/*
 * Copyright (C) 2009-2021 Lightbend Inc. <https://www.lightbend.com>
 */

package akka.cluster

import scala.collection.immutable
import scala.concurrent.Future
import scala.concurrent.Promise
import scala.concurrent.duration._
import scala.util.control.NonFatal

import scala.annotation.nowarn
import com.typesafe.config.Config

import akka.Done
import akka.actor._
import akka.actor.SupervisorStrategy.Stop
import akka.annotation.InternalApi
import akka.cluster.ClusterEvent._
import akka.cluster.MemberStatus._
import akka.dispatch.{ RequiresMessageQueue, UnboundedMessageQueueSemantics }
import akka.event.ActorWithLogClass
import akka.event.Logging
import akka.pattern.ask
import akka.remote.{ QuarantinedEvent => ClassicQuarantinedEvent }
import akka.remote.artery.QuarantinedEvent
import akka.util.Timeout
import akka.util.Version

/**
 * 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.
 */
@InternalApi
private[cluster] object ClusterUserAction {

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

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

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

  /**
   * Command to mark all nodes as shutting down
   */
  @SerialVersionUID(1L)
  case object PrepareForShutdown extends ClusterMessage
}

/**
 * INTERNAL API
 */
@InternalApi
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)
  final case class Join(node: UniqueAddress, roles: Set[String], appVersion: Version) extends ClusterMessage

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

  /**
   * Command to initiate the process to join the specified
   * seed nodes.
   */
  final 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 extends DeadLetterSuppression

  sealed trait ConfigCheck
  case object UncheckedConfig extends ConfigCheck
  case object IncompatibleConfig extends ConfigCheck

  /**
   * Node with version 2.5.9 or earlier is joining. The serialized
   * representation of `InitJoinAck` must be a plain `Address` for
   * such a joining node.
   */
  case object ConfigCheckUnsupportedByJoiningNode extends ConfigCheck

  final case class CompatibleConfig(clusterConfig: Config) extends ConfigCheck

  /**
   * see JoinSeedNode
   */
  @SerialVersionUID(1L)
  case class InitJoin(configOfJoiningNode: Config) extends ClusterMessage with DeadLetterSuppression

  /**
   * see JoinSeedNode
   */
  @SerialVersionUID(1L)
  final case class InitJoinAck(address: Address, configCheck: ConfigCheck)
      extends ClusterMessage
      with DeadLetterSuppression

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

  final case class ExitingConfirmed(node: UniqueAddress) extends ClusterMessage with DeadLetterSuppression

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

  case object GossipTick extends Tick

  case object GossipSpeedupTick extends Tick

  case object ReapUnreachableTick extends Tick

  case object LeaderActionsTick extends Tick

  case object PublishStatsTick extends Tick

  final case class SendGossipTo(address: Address)

  case object GetClusterCoreRef

  /**
   * Command to [[akka.cluster.ClusterDaemon]] to create a
   * [[akka.cluster.OnMemberStatusChangedListener]].
   */
  final case class AddOnMemberUpListener(callback: Runnable) extends NoSerializationVerificationNeeded

  final case class AddOnMemberRemovedListener(callback: Runnable) extends NoSerializationVerificationNeeded

  sealed trait SubscriptionMessage
  final case class Subscribe(subscriber: ActorRef, initialStateMode: SubscriptionInitialStateMode, to: Set[Class[_]])
      extends SubscriptionMessage
  final case class Unsubscribe(subscriber: ActorRef, to: Option[Class[_]])
      extends SubscriptionMessage
      with DeadLetterSuppression

  /**
   * @param receiver [[akka.cluster.ClusterEvent.CurrentClusterState]] will be sent to the `receiver`
   */
  final case class SendCurrentClusterState(receiver: ActorRef) extends SubscriptionMessage

  sealed trait PublishMessage
  final case class PublishChanges(state: MembershipState) extends PublishMessage
  final case class PublishEvent(event: ClusterDomainEvent) extends PublishMessage

  case object ExitingCompleted

}

/**
 * INTERNAL API.
 *
 * Supervisor managing the different Cluster daemons.
 */
@InternalApi
private[cluster] final class ClusterDaemon(joinConfigCompatChecker: JoinConfigCompatChecker)
    extends Actor
    with RequiresMessageQueue[UnboundedMessageQueueSemantics] {
  import InternalClusterAction._
  // Important - don't use Cluster(context.system) in constructor because that would
  // cause deadlock. The Cluster extension is currently being created and is waiting
  // for response from GetClusterCoreRef in its constructor.
  // Child actors are therefore created when GetClusterCoreRef is received
  var coreSupervisor: Option[ActorRef] = None

  val clusterShutdown = Promise[Done]()
  val coordShutdown = CoordinatedShutdown(context.system)
  coordShutdown.addTask(CoordinatedShutdown.PhaseClusterLeave, "leave") {
    val sys = context.system
    () =>
      if (Cluster(sys).isTerminated || Cluster(sys).selfMember.status == Down)
        Future.successful(Done)
      else {
        implicit val timeout = Timeout(coordShutdown.timeout(CoordinatedShutdown.PhaseClusterLeave))
        self.ask(CoordinatedShutdownLeave.LeaveReq).mapTo[Done]
      }
  }
  coordShutdown.addTask(CoordinatedShutdown.PhaseClusterShutdown, "wait-shutdown") { () =>
    clusterShutdown.future
  }

  override def postStop(): Unit = {
    clusterShutdown.trySuccess(Done)
    if (Cluster(context.system).settings.RunCoordinatedShutdownWhenDown) {
      // if it was stopped due to leaving CoordinatedShutdown was started earlier
      coordShutdown.run(CoordinatedShutdown.ClusterDowningReason)
    }
  }

  def createChildren(): Unit = {
    coreSupervisor = Some(
      context.actorOf(
        Props(classOf[ClusterCoreSupervisor], joinConfigCompatChecker).withDispatcher(context.props.dispatcher),
        name = "core"))
    context.actorOf(
      ClusterHeartbeatReceiver.props(() => Cluster(context.system)).withDispatcher(context.props.dispatcher),
      name = "heartbeatReceiver")
  }

  def receive = {
    case msg: GetClusterCoreRef.type =>
      if (coreSupervisor.isEmpty)
        createChildren()
      coreSupervisor.foreach(_.forward(msg))
    case AddOnMemberUpListener(code) =>
      context.actorOf(Props(classOf[OnMemberStatusChangedListener], code, Up).withDeploy(Deploy.local))
    case AddOnMemberRemovedListener(code) =>
      context.actorOf(Props(classOf[OnMemberStatusChangedListener], code, Removed).withDeploy(Deploy.local))
    case CoordinatedShutdownLeave.LeaveReq =>
      val ref = context.actorOf(CoordinatedShutdownLeave.props().withDispatcher(context.props.dispatcher))
      // forward the ask request
      ref.forward(CoordinatedShutdownLeave.LeaveReq)
  }

