher(UseDispatcher), name = "firstSeedNodeProcess"))
        } else {
          Some(context.actorOf(Props(classOf[JoinSeedNodeProcess], seedNodes).
            withDispatcher(UseDispatcher), name = "joinSeedNodeProcess"))
        }
    }
  }

  /**
   * Try to join this cluster node with the node specified by `address`.
   * It's only allowed to join from an empty state, i.e. when not already a member.
   * A `Join(selfUniqueAddress)` command is sent to the node to join,
   * which will reply with a `Welcome` message.
   */
  def join(address: Address): Unit = {
    if (address.protocol != selfAddress.protocol)
      log.warning("Trying to join member with wrong protocol, but was ignored, expected [{}] but was [{}]",
        selfAddress.protocol, address.protocol)
    else if (address.system != selfAddress.system)
      log.warning("Trying to join member with wrong ActorSystem name, but was ignored, expected [{}] but was [{}]",
        selfAddress.system, address.system)
    else {
      require(latestGossip.members.isEmpty, "Join can only be done from empty state")

      // to support manual join when joining to seed nodes is stuck (no seed nodes available)
      val snd = sender
      seedNodeProcess match {
        case Some(`snd`) ⇒
          // seedNodeProcess completed, it will stop itself
          seedNodeProcess = None
        case Some(s) ⇒
          // manual join, abort current seedNodeProcess
          context stop s
          seedNodeProcess = None
        case None ⇒ // no seedNodeProcess in progress
      }

      if (address == selfAddress) {
        becomeInitialized()
        joining(selfUniqueAddress, cluster.selfRoles)
      } else {
        val joinDeadline = RetryUnsuccessfulJoinAfter match {
          case d: FiniteDuration ⇒ Some(Deadline.now + d)
          case _                 ⇒ None
        }
        context.become(tryingToJoin(address, joinDeadline))
        clusterCore(address) ! Join(selfUniqueAddress, cluster.selfRoles)
      }
    }
  }

  /**
   * State transition to JOINING - new node joining.
   * Received `Join` message and replies with `Welcome` message, containing
   * current gossip state, including the new joining member.
   */
  def joining(node: UniqueAddress, roles: Set[String]): Unit = {
    if (node.address.protocol != selfAddress.protocol)
      log.warning("Member with wrong protocol tried to join, but was ignored, expected [{}] but was [{}]",
        selfAddress.protocol, node.address.protocol)
    else if (node.address.system != selfAddress.system)
      log.warning("Member with wrong ActorSystem name tried to join, but was ignored, expected [{}] but was [{}]",
        selfAddress.system, node.address.system)
    else {
      val localMembers = latestGossip.members
      val localUnreachable = latestGossip.overview.unreachable

      // check by address without uid to make sure that node with same host:port is not allowed
      // to join until previous node with that host:port has been removed from the cluster
      val alreadyMember = localMembers.exists(_.address == node.address)
      val isUnreachable = localUnreachable.exists(_.address == node.address)

      if (alreadyMember)
        logInfo("Existing member [{}] is trying to join, ignoring", node)
      else if (isUnreachable)
        logInfo("Unreachable member [{}] is trying to join, ignoring", node)
      else {

        // remove the node from the failure detector
        failureDetector.remove(node.address)

        // add joining node as Joining
        // add self in case someone else joins before self has joined (Set discards duplicates)
        val newMembers = localMembers + Member(node, roles) + Member(selfUniqueAddress, cluster.selfRoles)
        val newGossip = latestGossip copy (members = newMembers)

        updateLatestGossip(newGossip)

        logInfo("Node [{}] is JOINING, roles [{}]", node.address, roles.mkString(", "))
        if (node != selfUniqueAddress) {
          sender ! Welcome(selfUniqueAddress, latestGossip)
        }

        publish(latestGossip)
      }
    }
  }

  /**
   * Reply from Join request.
   */
  def welcome(joinWith: Address, from: UniqueAddress, gossip: Gossip): Unit = {
    require(latestGossip.members.isEmpty, "Join can only be done from empty state")
    if (joinWith != from.address)
      logInfo("Ignoring welcome from [{}] when trying to join with [{}]", from.address, joinWith)
    else {
      logInfo("Welcome from [{}]", from.address)
      latestGossip = gossip seen selfUniqueAddress
      publish(latestGossip)
      if (from != selfUniqueAddress)
        gossipTo(from, sender)
      becomeInitialized()
    }
  }

