diff --git a/akka-actor/src/main/scala/akka/routing/ConsistentHash.scala b/akka-actor/src/main/scala/akka/routing/ConsistentHash.scala
index 79c31cda33..d3bef92e6c 100644
--- a/akka-actor/src/main/scala/akka/routing/ConsistentHash.scala
+++ b/akka-actor/src/main/scala/akka/routing/ConsistentHash.scala
@@ -18,7 +18,7 @@ import java.util.Arrays
  * hash, i.e. make sure it is different for different nodes.
  *
  */
-class ConsistentHash[T: ClassTag] private (nodes: SortedMap[Int, T], virtualNodesFactor: Int) {
+class ConsistentHash[T: ClassTag] private (nodes: SortedMap[Int, T], val virtualNodesFactor: Int) {
 
   import ConsistentHash._
 
diff --git a/akka-cluster/src/main/scala/akka/cluster/ClusterHeartbeat.scala b/akka-cluster/src/main/scala/akka/cluster/ClusterHeartbeat.scala
index 561f3c85ba..325e0aae25 100644
--- a/akka-cluster/src/main/scala/akka/cluster/ClusterHeartbeat.scala
+++ b/akka-cluster/src/main/scala/akka/cluster/ClusterHeartbeat.scala
@@ -89,13 +89,9 @@ private[cluster] final class ClusterHeartbeatSender extends Actor with ActorLogg
 
   val selfHeartbeat = Heartbeat(selfAddress)
   val selfEndHeartbeat = EndHeartbeat(selfAddress)
-  val selfAddressStr = selfAddress.toString
 
-  var all = Set.empty[Address]
-  var current = Set.empty[Address]
-  var ending = Map.empty[Address, Int]
-  var joinInProgress = Map.empty[Address, Deadline]
-  var consistentHash = ConsistentHash(Seq.empty[Address], HeartbeatConsistentHashingVirtualNodesFactor)
+  var state = ClusterHeartbeatSenderState.empty(ConsistentHash(Seq.empty[Address], HeartbeatConsistentHashingVirtualNodesFactor),
+    selfAddress.toString, MonitoredByNrOfMembers)
 
   // start periodic heartbeat to other nodes in cluster
   val heartbeatTask = scheduler.schedule(PeriodicTasksInitialDelay.max(HeartbeatInterval).asInstanceOf[FiniteDuration],
@@ -115,47 +111,31 @@ private[cluster] final class ClusterHeartbeatSender extends Actor with ActorLogg
     context.actorFor(RootActorPath(address) / "system" / "cluster" / "heartbeatReceiver")
 
   def receive = {
-    case HeartbeatTick              ⇒ heartbeat()
-    case state: CurrentClusterState ⇒ init(state)
-    case MemberUnreachable(m)       ⇒ removeMember(m)
-    case MemberRemoved(m)           ⇒ removeMember(m)
-    case e: MemberEvent             ⇒ addMember(e.member)
-    case JoinInProgress(a, d)       ⇒ addJoinInProgress(a, d)
+    case HeartbeatTick          ⇒ heartbeat()
+    case s: CurrentClusterState ⇒ reset(s)
+    case MemberUnreachable(m)   ⇒ removeMember(m)
+    case MemberRemoved(m)       ⇒ removeMember(m)
+    case e: MemberEvent         ⇒ addMember(e.member)
+    case JoinInProgress(a, d)   ⇒ addJoinInProgress(a, d)
   }
 
-  def init(state: CurrentClusterState): Unit = {
-    all = state.members.collect { case m if m.address != selfAddress ⇒ m.address }
-    joinInProgress --= all
-    consistentHash = ConsistentHash(all, HeartbeatConsistentHashingVirtualNodesFactor)
-    update()
-  }
+  def reset(snapshot: CurrentClusterState): Unit =
+    state = state.reset(snapshot.members.collect { case m if m.address != selfAddress ⇒ m.address })
 
-  def addMember(m: Member): Unit = if (m.address != selfAddress) {
-    all += m.address
-    consistentHash = consistentHash :+ m.address
-    removeJoinInProgress(m.address)
-    update()
-  }
+  def addMember(m: Member): Unit = if (m.address != selfAddress)
+    state = state addMember m.address
 
-  def removeMember(m: Member): Unit = if (m.address != selfAddress) {
-    all -= m.address
-    consistentHash = consistentHash :- m.address
-    removeJoinInProgress(m.address)
-    update()
-  }
+  def removeMember(m: Member): Unit = if (m.address != selfAddress)
+    state = state removeMember m.address
 
-  def removeJoinInProgress(address: Address): Unit = if (joinInProgress contains address) {
-    joinInProgress -= address
-    ending += (address -> 0)
-  }
+  def removeJoinInProgress(address: Address): Unit = if (address != selfAddress)
+    state = state.removeJoinInProgress(address)
 