}

/**
 * INTERNAL API.
 *
 * ClusterCoreDaemon and ClusterDomainEventPublisher can't be restarted because the state
 * would be obsolete. Shutdown the member if any those actors crashed.
 */
@InternalApi
private[cluster] final class ClusterCoreSupervisor(joinConfigCompatChecker: JoinConfigCompatChecker)
    extends Actor
    with RequiresMessageQueue[UnboundedMessageQueueSemantics] {

  // Important - don't use Cluster(context.system) in constructor because that would
  // cause deadlock. The Cluster extension is currently being created and is waiting
  // for response from GetClusterCoreRef in its constructor.
  // Child actors are therefore created when GetClusterCoreRef is received

  var coreDaemon: Option[ActorRef] = None

  def createChildren(): Unit = {
    val publisher =
      context.actorOf(Props[ClusterDomainEventPublisher]().withDispatcher(context.props.dispatcher), name = "publisher")
    coreDaemon = Some(
      context.watch(context.actorOf(
        Props(classOf[ClusterCoreDaemon], publisher, joinConfigCompatChecker).withDispatcher(context.props.dispatcher),
        name = "daemon")))
  }

  override val supervisorStrategy =
    OneForOneStrategy() {
      case NonFatal(e) =>
        Cluster(context.system).ClusterLogger.logError(e, "crashed, [{}] - shutting down...", e.getMessage)
        self ! PoisonPill
        Stop
    }

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

  def receive = {
    case InternalClusterAction.GetClusterCoreRef =>
      if (coreDaemon.isEmpty)
        createChildren()
      coreDaemon.foreach(sender() ! _)
  }
}

/**
 * INTERNAL API.
 */
@InternalApi
private[cluster] object ClusterCoreDaemon {
  val NumberOfGossipsBeforeShutdownWhenLeaderExits = 5
  val MaxGossipsBeforeShuttingDownMyself = 5
  val MaxTicksBeforeShuttingDownMyself = 4

}

/**
 * INTERNAL API.
 */
@InternalApi
private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatChecker: JoinConfigCompatChecker)
    extends Actor
    with RequiresMessageQueue[UnboundedMessageQueueSemantics] {
  import ClusterCoreDaemon._
  import InternalClusterAction._
  import MembershipState._

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

  val selfDc = cluster.selfDataCenter

  private val gossipLogger: cluster.ClusterLogger =
    new cluster.ClusterLogger(
      Logging.withMarker(context.system, ActorWithLogClass(this, ClusterLogClass.ClusterGossip)))

  protected def selfUniqueAddress = cluster.selfUniqueAddress

  val vclockNode = VectorClock.Node(Gossip.vclockName(selfUniqueAddress))
  val gossipTargetSelector = new GossipTargetSelector(
    ReduceGossipDifferentViewProbability,
    cluster.settings.MultiDataCenter.CrossDcGossipProbability)

  // note that self is not initially member,
  // and the Gossip is not versioned for this 'Node' yet
  var membershipState = MembershipState(
    Gossip.empty,
    cluster.selfUniqueAddress,
    cluster.settings.SelfDataCenter,
    cluster.settings.MultiDataCenter.CrossDcConnections)

  var isCurrentlyLeader = false

  def latestGossip: Gossip = membershipState.latestGossip

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

  var seedNodes = SeedNodes
  var seedNodeProcess: Option[ActorRef] = None
  var seedNodeProcessCounter = 0 // for unique names
  var joinSeedNodesDeadline: Option[Deadline] = None
  var leaderActionCounter = 0
  var selfDownCounter = 0
  var preparingForShutdown = false

  var exitingTasksInProgress = false
  val selfExiting = Promise[Done]()
  val coordShutdown = CoordinatedShutdown(context.system)
  coordShutdown.addTask(CoordinatedShutdown.PhaseClusterExiting, "wait-exiting") { () =>
    if (latestGossip.members.isEmpty)
      Future.successful(Done) // not joined yet
    else
      selfExiting.future
  }
  coordShutdown.addTask(CoordinatedShutdown.PhaseClusterExitingDone, "exiting-completed") {
    val sys = context.system
    () =>
      if (Cluster(sys).isTerminated || Cluster(sys).selfMember.status == Down)
        Future.successful(Done)
      else {
        implicit val timeout = Timeout(coordShutdown.timeout(CoordinatedShutdown.PhaseClusterExitingDone))
        self.ask(ExitingCompleted).mapTo[Done]
      }
  }
  var exitingConfirmed = Set.empty[UniqueAddress]

  /**
   * 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.scheduleWithFixedDelay(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.scheduleWithFixedDelay(
    PeriodicTasksInitialDelay.max(UnreachableNodesReaperInterval),
    UnreachableNodesReaperInterval,
    self,
    ReapUnreachableTick)

  // start periodic leader action management (only applies for the current leader)
  val leaderActionsTask = scheduler.scheduleWithFixedDelay(
    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.scheduleWithFixedDelay(PeriodicTasksInitialDelay.max(d), d, self, PublishStatsTick))
  }

  override def preStart(): Unit = {
    subscribeQuarantinedEvent()

    cluster.downingProvider.downingActorProps match {
      case Some(props) =>
        val propsWithDispatcher =
          if (props.dispatcher == Deploy.NoDispatcherGiven) props.withDispatcher(context.props.dispatcher)
          else props

        context.actorOf(propsWithDispatcher, name = "downingProvider")
      case None =>
        logInfo(
          "No downing-provider-class configured, manual cluster downing required, see " +
          "https://doc.akka.io/docs/akka/current/typed/cluster.html#downing")
    }

    if (seedNodes.isEmpty) {
      if (isClusterBootstrapAvailable)
        logInfo("No seed nodes found in configuration, relying on Cluster Bootstrap for joining")
      else
        logInfo(
          "No seed-nodes configured, manual cluster join required, see " +
          "https://doc.akka.io/docs/akka/current/typed/cluster.html#joining")
    } else {
      self ! JoinSeedNodes(seedNodes)
    }
  }