  /**
   * State transition to LEAVING.
   * The node will eventually be removed by the leader, after hand-off in EXITING, and only after
   * removal a new node with same address can join the cluster through the normal joining procedure.
   */
  def leaving(address: Address): Unit = {
    // only try to update if the node is available (in the member ring)
    if (latestGossip.members.exists(m ⇒ m.address == address && m.status == Up)) {
      val newMembers = latestGossip.members map { m ⇒ if (m.address == address) m.copy(status = Leaving) else m } // mark node as LEAVING
      val newGossip = latestGossip copy (members = newMembers)

      updateLatestGossip(newGossip)

      logInfo("Marked address [{}] as [{}]", address, Leaving)
      publish(latestGossip)
    }
  }

  /**
   * This method is called when a member sees itself as Exiting.
   */
  def shutdown(): Unit = cluster.shutdown()

  /**
   * State transition to DOW.
   * The node to DOWN is removed from the `members` set and put in the `unreachable` set (if not already there)
   * and its status is set to DOWN. The node is also removed from the `seen` table.
   *
   * The node will eventually be removed by the leader, and only after removal a new node with same address can
   * join the cluster through the normal joining procedure.
   */
  def downing(address: Address): Unit = {
    val localGossip = latestGossip
    val localMembers = localGossip.members
    val localOverview = localGossip.overview
    val localSeen = localOverview.seen
    val localUnreachableMembers = localOverview.unreachable

    // 1. check if the node to DOWN is in the `members` set
    val downedMember: Option[Member] =
      localMembers.collectFirst { case m if m.address == address ⇒ m.copy(status = Down) }

    val newMembers = downedMember match {
      case Some(m) ⇒
        logInfo("Marking node [{}] as [{}]", m.address, Down)
        localMembers - m
      case None ⇒ localMembers
    }

    // 2. check if the node to DOWN is in the `unreachable` set
    val newUnreachableMembers =
      localUnreachableMembers.map { member ⇒
        // no need to DOWN members already DOWN
        if (member.address == address && member.status != Down) {
          logInfo("Marking unreachable node [{}] as [{}]", member.address, Down)
          member copy (status = Down)
        } else member
      }

    // 3. add the newly DOWNED members from the `members` (in step 1.) to the `newUnreachableMembers` set.
    val newUnreachablePlusNewlyDownedMembers = newUnreachableMembers ++ downedMember

    // 4. remove nodes marked as DOWN from the `seen` table
    val newSeen = localSeen -- newUnreachablePlusNewlyDownedMembers.collect { case m if m.status == Down ⇒ m.uniqueAddress }

    // update gossip overview
    val newOverview = localOverview copy (seen = newSeen, unreachable = newUnreachablePlusNewlyDownedMembers)
    val newGossip = localGossip copy (overview = newOverview, members = newMembers) // update gossip
    updateLatestGossip(newGossip)

    publish(latestGossip)
  }

  def receiveGossipStatus(status: GossipStatus): Unit = {
    val from = status.from
    if (latestGossip.overview.unreachable.exists(_.uniqueAddress == from))
      logInfo("Ignoring received gossip status from unreachable [{}] ", from)
    else if (latestGossip.members.forall(_.uniqueAddress != from))
      log.debug("Cluster Node [{}] - Ignoring received gossip status from unknown [{}]", selfAddress, from)
    else {
      (status.version compareTo latestGossip.version) match {
        case VectorClock.Same  ⇒ // same version
        case VectorClock.After ⇒ gossipStatusTo(from, sender) // remote is newer
        case _                 ⇒ gossipTo(from, sender) // conflicting or local is newer
      }
    }
  }

  /**
   * The types of gossip actions that receive gossip has performed.
   */
  sealed trait ReceiveGossipType
  case object Ignored extends ReceiveGossipType
  case object Older extends ReceiveGossipType
  case object Newer extends ReceiveGossipType
  case object Same extends ReceiveGossipType
  case object Merge extends ReceiveGossipType

  /**
   * Receive new gossip.
   */
  def receiveGossip(envelope: GossipEnvelope): ReceiveGossipType = {
    val from = envelope.from
    val remoteGossip = envelope.gossip
    val localGossip = latestGossip

    if (envelope.to != selfUniqueAddress) {
      logInfo("Ignoring received gossip intended for someone else, from [{}] to [{}]", from.address, envelope.to)
      Ignored
    } else if (remoteGossip.overview.unreachable.exists(_.address == selfAddress)) {
      logInfo("Ignoring received gossip with myself as unreachable, from [{}]", from.address)
      Ignored
    } else if (localGossip.overview.unreachable.exists(_.uniqueAddress == from)) {
      logInfo("Ignoring received gossip from unreachable [{}] ", from)
      Ignored
    } else if (localGossip.members.forall(_.uniqueAddress != from)) {
      log.debug("Cluster Node [{}] - Ignoring received gossip from unknown [{}]", selfAddress, from)
      Ignored
    } else if (remoteGossip.members.forall(_.uniqueAddress != selfUniqueAddress)) {
      logInfo("Ignoring received gossip that does not contain myself, from [{}]", from)
      Ignored
    } else {
      val comparison = remoteGossip.version compareTo localGossip.version

      val (winningGossip, talkback, gossipType) = comparison match {
        case VectorClock.Concurrent ⇒
          // conflicting versions, merge
          (remoteGossip merge localGossip, true, Merge)
        case VectorClock.Same ⇒
          // same version
          (remoteGossip mergeSeen localGossip, !remoteGossip.seenByNode(selfUniqueAddress), Same)
        case VectorClock.Before ⇒
          // local is newer
          (localGossip, true, Older)
        case VectorClock.After ⇒
          // remote is newer
          (remoteGossip, !remoteGossip.seenByNode(selfUniqueAddress), Newer)
      }

      latestGossip = winningGossip seen selfUniqueAddress

      // for all new joining nodes we remove them from the failure detector
      latestGossip.members foreach {
        node ⇒ if (node.status == Joining && !localGossip.members(node)) failureDetector.remove(node.address)
      }

      log.debug("Cluster Node [{}] - Receiving gossip from [{}]", selfAddress, from)

      if (comparison == VectorClock.Concurrent) {
        log.debug(
          """Couldn't establish a causal relationship between "remote" gossip and "local" gossip - Remote[{}] - Local[{}] - merged them into [{}]""",
          remoteGossip, localGossip, winningGossip)
      }

      if (statsEnabled) {
        gossipStats = gossipType match {
          case Merge ⇒ gossipStats.incrementMergeCount
          case Same  ⇒ gossipStats.incrementSameCount
          case Newer ⇒ gossipStats.incrementNewerCount
          case Older ⇒ gossipStats.incrementOlderCount
        }
      }

      publish(latestGossip)

      if (latestGossip.member(selfUniqueAddress).status == Exiting)
        shutdown()
      else if (talkback) {
        // send back gossip to sender when sender had different view, i.e. merge, or sender had
        // older or sender had newer
        gossipTo(from, sender)
      }
      gossipType
    }
  }

  def mergeRate(count: Long): Double = (count * 1000.0) / GossipInterval.toMillis

  /**
   * Initiates a new round of gossip.
   */
  def gossip(): Unit = {
    log.debug("Cluster Node [{}] - Initiating new round of gossip", selfAddress)

    if (!isSingletonCluster && isAvailable) {
      val localGossip = latestGossip

      val preferredGossipTargets: Vector[UniqueAddress] =
        if (ThreadLocalRandom.current.nextDouble() < GossipDifferentViewProbability) { // If it's time to try to gossip to some nodes with a different view
          // gossip to a random alive member with preference to a member with older or newer gossip version
          val localMemberAddressesSet = localGossip.members map { _.uniqueAddress }
          val nodesWithDifferentView = for {
            (node, version) ← localGossip.overview.seen
            if localMemberAddressesSet contains node
            if version != localGossip.version
          } yield node

          nodesWithDifferentView.toVector
        } else Vector.empty[UniqueAddress]

      if (preferredGossipTargets.nonEmpty) {
        val peer = selectRandomNode(preferredGossipTargets filterNot (_ == selfUniqueAddress))
        // send full gossip because it has different view
        peer foreach gossipTo
      } else {
        // Fall back to localGossip; important to not accidentally use `map` of the SortedSet, since the original order is not preserved)
        val peer = selectRandomNode(localGossip.members.toIndexedSeq.collect {
          case m if m.uniqueAddress != selfUniqueAddress ⇒ m.uniqueAddress
        })
        peer foreach { node ⇒
          if (localGossip.seenByNode(node)) gossipStatusTo(node)
          else gossipTo(node)
        }
      }
    }
  }