  @nowarn("msg=deprecated")
  private def subscribeQuarantinedEvent(): Unit = {
    context.system.eventStream.subscribe(self, classOf[QuarantinedEvent])
    context.system.eventStream.subscribe(self, classOf[ClassicQuarantinedEvent])
  }

  private def isClusterBootstrapAvailable: Boolean =
    context.system.settings.config.hasPath("akka.management.cluster.bootstrap")

  override def postStop(): Unit = {
    context.system.eventStream.unsubscribe(self)
    gossipTask.cancel()
    failureDetectorReaperTask.cancel()
    leaderActionsTask.cancel()
    publishStatsTask.foreach { _.cancel() }
    selfExiting.trySuccess(Done)
  }

  def uninitialized: Actor.Receive =
    ({
      case InitJoin(_) =>
        logInfo("Received InitJoin message from [{}], but this node is not initialized yet", sender())
        sender() ! InitJoinNack(selfAddress)
      case ClusterUserAction.JoinTo(address) =>
        join(address)
      case JoinSeedNodes(newSeedNodes) =>
        resetJoinSeedNodesDeadline()
        joinSeedNodes(newSeedNodes)
      case msg: SubscriptionMessage =>
        publisher.forward(msg)
      case Welcome(from, gossip) =>
        welcome(from.address, from, gossip)
      case _: Tick =>
        if (joinSeedNodesDeadline.exists(_.isOverdue()))
          joinSeedNodesWasUnsuccessful()
    }: Actor.Receive).orElse(receiveExitingCompleted)

  def tryingToJoin(joinWith: Address, deadline: Option[Deadline]): Actor.Receive =
    ({
      case Welcome(from, gossip) =>
        welcome(joinWith, from, gossip)
      case InitJoin(_) =>
        logInfo("Received InitJoin message from [{}], but this node is not a member yet", sender())
        sender() ! InitJoinNack(selfAddress)
      case ClusterUserAction.JoinTo(address) =>
        becomeUninitialized()
        join(address)
      case JoinSeedNodes(newSeedNodes) =>
        resetJoinSeedNodesDeadline()
        becomeUninitialized()
        joinSeedNodes(newSeedNodes)
      case msg: SubscriptionMessage => publisher.forward(msg)
      case _: Tick =>
        if (joinSeedNodesDeadline.exists(_.isOverdue()))
          joinSeedNodesWasUnsuccessful()
        else if (deadline.exists(_.isOverdue())) {
          // join attempt failed, retry
          becomeUninitialized()
          if (seedNodes.nonEmpty) joinSeedNodes(seedNodes)
          else join(joinWith)
        }
    }: Actor.Receive).orElse(receiveExitingCompleted)

  private def resetJoinSeedNodesDeadline(): Unit = {
    joinSeedNodesDeadline = ShutdownAfterUnsuccessfulJoinSeedNodes match {
      case d: FiniteDuration => Some(Deadline.now + d)
      case _                 => None // off
    }
  }

  private def joinSeedNodesWasUnsuccessful(): Unit = {
    logWarning(
      "Joining of seed-nodes [{}] was unsuccessful after configured " +
      "shutdown-after-unsuccessful-join-seed-nodes [{}]. Running CoordinatedShutdown.",
      seedNodes.mkString(", "),
      ShutdownAfterUnsuccessfulJoinSeedNodes)
    joinSeedNodesDeadline = None
    CoordinatedShutdown(context.system).run(CoordinatedShutdown.ClusterJoinUnsuccessfulReason)
  }

  def becomeUninitialized(): Unit = {
    // make sure that join process is stopped
    stopSeedNodeProcess()
    context.become(uninitialized)
  }

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

    val externalHeartbeatProps = Props(new CrossDcHeartbeatSender()).withDispatcher(UseDispatcher)
    context.actorOf(externalHeartbeatProps, name = "crossDcHeartbeatSender")

    // make sure that join process is stopped
    stopSeedNodeProcess()
    joinSeedNodesDeadline = None
    context.become(initialized)
  }

  def initialized: Actor.Receive =
    ({
      case msg: GossipEnvelope => receiveGossip(msg)
      case msg: GossipStatus   => receiveGossipStatus(msg)
      case GossipTick          => gossipTick()
      case GossipSpeedupTick   => gossipSpeedupTick()
      case ReapUnreachableTick => reapUnreachableMembers()
      case LeaderActionsTick   => leaderActions()
      case PublishStatsTick    => publishInternalStats()
      case InitJoin(joiningNodeConfig) =>
        logInfo("Received InitJoin message from [{}] to [{}]", sender(), selfAddress)
        initJoin(joiningNodeConfig)
      case Join(node, roles, appVersion)         => joining(node, roles, appVersion)
      case ClusterUserAction.Down(address)       => downing(address)
      case ClusterUserAction.Leave(address)      => leaving(address)
      case ClusterUserAction.PrepareForShutdown  => startPrepareForShutdown()
      case SendGossipTo(address)                 => sendGossipTo(address)
      case msg: SubscriptionMessage              => publisher.forward(msg)
      case QuarantinedEvent(ua)                  => quarantined(UniqueAddress(ua))
      case ClassicQuarantinedEvent(address, uid) => quarantined(UniqueAddress(address, uid))
      case ClusterUserAction.JoinTo(address) =>
        logInfo("Trying to join [{}] when already part of a cluster, ignoring", address)
      case JoinSeedNodes(nodes) =>
        logInfo("Trying to join seed nodes [{}] when already part of a cluster, ignoring", nodes.mkString(", "))
      case ExitingConfirmed(address) => receiveExitingConfirmed(address)
    }: Actor.Receive).orElse(receiveExitingCompleted)

  def receiveExitingCompleted: Actor.Receive = {
    case ExitingCompleted =>
      exitingCompleted()
      sender() ! Done // reply to ask
  }

  def receive = uninitialized

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

  def initJoin(joiningNodeConfig: Config): Unit = {
    val joiningNodeVersion =
      if (joiningNodeConfig.hasPath("akka.version")) joiningNodeConfig.getString("akka.version")
      else "unknown"
    // When joiningNodeConfig is empty the joining node has version 2.5.9 or earlier.
    val configCheckUnsupportedByJoiningNode = joiningNodeConfig.isEmpty

    val selfStatus = latestGossip.member(selfUniqueAddress).status

    if (removeUnreachableWithMemberStatus.contains(selfStatus)) {
      // prevents a Down and Exiting node from being used for joining
      logInfo(
        "Sending InitJoinNack message from node [{}] to [{}] (version [{}])",
        selfAddress,
        sender(),
        joiningNodeVersion)
      sender() ! InitJoinNack(selfAddress)
    } else {
      logInfo(
        "Sending InitJoinAck message from node [{}] to [{}] (version [{}])",
        selfAddress,
        sender(),
        joiningNodeVersion)
      // run config compatibility check using config provided by
      // joining node and current (full) config on cluster side

      val configWithoutSensitiveKeys = {
        val allowedConfigPaths =
          JoinConfigCompatChecker.removeSensitiveKeys(context.system.settings.config, cluster.settings)
        // build a stripped down config instead where sensitive config paths are removed
        // we don't want any check to happen on those keys
        JoinConfigCompatChecker.filterWithKeys(allowedConfigPaths, context.system.settings.config)
      }

      val configCheckReply =
        joinConfigCompatChecker.check(joiningNodeConfig, configWithoutSensitiveKeys) match {
          case Valid =>
            if (configCheckUnsupportedByJoiningNode)
              ConfigCheckUnsupportedByJoiningNode
            else {
              val nonSensitiveKeys = JoinConfigCompatChecker.removeSensitiveKeys(joiningNodeConfig, cluster.settings)
              // Send back to joining node a subset of current configuration
              // containing the keys initially sent by the joining node minus
              // any sensitive keys as defined by this node configuration
              val clusterConfig =
                JoinConfigCompatChecker.filterWithKeys(nonSensitiveKeys, context.system.settings.config)
              CompatibleConfig(clusterConfig)
            }
          case Invalid(messages) =>
            // messages are only logged on the cluster side
            logWarning(
              "Found incompatible settings when [{}] tried to join: {}. " +
              s"Self version [{}], Joining version [$joiningNodeVersion].",
              sender().path.address,
              messages.mkString(", "),
              context.system.settings.ConfigVersion)
            if (configCheckUnsupportedByJoiningNode)
              ConfigCheckUnsupportedByJoiningNode
            else
              IncompatibleConfig
        }

      sender() ! InitJoinAck(selfAddress, configCheckReply)

    }
  }

  def joinSeedNodes(newSeedNodes: immutable.IndexedSeq[Address]): Unit = {
    if (newSeedNodes.nonEmpty) {
      stopSeedNodeProcess()

      seedNodes = newSeedNodes // keep them for retry
      seedNodeProcess = if (newSeedNodes == immutable.IndexedSeq(selfAddress)) {
        self ! ClusterUserAction.JoinTo(selfAddress)
        None
      } else {
        // use unique name of this actor, stopSeedNodeProcess doesn't wait for termination
        seedNodeProcessCounter += 1
        if (newSeedNodes.head == selfAddress) {
          Some(
            context.actorOf(
              Props(classOf[FirstSeedNodeProcess], newSeedNodes, joinConfigCompatChecker).withDispatcher(UseDispatcher),
              name = "firstSeedNodeProcess-" + seedNodeProcessCounter))
        } else {
          Some(
            context.actorOf(
              Props(classOf[JoinSeedNodeProcess], newSeedNodes, joinConfigCompatChecker).withDispatcher(UseDispatcher),
              name = "joinSeedNodeProcess-" + seedNodeProcessCounter))
        }
      }
    }
  }

  /**
   * 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)
      logWarning(
        "Trying to join member with wrong protocol, but was ignored, expected [{}] but was [{}]",
        selfAddress.protocol,
        address.protocol)
    else if (address.system != selfAddress.system)
      logWarning(
        "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)
      stopSeedNodeProcess()

      if (address == selfAddress) {
        becomeInitialized()
        joining(selfUniqueAddress, cluster.selfRoles, cluster.settings.AppVersion)
      } 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, cluster.settings.AppVersion)
      }
    }
  }

  def stopSeedNodeProcess(): Unit = {
    seedNodeProcess match {
      case Some(s) =>
        // manual join, abort current seedNodeProcess
        context.stop(s)
        seedNodeProcess = None
      case None => // no seedNodeProcess in progress
    }
  }

  /**
   * 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(joiningNode: UniqueAddress, roles: Set[String], appVersion: Version): Unit = {
    if (!preparingForShutdown) {
      val selfStatus = latestGossip.member(selfUniqueAddress).status
      if (joiningNode.address.protocol != selfAddress.protocol)
        logWarning(
          "Member with wrong protocol tried to join, but was ignored, expected [{}] but was [{}]",
          selfAddress.protocol,
          joiningNode.address.protocol)
      else if (joiningNode.address.system != selfAddress.system)
        logWarning(
          "Member with wrong ActorSystem name tried to join, but was ignored, expected [{}] but was [{}]",
          selfAddress.system,
          joiningNode.address.system)
      else if (removeUnreachableWithMemberStatus.contains(selfStatus))
        logInfo(
          "Trying to join [{}] to [{}] member, ignoring. Use a member that is Up instead.",
          joiningNode,
          selfStatus)
      else {
        val localMembers = latestGossip.members

        // 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
        localMembers.find(_.address == joiningNode.address) match {
          case Some(m) if m.uniqueAddress == joiningNode =>
            // node retried join attempt, probably due to lost Welcome message
            logInfo("Existing member [{}] is joining again.", m)
            if (joiningNode != selfUniqueAddress)
              sender() ! Welcome(selfUniqueAddress, latestGossip)
          case Some(m) =>
            // node restarted, same host:port as existing member, but with different uid
            // safe to down and later remove existing member
            // new node will retry join
            logInfo(
              "New incarnation of existing member [{}] is trying to join. " +
              "Existing will be removed from the cluster and then new member will be allowed to join.",
              m)
            if (m.status != Down) {
              // we can confirm it as terminated/unreachable immediately
              val newReachability = latestGossip.overview.reachability.terminated(selfUniqueAddress, m.uniqueAddress)
              val newOverview = latestGossip.overview.copy(reachability = newReachability)
              val newGossip = latestGossip.copy(overview = newOverview)
              updateLatestGossip(newGossip)

              downing(m.address)
            }
          case None =>
            // remove the node from the failure detector
            failureDetector.remove(joiningNode.address)
            crossDcFailureDetector.remove(joiningNode.address)

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

            updateLatestGossip(newGossip)

            if (joiningNode == selfUniqueAddress) {
              logInfo(
                ClusterLogMarker.memberChanged(joiningNode, MemberStatus.Joining),
                "Node [{}] is JOINING itself (with roles [{}], version [{}]) and forming new cluster",
                joiningNode.address,
                roles.mkString(", "),
                appVersion)
              if (localMembers.isEmpty)
                leaderActions() // important for deterministic oldest when bootstrapping
            } else {
              logInfo(
                ClusterLogMarker.memberChanged(joiningNode, MemberStatus.Joining),
                "Node [{}] is JOINING, roles [{}], version [{}]",
                joiningNode.address,
                roles.mkString(", "),
                appVersion)
              sender() ! Welcome(selfUniqueAddress, latestGossip)
            }

            publishMembershipState()
        }
      }
    } else {
      logInfo("Ignoring join request from [{}] as preparing for shutdown", joiningNode)
    }
  }

  /**
   * Accept 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 {
      membershipState = membershipState.copy(latestGossip = gossip).seen()
      logInfo("Welcome from [{}]", from.address)
      assertLatestGossip()
      publishMembershipState()
      if (from != selfUniqueAddress)
        gossipTo(from, sender())
      becomeInitialized()
    }
  }

  def startPrepareForShutdown(): Unit = {
    if (!preparingForShutdown) {
      preparingForShutdown = true
      val changedMembers = latestGossip.members.collect {
        case m if MembershipState.allowedToPrepareToShutdown(m.status) =>
          m.copy(status = PreparingForShutdown)
      }
      val newGossip = latestGossip.update(changedMembers)
      updateLatestGossip(newGossip)
      changedMembers.foreach { member =>
        logInfo(
          ClusterLogMarker.memberChanged(member.uniqueAddress, MemberStatus.PreparingForShutdown),
          "Preparing for shutdown [{}] as [{}]",
          member.address,
          PreparingForShutdown)
      }
      publishMembershipState()
      gossip()
    }
  }