  /**
   * Runs periodic leader actions, such as member status transitions, auto-downing unreachable nodes,
   * assigning partitions etc.
   */
  def leaderActions(): Unit =
    if (latestGossip.isLeader(selfUniqueAddress) && isAvailable) {
      // only run the leader actions if we are the LEADER and available

      if (AutoDown)
        leaderAutoDownActions()

      if (latestGossip.convergence)
        leaderActionsOnConvergence()
    }

  /**
   * Leader actions are as follows:
   * 1. Move JOINING     => UP                   -- When a node joins the cluster
   * 2. Move LEAVING     => EXITING              -- When all partition handoff has completed
   * 3. Non-exiting remain                       -- When all partition handoff has completed
   * 4. Move unreachable EXITING => REMOVED      -- When all nodes have seen the EXITING node as unreachable (convergence) -
   *                                                remove the node from the node ring and seen table
   * 5. Move unreachable DOWN/EXITING => REMOVED -- When all nodes have seen that the node is DOWN/EXITING (convergence) -
   *                                                remove the node from the node ring and seen table
   * 7. Updating the vclock version for the changes
   * 8. Updating the `seen` table
   * 9. Update the state with the new gossip
   */
  def leaderActionsOnConvergence(): Unit = {
    val localGossip = latestGossip
    val localMembers = localGossip.members
    val localOverview = localGossip.overview
    val localSeen = localOverview.seen
    val localUnreachableMembers = localOverview.unreachable

    val hasPartionHandoffCompletedSuccessfully: Boolean = {
      // TODO implement partion handoff and a check if it is completed - now just returns TRUE - e.g. has completed successfully
      true
    }

    def enoughMembers: Boolean = {
      localMembers.size >= MinNrOfMembers && MinNrOfMembersOfRole.forall {
        case (role, threshold) ⇒ localMembers.count(_.hasRole(role)) >= threshold
      }
    }
    def isJoiningToUp(m: Member): Boolean = m.status == Joining && enoughMembers

    val (removedUnreachable, newUnreachable) = localUnreachableMembers partition { m ⇒
      Gossip.removeUnreachableWithMemberStatus(m.status)
    }

    val changedMembers = localMembers collect {
      var upNumber = 0

      {
        case m if isJoiningToUp(m) ⇒
          // Move JOINING => UP (once all nodes have seen that this node is JOINING, i.e. we have a convergence)
          // and minimum number of nodes have joined the cluster
          if (upNumber == 0) {
            // It is alright to use same upNumber as already used by a removed member, since the upNumber
            // is only used for comparing age of current cluster members (Member.isOlderThan)
            val youngest = localGossip.youngestMember
            upNumber = 1 + (if (youngest.upNumber == Int.MaxValue) 0 else youngest.upNumber)
          } else {
            upNumber += 1
          }
          m.copyUp(upNumber)

        case m if m.status == Leaving && hasPartionHandoffCompletedSuccessfully ⇒
          // Move LEAVING => EXITING (once we have a convergence on LEAVING
          // *and* if we have a successful partition handoff)
          m copy (status = Exiting)
      }
    }

    if (removedUnreachable.nonEmpty || changedMembers.nonEmpty) {
      // handle changes

      // replace changed members
      val newMembers = localMembers -- changedMembers ++ changedMembers

      // removing REMOVED nodes from the `seen` table
      val newSeen = localSeen -- removedUnreachable.map(_.uniqueAddress)
      val newOverview = localOverview copy (seen = newSeen, unreachable = newUnreachable) // update gossip overview
      val newGossip = localGossip copy (members = newMembers, overview = newOverview) // update gossip

      updateLatestGossip(newGossip)

      // log status changes
      changedMembers foreach { m ⇒
        logInfo("Leader is moving node [{}] to [{}]", m.address, m.status)
      }

      // log the removal of the unreachable nodes
      removedUnreachable foreach { m ⇒
        val status = if (m.status == Exiting) "exiting" else "unreachable"
        logInfo("Leader is removing {} node [{}]", status, m.address)
      }

      publish(latestGossip)

      if (latestGossip.member(selfUniqueAddress).status == Exiting) {
        // Leader is moving itself from Leaving to Exiting. Let others know (best effort)
        // before shutdown. Otherwise they will not see the Exiting state change
        // and there will not be convergence until they have detected this node as
        // unreachable and the required downing has finished. They will still need to detect
        // unreachable, but Exiting unreachable will be removed without downing, i.e.
        // normally the leaving of a leader will be graceful without the need
        // for downing. However, if those final gossip messages never arrive it is
        // alright to require the downing, because that is probably caused by a
        // network failure anyway.
        for (_ ← 1 to NumberOfGossipsBeforeShutdownWhenLeaderExits) gossip()
        shutdown()
      }

    }
  }

  /**
   * When the node is in the UNREACHABLE set it can be auto-down by leader
   */
  def leaderAutoDownActions(): Unit = {
    val localGossip = latestGossip
    val localOverview = localGossip.overview
    val localSeen = localOverview.seen
    val localUnreachableMembers = localOverview.unreachable

    val changedUnreachableMembers = localUnreachableMembers collect {
      case m if !Gossip.convergenceSkipUnreachableWithMemberStatus(m.status) ⇒ m copy (status = Down)
    }

    if (changedUnreachableMembers.nonEmpty) {
      // handle changes

      // replace changed unreachable
      val newUnreachableMembers = localUnreachableMembers -- changedUnreachableMembers ++ changedUnreachableMembers

      // removing nodes marked as Down/Exiting from the `seen` table
      val newSeen = localSeen -- changedUnreachableMembers.map(_.uniqueAddress)

      val newOverview = localOverview copy (seen = newSeen, unreachable = newUnreachableMembers) // update gossip overview
      val newGossip = localGossip copy (overview = newOverview) // update gossip

      updateLatestGossip(newGossip)

      // log the auto-downing of the unreachable nodes
      changedUnreachableMembers foreach { m ⇒
        logInfo("Leader is marking unreachable node [{}] as [{}]", m.address, m.status)
      }

      publish(latestGossip)
    }
  }

  /**
   * Reaps the unreachable members (moves them to the `unreachable` list in the cluster overview) according to the failure detector's verdict.
   */
  def reapUnreachableMembers(): Unit = {
    if (!isSingletonCluster && isAvailable) {
      // only scrutinize if we are a non-singleton cluster and available

      val localGossip = latestGossip
      val localOverview = localGossip.overview
      val localMembers = localGossip.members
      val localUnreachableMembers = localGossip.overview.unreachable

      val newlyDetectedUnreachableMembers = localMembers filterNot { member ⇒
        member.uniqueAddress == selfUniqueAddress || failureDetector.isAvailable(member.address)
      }

      if (newlyDetectedUnreachableMembers.nonEmpty) {

        val newMembers = localMembers -- newlyDetectedUnreachableMembers
        val newUnreachableMembers = localUnreachableMembers ++ newlyDetectedUnreachableMembers

        val newOverview = localOverview copy (unreachable = newUnreachableMembers)
        val newGossip = localGossip copy (overview = newOverview, members = newMembers)

        updateLatestGossip(newGossip)

        val (exiting, nonExiting) = newlyDetectedUnreachableMembers.partition(_.status == Exiting)
        if (nonExiting.nonEmpty)
          log.error("Cluster Node [{}] - Marking node(s) as UNREACHABLE [{}]", selfAddress, nonExiting.mkString(", "))
        if (exiting.nonEmpty)
          logInfo("Marking exiting node(s) as UNREACHABLE [{}]. This is expected and they will be removed.",
            exiting.mkString(", "))

        publish(latestGossip)
      }
    }
  }

  def selectRandomNode(nodes: IndexedSeq[UniqueAddress]): Option[UniqueAddress] =
    if (nodes.isEmpty) None
    else Some(nodes(ThreadLocalRandom.current nextInt nodes.size))

  def isSingletonCluster: Boolean = latestGossip.isSingletonCluster

  def isAvailable: Boolean = !latestGossip.isUnreachable(selfUniqueAddress)

  // needed for tests
  def sendGossipTo(address: Address): Unit = {
    latestGossip.members.foreach(m ⇒
      if (m.address == address)
        gossipTo(m.uniqueAddress))
  }