  /**
   * 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)
    latestGossip.members.find(_.address == address).foreach { existingMember =>
      if (existingMember.status == Joining || existingMember.status == WeaklyUp || existingMember.status == Up || existingMember.status == PreparingForShutdown || existingMember.status == ReadyForShutdown) {
        // mark node as LEAVING
        val newMembers = latestGossip.members - existingMember + existingMember.copy(status = Leaving)
        val newGossip = latestGossip.copy(members = newMembers)

        updateLatestGossip(newGossip)

        logInfo(
          ClusterLogMarker.memberChanged(existingMember.uniqueAddress, MemberStatus.Leaving),
          "Marked address [{}] as [{}]",
          address,
          Leaving)
        publishMembershipState()
        // immediate gossip to speed up the leaving process
        gossip()
      }
    }
  }

  def exitingCompleted() = {
    logInfo("Exiting completed")
    // ExitingCompleted sent via CoordinatedShutdown to continue the leaving process.
    exitingTasksInProgress = false
    // status Removed also before joining
    if (membershipState.selfMember.status != MemberStatus.Removed) {
      // mark as seen
      membershipState = membershipState.seen()
      assertLatestGossip()
      publishMembershipState()

      // Let others know (best effort) before shutdown. Otherwise they will not see
      // convergence of the Exiting state 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.
      gossipRandomN(NumberOfGossipsBeforeShutdownWhenLeaderExits)

      // send ExitingConfirmed to two potential leaders
      val membersExceptSelf = latestGossip.members.filter(_.uniqueAddress != selfUniqueAddress)

      membershipState.leaderOf(membersExceptSelf) match {
        case Some(node1) =>
          clusterCore(node1.address) ! ExitingConfirmed(selfUniqueAddress)
          membershipState.leaderOf(membersExceptSelf.filterNot(_.uniqueAddress == node1)) match {
            case Some(node2) =>
              clusterCore(node2.address) ! ExitingConfirmed(selfUniqueAddress)
            case None => // no more potential leader
          }
        case None => // no leader
      }
    }

    shutdown()
  }

  def receiveExitingConfirmed(node: UniqueAddress): Unit = {
    logInfo("Exiting confirmed [{}]", node.address)
    exitingConfirmed += node
  }

  def cleanupExitingConfirmed(): Unit = {
    // in case the actual removal was performed by another leader node we
    if (exitingConfirmed.nonEmpty)
      exitingConfirmed = exitingConfirmed.filter(n => latestGossip.members.exists(_.uniqueAddress == n))
  }

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

  /**
   * State transition to DOWN.
   * 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 localReachability = membershipState.dcReachability

    // check if the node to DOWN is in the `members` set
    localMembers.find(_.address == address) match {
      case Some(m) if m.status != Down =>
        if (localReachability.isReachable(m.uniqueAddress))
          logInfo(
            ClusterLogMarker.memberChanged(m.uniqueAddress, MemberStatus.Down),
            "Marking node [{}] as [{}]",
            m.address,
            Down)
        else
          logInfo(
            ClusterLogMarker.memberChanged(m.uniqueAddress, MemberStatus.Down),
            "Marking unreachable node [{}] as [{}]",
            m.address,
            Down)

        val newGossip = localGossip.markAsDown(m)
        updateLatestGossip(newGossip)
        publishMembershipState()
        if (address == cluster.selfAddress) {
          // spread the word quickly, without waiting for next gossip tick
          gossipRandomN(MaxGossipsBeforeShuttingDownMyself)
        } else {
          // try to gossip immediately to downed node, as a STONITH signal
          gossipTo(m.uniqueAddress)
        }
      case Some(_) => // already down
      case None =>
        logInfo("Ignoring down of unknown node [{}]", address)
    }

  }

  def quarantined(node: UniqueAddress): Unit = {
    val localGossip = latestGossip
    if (localGossip.hasMember(node)) {
      val newReachability = latestGossip.overview.reachability.terminated(selfUniqueAddress, node)
      val newOverview = localGossip.overview.copy(reachability = newReachability)
      val newGossip = localGossip.copy(overview = newOverview)
      updateLatestGossip(newGossip)
      logWarning(
        ClusterLogMarker.unreachable(node.address),
        "Marking node as TERMINATED [{}], due to quarantine. Node roles [{}]. " +
        "It must still be marked as down before it's removed.",
        node.address,
        selfRoles.mkString(","))
      publishMembershipState()
    }
  }

  def receiveGossipStatus(status: GossipStatus): Unit = {
    val from = status.from
    if (!latestGossip.hasMember(from))
      gossipLogger.logInfo("Ignoring received gossip status from unknown [{}]", from)
    else if (!latestGossip.isReachable(selfUniqueAddress, from))
      gossipLogger.logInfo("Ignoring received gossip status from unreachable [{}] ", from)
    else {
      val localSeenDigest = latestGossip.seenDigest
      val seenSame =
        if (status.seenDigest.isEmpty || localSeenDigest.isEmpty) true
        else java.util.Arrays.equals(status.seenDigest, localSeenDigest)
      if (!seenSame) {
        gossipTo(from, sender())
      } 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 = try {
      envelope.gossip
    } catch {
      case NonFatal(t) =>
        gossipLogger.logWarning("Invalid Gossip. This should only happen during a rolling upgrade. {}", t.getMessage)
        Gossip.empty

    }
    val localGossip = latestGossip

    if (remoteGossip eq Gossip.empty) {
      gossipLogger.logDebug("Ignoring received gossip from [{}] to protect against overload", from)
      Ignored
    } else if (envelope.to != selfUniqueAddress) {
      gossipLogger.logInfo(
        "Ignoring received gossip intended for someone else, from [{}] to [{}]",
        from.address,
        envelope.to)
      Ignored
    } else if (!localGossip.hasMember(from)) {
      gossipLogger.logInfo("Ignoring received gossip from unknown [{}]", from)
      Ignored
    } else if (!localGossip.isReachable(selfUniqueAddress, from)) {
      gossipLogger.logInfo("Ignoring received gossip from unreachable [{}] ", from)
      Ignored
    } else if (remoteGossip.members.forall(_.uniqueAddress != selfUniqueAddress)) {
      gossipLogger.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.Same =>
          // same version
          val talkback = !exitingTasksInProgress && !remoteGossip.seenByNode(selfUniqueAddress)
          (remoteGossip.mergeSeen(localGossip), talkback, Same)
        case VectorClock.Before =>
          // local is newer
          (localGossip, true, Older)
        case VectorClock.After =>
          // remote is newer
          val talkback = !exitingTasksInProgress && !remoteGossip.seenByNode(selfUniqueAddress)
          (remoteGossip, talkback, Newer)
        case _ =>
          // conflicting versions, merge
          // We can see that a removal was done when it is not in one of the gossips has status
          // Down or Exiting in the other gossip.
          // Perform the same pruning (clear of VectorClock) as the leader did when removing a member.
          // Removal of member itself is handled in merge (pickHighestPriority)
          val prunedLocalGossip = localGossip.members.foldLeft(localGossip) { (g, m) =>
            if (removeUnreachableWithMemberStatus(m.status) && !remoteGossip.members.contains(m)) {
              gossipLogger.logDebug("Pruned conflicting local gossip: {}", m)
              g.prune(VectorClock.Node(Gossip.vclockName(m.uniqueAddress)))
            } else
              g
          }
          val prunedRemoteGossip = remoteGossip.members.foldLeft(remoteGossip) { (g, m) =>
            if (removeUnreachableWithMemberStatus(m.status) && !localGossip.members.contains(m)) {
              gossipLogger.logDebug("Pruned conflicting remote gossip: {}", m)
              g.prune(VectorClock.Node(Gossip.vclockName(m.uniqueAddress)))
            } else
              g
          }

          (prunedRemoteGossip.merge(prunedLocalGossip), true, Merge)
      }

      // Don't mark gossip state as seen while exiting is in progress, e.g.
      // shutting down singleton actors. This delays removal of the member until
      // the exiting tasks have been completed.
      membershipState = membershipState.copy(
        latestGossip =
          if (exitingTasksInProgress) winningGossip
          else winningGossip.seen(selfUniqueAddress))
      assertLatestGossip()

      // for all new nodes we remove them from the failure detector
      latestGossip.members.foreach { node =>
        if (!localGossip.members(node)) {
          failureDetector.remove(node.address)
          crossDcFailureDetector.remove(node.address)
        }
      }

      gossipLogger.logDebug("Receiving gossip from [{}]", from)

      if (comparison == VectorClock.Concurrent && cluster.settings.Debug.VerboseGossipLogging) {
        gossipLogger.logDebug(
          """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()
          case Ignored => gossipStats // included in receivedGossipCount
        }
      }

      publishMembershipState()

      val selfStatus = latestGossip.member(selfUniqueAddress).status
      if (selfStatus == Exiting && !exitingTasksInProgress) {
        // ExitingCompleted will be received via CoordinatedShutdown to continue
        // the leaving process. Meanwhile the gossip state is not marked as seen.
        exitingTasksInProgress = true
        if (coordShutdown.shutdownReason().isEmpty)
          logInfo("Exiting, starting coordinated shutdown")
        selfExiting.trySuccess(Done)
        coordShutdown.run(CoordinatedShutdown.ClusterLeavingReason)
      }

      if (selfStatus == Down && localGossip.member(selfUniqueAddress).status != Down) {
        logWarning("Received gossip where this member has been downed, from [{}]", from.address)
        shutdownSelfWhenDown()
      }

      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 gossipTick(): Unit = {
    gossip()
    if (isGossipSpeedupNeeded) {
      scheduler.scheduleOnce(GossipInterval / 3, self, GossipSpeedupTick)
      scheduler.scheduleOnce(GossipInterval * 2 / 3, self, GossipSpeedupTick)
    }
  }

  def gossipSpeedupTick(): Unit =
    if (isGossipSpeedupNeeded) gossip()

  def isGossipSpeedupNeeded: Boolean = {
    if (latestGossip.isMultiDc) {
      latestGossip.members.exists(m => m.status == Down || m.dataCenter == cluster.selfDataCenter) ||
      latestGossip.overview.seen
        .count(membershipState.isInSameDc) < latestGossip.members.count(_.dataCenter == cluster.selfDataCenter) / 2
    } else {
      latestGossip.members.exists(m => m.status == Down) ||
      latestGossip.overview.seen.size < latestGossip.members.size / 2
    }
  }

  /**
   * Sends full gossip to `n` other random members.
   */
  def gossipRandomN(n: Int): Unit = {
    if (!isSingletonCluster && n > 0) {
      gossipTargetSelector.randomNodesForFullGossip(membershipState, n).foreach(gossipTo)
    }
  }

  /**
   * Initiates a new round of gossip.
   */
  def gossip(): Unit =
    if (!isSingletonCluster) {
      gossipTargetSelector.gossipTarget(membershipState) match {
        case Some(peer) =>
          if (!membershipState.isInSameDc(peer) || latestGossip.seenByNode(peer))
            // avoid transferring the full state if possible
            gossipStatusTo(peer)
          else
            gossipTo(peer)
        case None => // nothing to see here
          if (cluster.settings.Debug.VerboseGossipLogging)
            gossipLogger.logDebug("will not gossip this round")

      }
    }

  /**
   * Runs periodic leader actions, such as member status transitions, assigning partitions etc.
   */
  def leaderActions(): Unit = {
    if (membershipState.isLeader(selfUniqueAddress)) {
      // only run the leader actions if we are the LEADER of the data center
      if (!isCurrentlyLeader) {
        logInfo("is the new leader among reachable nodes (more leaders may exist)")
        isCurrentlyLeader = true
      }
      val firstNotice = 20
      val periodicNotice = 60
      if (membershipState.convergence(exitingConfirmed)) {
        if (leaderActionCounter >= firstNotice)
          logInfo(ClusterLogMarker.leaderRestored, "Leader can perform its duties again")
        leaderActionCounter = 0
        leaderActionsOnConvergence()
      } else {
        leaderActionCounter += 1
        import cluster.settings.{ AllowWeaklyUpMembers, LeaderActionsInterval, WeaklyUpAfter }
        if (AllowWeaklyUpMembers && LeaderActionsInterval * leaderActionCounter >= WeaklyUpAfter && !preparingForShutdown)
          moveJoiningToWeaklyUp()

        if (leaderActionCounter == firstNotice || leaderActionCounter % periodicNotice == 0)
          logInfo(
            ClusterLogMarker.leaderIncapacitated,
            "Leader can currently not perform its duties, reachability status: [{}], member status: [{}]",
            membershipState.dcReachabilityExcludingDownedObservers,
            latestGossip.members
              .collect {
                case m if m.dataCenter == selfDc =>
                  s"${m.address} ${m.status} seen=${latestGossip.seenByNode(m.uniqueAddress)}"
              }
              .mkString(", "))
      }
    } else if (isCurrentlyLeader) {
      logInfo("is no longer leader")
      isCurrentlyLeader = false
    }
    cleanupExitingConfirmed()
    checkForPrepareForShutdown()
    shutdownSelfWhenDown()
  }