  /**
   * Gossips latest gossip to a node.
   */
  def gossipTo(node: UniqueAddress): Unit =
    if (validNodeForGossip(node))
      clusterCore(node.address) ! GossipEnvelope(selfUniqueAddress, node, latestGossip)

  def gossipTo(node: UniqueAddress, destination: ActorRef): Unit =
    if (validNodeForGossip(node))
      destination ! GossipEnvelope(selfUniqueAddress, node, latestGossip)

  def gossipStatusTo(node: UniqueAddress, destination: ActorRef): Unit =
    if (validNodeForGossip(node))
      destination ! GossipStatus(selfUniqueAddress, latestGossip.version)

  def gossipStatusTo(node: UniqueAddress): Unit =
    if (validNodeForGossip(node))
      clusterCore(node.address) ! GossipStatus(selfUniqueAddress, latestGossip.version)

  def validNodeForGossip(node: UniqueAddress): Boolean =
    (node != selfUniqueAddress && latestGossip.members.exists(_.uniqueAddress == node))

  def updateLatestGossip(newGossip: Gossip): Unit = {
    // Updating the vclock version for the changes
    val versionedGossip = newGossip :+ vclockNode
    // Updating the `seen` table
    val seenVersionedGossip = versionedGossip seen selfUniqueAddress
    // Update the state with the new gossip
    latestGossip = seenVersionedGossip
  }

  def publish(newGossip: Gossip): Unit = {
    publisher ! PublishChanges(newGossip)
    if (PublishStatsInterval == Duration.Zero) publishInternalStats()
  }

  def publishInternalStats(): Unit = {
    val vclockStats = VectorClockStats(
      versionSize = latestGossip.version.versions.size,
      seenLatest = latestGossip.members.count(m ⇒ latestGossip.seenByNode(m.uniqueAddress)))
    publisher ! CurrentInternalStats(gossipStats, vclockStats)
  }

}

/**
 * INTERNAL API.
 *
 * Used only for the first seed node.
 * Sends InitJoin to all seed nodes (except itself).
 * If other seed nodes are not part of the cluster yet they will reply with
 * InitJoinNack or not respond at all and then the first seed node
 * will join itself to initialize the new cluster. When the first
 * seed node is restarted, and some other seed node is part of the cluster
 * it will reply with InitJoinAck and then the first seed node will join
 * that other seed node to join existing cluster.
 */
private[cluster] final class FirstSeedNodeProcess(seedNodes: immutable.IndexedSeq[Address]) extends Actor with ActorLogging {
  import InternalClusterAction._
  import ClusterUserAction.JoinTo

  val cluster = Cluster(context.system)
  def selfAddress = cluster.selfAddress

  if (seedNodes.size <= 1 || seedNodes.head != selfAddress)
    throw new IllegalArgumentException("Join seed node should not be done")

  val timeout = Deadline.now + cluster.settings.SeedNodeTimeout

  var remainingSeedNodes = seedNodes.toSet - selfAddress

  // retry until one ack, or all nack, or timeout
  import context.dispatcher
  val retryTask = cluster.scheduler.schedule(1.second, 1.second, self, JoinSeedNode)
  self ! JoinSeedNode

  override def postStop(): Unit = retryTask.cancel()

  def receive = {
    case JoinSeedNode ⇒
      if (timeout.hasTimeLeft) {
        // send InitJoin to remaining seed nodes (except myself)
        remainingSeedNodes foreach { a ⇒ context.actorSelection(context.parent.path.toStringWithAddress(a)) ! InitJoin }
      } else {
        // no InitJoinAck received, initialize new cluster by joining myself
        context.parent ! JoinTo(selfAddress)
        context.stop(self)
      }
    case InitJoinAck(address) ⇒
      // first InitJoinAck reply, join existing cluster
      context.parent ! JoinTo(address)
      context.stop(self)
    case InitJoinNack(address) ⇒
      remainingSeedNodes -= address
      if (remainingSeedNodes.isEmpty) {
        // initialize new cluster by joining myself when nacks from all other seed nodes
        context.parent ! JoinTo(selfAddress)
        context.stop(self)
      }
  }

}

/**
 * INTERNAL API.
 *
 * Sends InitJoin to all seed nodes (except itself) and expect
 * InitJoinAck reply back. The seed node that replied first
 * will be used, joined to. InitJoinAck replies received after the
 * first one are ignored.
 *
 * Retries if no InitJoinAck replies are received within the
 * SeedNodeTimeout.
 * When at least one reply has been received it stops itself after
 * an idle SeedNodeTimeout.
 *
 * The seed nodes can be started in any order, but they will not be "active",
 * until they have been able to join another seed node (seed1).
 * They will retry the join procedure.
 * So one possible startup scenario is:
 * 1. seed2 started, but doesn't get any ack from seed1 or seed3
 * 2. seed3 started, doesn't get any ack from seed1 or seed3 (seed2 doesn't reply)
 * 3. seed1 is started and joins itself
 * 4. seed2 retries the join procedure and gets an ack from seed1, and then joins to seed1
 * 5. seed3 retries the join procedure and gets acks from seed2 first, and then joins to seed2
 *
 */
private[cluster] final class JoinSeedNodeProcess(seedNodes: immutable.IndexedSeq[Address]) extends Actor with ActorLogging {
  import InternalClusterAction._
  import ClusterUserAction.JoinTo

  def selfAddress = Cluster(context.system).selfAddress

  if (seedNodes.isEmpty || seedNodes.head == selfAddress)
    throw new IllegalArgumentException("Join seed node should not be done")

  context.setReceiveTimeout(Cluster(context.system).settings.SeedNodeTimeout)

  override def preStart(): Unit = self ! JoinSeedNode

  def receive = {
    case JoinSeedNode ⇒
      // send InitJoin to all seed nodes (except myself)
      seedNodes.collect {
        case a if a != selfAddress ⇒ context.actorSelection(context.parent.path.toStringWithAddress(a))
      } foreach { _ ! InitJoin }
    case InitJoinAck(address) ⇒
      // first InitJoinAck reply
      context.parent ! JoinTo(address)
      context.become(done)
    case InitJoinNack(_) ⇒ // that seed was uninitialized
    case ReceiveTimeout ⇒
      // no InitJoinAck received, try again
      self ! JoinSeedNode
  }

  def done: Actor.Receive = {
    case InitJoinAck(_) ⇒ // already received one, skip rest
    case ReceiveTimeout ⇒ context.stop(self)
  }
}

/**
 * INTERNAL API
 *
 * The supplied callback will be run, once, when current cluster member is `Up`.
 */
private[cluster] class OnMemberUpListener(callback: Runnable) extends Actor with ActorLogging {
  import ClusterEvent._
  val cluster = Cluster(context.system)
  // subscribe to MemberUp, re-subscribe when restart
  override def preStart(): Unit =
    cluster.subscribe(self, classOf[MemberUp])
  override def postStop(): Unit =
    cluster.unsubscribe(self)

  def receive = {
    case state: CurrentClusterState ⇒
      if (state.members.exists(isSelfUp(_)))
        done()
    case MemberUp(m) ⇒
      if (isSelfUp(m))
        done()
  }

  def done(): Unit = {
    try callback.run() catch {
      case NonFatal(e) ⇒ log.error(e, "OnMemberUp callback failed with [{}]", e.getMessage)
    } finally {
      context stop self
    }
  }

  def isSelfUp(m: Member): Boolean =
    m.uniqueAddress == cluster.selfUniqueAddress && m.status == MemberStatus.Up

}

/**
 * INTERNAL API
 */
@SerialVersionUID(1L)
private[cluster] case class GossipStats(
  receivedGossipCount: Long = 0L,
  mergeCount: Long = 0L,
  sameCount: Long = 0L,
  newerCount: Long = 0L,
  olderCount: Long = 0L) {

  def incrementMergeCount(): GossipStats =
    copy(mergeCount = mergeCount + 1, receivedGossipCount = receivedGossipCount + 1)

  def incrementSameCount(): GossipStats =
    copy(sameCount = sameCount + 1, receivedGossipCount = receivedGossipCount + 1)

  def incrementNewerCount(): GossipStats =
    copy(newerCount = newerCount + 1, receivedGossipCount = receivedGossipCount + 1)

  def incrementOlderCount(): GossipStats =
    copy(olderCount = olderCount + 1, receivedGossipCount = receivedGossipCount + 1)

  def :+(that: GossipStats): GossipStats = {
    GossipStats(
      this.receivedGossipCount + that.receivedGossipCount,
      this.mergeCount + that.mergeCount,
      this.sameCount + that.sameCount,
      this.newerCount + that.newerCount,
      this.olderCount + that.olderCount)
  }

  def :-(that: GossipStats): GossipStats = {
    GossipStats(
      this.receivedGossipCount - that.receivedGossipCount,
      this.mergeCount - that.mergeCount,
      this.sameCount - that.sameCount,
      this.newerCount - that.newerCount,
      this.olderCount - that.olderCount)
  }

}

/**
 * INTERNAL API
 */
@SerialVersionUID(1L)
private[cluster] case class VectorClockStats(
  versionSize: Int = 0,
  seenLatest: Int = 0)