  def checkForPrepareForShutdown(): Unit = {
    if (MembershipState.allowedToPrepareToShutdown(latestGossip.member(selfUniqueAddress).status) && latestGossip.members
          .exists(m => MembershipState.prepareForShutdownStates(m.status))) {
      logDebug("Detected full cluster shutdown")
      self ! ClusterUserAction.PrepareForShutdown
    }
  }

  def shutdownSelfWhenDown(): Unit = {
    if (latestGossip.member(selfUniqueAddress).status == Down) {
      // When all reachable have seen the state this member will shutdown itself when it has
      // status Down. The down commands should spread before we shutdown.
      val unreachable = membershipState.dcReachability.allUnreachableOrTerminated
      val downed = membershipState.dcMembers.collect { case m if m.status == Down => m.uniqueAddress }
      if (selfDownCounter >= MaxTicksBeforeShuttingDownMyself || downed.forall(
            node => unreachable(node) || latestGossip.seenByNode(node))) {
        // the reason for not shutting down immediately is to give the gossip a chance to spread
        // the downing information to other downed nodes, so that they can shutdown themselves
        logInfo("Node has been marked as DOWN. Shutting down myself")
        // not crucial to send gossip, but may speedup removal since fallback to failure detection is not needed
        // if other downed know that this node has seen the version
        gossipRandomN(MaxGossipsBeforeShuttingDownMyself)
        shutdown()
      } else {
        selfDownCounter += 1
      }
    }
  }

  def isMinNrOfMembersFulfilled: Boolean = {
    latestGossip.members.size >= MinNrOfMembers && MinNrOfMembersOfRole.forall {
      case (role, threshold) => latestGossip.members.count(_.hasRole(role)) >= threshold
    }
  }

  /**
   * Leader actions are as follows:
   * 1. Move JOINING     => UP                   -- When a node joins the cluster
   * 2. Move LEAVING     => EXITING              --
   * 3. Non-exiting remain                       --
   * 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 removedUnreachable = for {
      node <- membershipState.dcReachability.allUnreachableOrTerminated
      m = latestGossip.member(node)
      if m.dataCenter == selfDc && removeUnreachableWithMemberStatus(m.status)
    } yield m

    val removedExitingConfirmed = exitingConfirmed.filter { n =>
      val member = latestGossip.member(n)
      member.dataCenter == selfDc && member.status == Exiting
    }

    val removedOtherDc =
      if (latestGossip.isMultiDc) {
        latestGossip.members.filter { m =>
          m.dataCenter != selfDc && removeUnreachableWithMemberStatus(m.status)
        }
      } else
        Set.empty[Member]

    val changedMembers = {
      val enoughMembers: Boolean = isMinNrOfMembersFulfilled
      def isJoiningToUp(m: Member): Boolean = (m.status == Joining || m.status == WeaklyUp) && enoughMembers

      latestGossip.members.collect {
        var upNumber = 0

        {
          case m if m.dataCenter == selfDc && isJoiningToUp(m) && !preparingForShutdown =>
            // 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
            // don't move members to up when preparing for shutdown
            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 = membershipState.youngestMember
              upNumber = 1 + (if (youngest.upNumber == Int.MaxValue) 0 else youngest.upNumber)
            } else {
              upNumber += 1
            }
            m.copyUp(upNumber)

          case m if m.dataCenter == selfDc && m.status == Leaving =>
            // Move LEAVING => EXITING (once we have a convergence on LEAVING)
            m.copy(status = Exiting)

          case m if m.dataCenter == selfDc & m.status == PreparingForShutdown =>
            // Move PreparingForShutdown => ReadyForShutdown (once we have a convergence on PreparingForShutdown)
            m.copy(status = ReadyForShutdown)
        }
      }
    }

    val updatedGossip: Gossip =
      if (removedUnreachable.nonEmpty || removedExitingConfirmed.nonEmpty || changedMembers.nonEmpty ||
          removedOtherDc.nonEmpty) {

        // replace changed members
        val removed = removedUnreachable
          .map(_.uniqueAddress)
          .union(removedExitingConfirmed)
          .union(removedOtherDc.map(_.uniqueAddress))
        val newGossip =
          latestGossip.update(changedMembers).removeAll(removed, System.currentTimeMillis())

        if (!exitingTasksInProgress && newGossip.member(selfUniqueAddress).status == Exiting) {
          // Leader is moving itself from Leaving to Exiting.
          // ExitingCompleted will be received via CoordinatedShutdown to continue
          // the leaving process. Meanwhile the gossip state is not marked as seen.
          exitingTasksInProgress = true
          if (coordShutdown.shutdownReason().isEmpty)
            logInfo("Exiting (leader), starting coordinated shutdown")
          selfExiting.trySuccess(Done)
          coordShutdown.run(CoordinatedShutdown.ClusterLeavingReason)
        }

        exitingConfirmed = exitingConfirmed.filterNot(removedExitingConfirmed)

        changedMembers.foreach { m =>
          logInfo(
            ClusterLogMarker.memberChanged(m.uniqueAddress, m.status),
            "Leader is moving node [{}] to [{}]",
            m.address,
            m.status)
        }
        removedUnreachable.foreach { m =>
          val status = if (m.status == Exiting) "exiting" else "unreachable"
          logInfo(
            ClusterLogMarker.memberChanged(m.uniqueAddress, MemberStatus.Removed),
            "Leader is removing {} node [{}]",
            status,
            m.address)
        }
        removedExitingConfirmed.foreach { n =>
          logInfo(
            ClusterLogMarker.memberChanged(n, MemberStatus.Removed),
            "Leader is removing confirmed Exiting node [{}]",
            n.address)
        }
        removedOtherDc.foreach { m =>
          logInfo(
            ClusterLogMarker.memberChanged(m.uniqueAddress, MemberStatus.Removed),
            "Leader is removing {} node [{}] in DC [{}]",
            m.status,
            m.address,
            m.dataCenter)
        }

        newGossip
      } else
        latestGossip

    val pruned = updatedGossip.pruneTombstones(System.currentTimeMillis() - PruneGossipTombstonesAfter.toMillis)
    if (pruned ne latestGossip) {
      updateLatestGossip(pruned)
      publishMembershipState()
      gossipExitingMembersToOldest(changedMembers.filter(_.status == Exiting))
    }
  }

  /**
   * Gossip the Exiting change to the two oldest nodes for quick dissemination to potential Singleton nodes
   */
  private def gossipExitingMembersToOldest(exitingMembers: Set[Member]): Unit = {
    val targets = membershipState.gossipTargetsForExitingMembers(exitingMembers)
    if (targets.nonEmpty) {

      if (isDebugEnabled)
        gossipLogger.logDebug(
          "Gossip exiting members [{}] to the two oldest (per role) [{}] (singleton optimization).",
          exitingMembers.mkString(", "),
          targets.mkString(", "))

      targets.foreach(m => gossipTo(m.uniqueAddress))
    }
  }

  def moveJoiningToWeaklyUp(): Unit = {
    val localGossip = latestGossip
    val localMembers = localGossip.members

    val enoughMembers: Boolean = isMinNrOfMembersFulfilled
    def isJoiningToWeaklyUp(m: Member): Boolean =
      m.dataCenter == selfDc &&
      m.status == Joining &&
      enoughMembers &&
      membershipState.dcReachabilityExcludingDownedObservers.isReachable(m.uniqueAddress)
    val changedMembers = localMembers.collect {
      case m if isJoiningToWeaklyUp(m) => m.copy(status = WeaklyUp)
    }

    if (changedMembers.nonEmpty) {
      val newGossip = localGossip.update(changedMembers)
      updateLatestGossip(newGossip)

      // log status changes
      changedMembers.foreach { m =>
        logInfo(
          ClusterLogMarker.memberChanged(m.uniqueAddress, m.status),
          "Leader is moving node [{}] to [{}]",
          m.address,
          m.status)
      }

      publishMembershipState()
    }

  }

  /**
   * Reaps the unreachable members according to the failure detector's verdict.
   */
  def reapUnreachableMembers(): Unit = {
    if (!isSingletonCluster) {
      // only scrutinize if we are a non-singleton cluster

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

      def isAvailable(member: Member): Boolean = {
        if (member.dataCenter == SelfDataCenter) failureDetector.isAvailable(member.address)
        else crossDcFailureDetector.isAvailable(member.address)
      }

      val newlyDetectedUnreachableMembers = localMembers.filterNot { member =>
        member.uniqueAddress == selfUniqueAddress ||
        localOverview.reachability.status(selfUniqueAddress, member.uniqueAddress) == Reachability.Unreachable ||
        localOverview.reachability.status(selfUniqueAddress, member.uniqueAddress) == Reachability.Terminated ||
        isAvailable(member)
      }

      val newlyDetectedReachableMembers = localOverview.reachability.allUnreachableFrom(selfUniqueAddress).collect {
        case node if node != selfUniqueAddress && isAvailable(localGossip.member(node)) =>
          localGossip.member(node)
      }

      if (newlyDetectedUnreachableMembers.nonEmpty || newlyDetectedReachableMembers.nonEmpty) {

        val newReachability1 = newlyDetectedUnreachableMembers.foldLeft(localOverview.reachability) {
          (reachability, m) =>
            reachability.unreachable(selfUniqueAddress, m.uniqueAddress)
        }
        val newReachability2 = newlyDetectedReachableMembers.foldLeft(newReachability1) { (reachability, m) =>
          reachability.reachable(selfUniqueAddress, m.uniqueAddress)
        }

        if (newReachability2 ne localOverview.reachability) {
          val newOverview = localOverview.copy(reachability = newReachability2)
          val newGossip = localGossip.copy(overview = newOverview)

          updateLatestGossip(newGossip)

          val (exiting, nonExiting) = newlyDetectedUnreachableMembers.partition(_.status == Exiting)
          nonExiting.foreach { node =>
            logWarning(ClusterLogMarker.unreachable(node.address), "Marking node as UNREACHABLE [{}].", node)
          }
          if (exiting.nonEmpty)
            logInfo(
              "Marking exiting node(s) as UNREACHABLE [{}]. This is expected and they will be removed.",
              exiting.mkString(", "))
          newlyDetectedReachableMembers.foreach { node =>
            logInfo(ClusterLogMarker.reachable(node.address), "Marking node as REACHABLE [{}].", node)
          }

          publishMembershipState()
        }
      }
    }
  }

  def isSingletonCluster: Boolean = latestGossip.isSingletonCluster

  // 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 (membershipState.validNodeForGossip(node))
      clusterCore(node.address) ! GossipEnvelope(selfUniqueAddress, node, latestGossip)

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

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

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

  def updateLatestGossip(gossip: Gossip): Unit = {
    // Updating the vclock version for the changes
    val versionedGossip = gossip :+ vclockNode

    // Don't mark gossip state as seen while exiting is in progress, e.g.
    // shutting down singleton actors. This delays removal of the member until
    // the exiting tasks have been completed.
    val newGossip =
      if (exitingTasksInProgress)
        versionedGossip.clearSeen()
      else {
        // Nobody else has seen this gossip but us
        val seenVersionedGossip = versionedGossip.onlySeen(selfUniqueAddress)
        // Update the state with the new gossip
        seenVersionedGossip
      }
    membershipState = membershipState.copy(newGossip)
    assertLatestGossip()
  }

  def assertLatestGossip(): Unit =
    if (Cluster.isAssertInvariantsEnabled && latestGossip.version.versions.size > latestGossip.members.size)
      throw new IllegalStateException(s"Too many vector clock entries in gossip state $latestGossip")

  def publishMembershipState(): Unit = {
    if (cluster.settings.Debug.VerboseGossipLogging)
      gossipLogger.logDebug("New gossip published [{}]", membershipState.latestGossip)

    publisher ! PublishChanges(membershipState)
    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
 *
 * The supplied callback will be run, once, when current cluster member come up with the same status.
 */
@InternalApi
private[cluster] class OnMemberStatusChangedListener(callback: Runnable, status: MemberStatus) extends Actor {
  import ClusterEvent._
  private val cluster = Cluster(context.system)
  import cluster.ClusterLogger._

  private val to = status match {
    case Up      => classOf[MemberUp]
    case Removed => classOf[MemberRemoved]
    case other =>
      throw new IllegalArgumentException(s"Expected Up or Removed in OnMemberStatusChangedListener, got [$other]")
  }

  override def preStart(): Unit =
    cluster.subscribe(self, to)

  override def postStop(): Unit = {
    if (status == Removed)
      done()
    cluster.unsubscribe(self)
  }

  def receive = {
    case state: CurrentClusterState =>
      if (state.members.exists(isTriggered))
        done()
    case MemberUp(member) =>
      if (isTriggered(member))
        done()
    case MemberRemoved(member, _) =>
      if (isTriggered(member))
        done()
  }

  private def done(): Unit = {
    try callback.run()
    catch {
      case NonFatal(e) => logError(e, "[{}] callback failed with [{}]", s"On${to.getSimpleName}", e.getMessage)
    } finally {
      context.stop(self)
    }
  }

  private def isTriggered(m: Member): Boolean =
    m.uniqueAddress == cluster.selfUniqueAddress && m.status == status

}

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