-  def addJoinInProgress(address: Address, deadline: Deadline): Unit = {
-    if (address != selfAddress && !all.contains(address))
-      joinInProgress += (address -> deadline)
-  }
+  def addJoinInProgress(address: Address, deadline: Deadline): Unit = if (address != selfAddress)
+    state = state.addJoinInProgress(address, deadline)
 
   def heartbeat(): Unit = {
-    removeOverdueJoinInProgress()
+    state = state.removeOverdueJoinInProgress()
 
     def connection(to: Address): ActorRef = {
       // URL encoded target address as child actor name
@@ -168,67 +148,145 @@ private[cluster] final class ClusterHeartbeatSender extends Actor with ActorLogg
     }
 
     val deadline = Deadline.now + HeartbeatInterval
-    (current ++ joinInProgress.keys) foreach { to ⇒ connection(to) ! SendHeartbeat(selfHeartbeat, to, deadline) }
+    state.active foreach { to ⇒ connection(to) ! SendHeartbeat(selfHeartbeat, to, deadline) }
 
     // When sending heartbeats to a node is stopped a few `EndHeartbeat` messages is
     // sent to notify it that no more heartbeats will be sent.
-    for ((to, count) ← ending) {
+    for ((to, count) ← state.ending) {
       val c = connection(to)
       c ! SendEndHeartbeat(selfEndHeartbeat, to)
       if (count == NumberOfEndHeartbeats) {
-        ending -= to
+        state = state.removeEnding(to)
         c ! PoisonPill
-      } else {
-        ending += (to -> (count + 1))
-      }
+      } else
+        state = state.increaseEndingCount(to)
     }
   }
 
+}
+
+/**
+ * INTERNAL API
+ */
+private[cluster] object ClusterHeartbeatSenderState {
   /**
-   * Update current peers to send heartbeats to, and
+   * Initial, empty state
+   */
+  def empty(consistentHash: ConsistentHash[Address], selfAddressStr: String,
+            monitoredByNrOfMembers: Int): ClusterHeartbeatSenderState =
+    ClusterHeartbeatSenderState(consistentHash, selfAddressStr, monitoredByNrOfMembers)
+
+  /**
+   * Create a new state based on previous state, and
    * keep track of which nodes to stop sending heartbeats to.
    */
-  def update(): Unit = {
-    val previous = current
-    current = selectPeers
+  private def apply(
+    old: ClusterHeartbeatSenderState,
+    consistentHash: ConsistentHash[Address],
+    all: Set[Address]): ClusterHeartbeatSenderState = {
+
+    /**
+     * Select a few peers that heartbeats will be sent to, i.e. that will
+     * monitor this node. Try to send heartbeats to same nodes as much
+     * as possible, but re-balance with consistent hashing algorithm when
+     * new members are added or removed.
+     */
+    def selectPeers: Set[Address] = {
+      val allSize = all.size
+      val nrOfPeers = math.min(allSize, old.monitoredByNrOfMembers)
+      // try more if consistentHash results in same node as already selected
+      val attemptLimit = nrOfPeers * 2
+      @tailrec def select(acc: Set[Address], n: Int): Set[Address] = {
+        if (acc.size == nrOfPeers || n == attemptLimit) acc
+        else select(acc + consistentHash.nodeFor(old.selfAddressStr + n), n + 1)
+      }
+      if (nrOfPeers >= allSize) all
+      else select(Set.empty[Address], 0)
+    }
+
+    val curr = selectPeers
     // start ending process for nodes not selected any more
-    ending ++= (previous -- current).map(_ -> 0)
     // abort ending process for nodes that have been selected again
-    ending --= current
+    val end = old.ending ++ (old.current -- curr).map(_ -> 0) -- curr
+    old.copy(consistentHash = consistentHash, all = all, current = curr, ending = end,
+      joinInProgress = old.joinInProgress -- all)
   }
 
-  /**
-   * Select a few peers that heartbeats will be sent to, i.e. that will
-   * monitor this node. Try to send heartbeats to same nodes as much
-   * as possible, but re-balance with consistent hashing algorithm when
-   * new members are added or removed.
-   */
-  def selectPeers: Set[Address] = {
-    val allSize = all.size
-    val nrOfPeers = math.min(allSize, MonitoredByNrOfMembers)
-    // try more if consistentHash results in same node as already selected
-    val attemptLimit = nrOfPeers * 2
-    @tailrec def select(acc: Set[Address], n: Int): Set[Address] = {
-      if (acc.size == nrOfPeers || n == attemptLimit) acc
-      else select(acc + consistentHash.nodeFor(selfAddressStr + n), n + 1)
-    }
-    if (nrOfPeers >= allSize) all
-    else select(Set.empty[Address], 0)
+}
+
+/**
+ * INTERNAL API
+ *
+ * State used by [akka.cluster.ClusterHeartbeatSender].
+ * The initial state is created with `empty` in the of
+ * the companion object, thereafter the state is modified
+ * with the methods, such as `addMember`. It is immutable,
+ * i.e. the methods return new instances.
+ */
+private[cluster] case class ClusterHeartbeatSenderState private (
+  consistentHash: ConsistentHash[Address],
+  selfAddressStr: String,
+  monitoredByNrOfMembers: Int,
+  all: Set[Address] = Set.empty,
+  current: Set[Address] = Set.empty,
+  ending: Map[Address, Int] = Map.empty,
+  joinInProgress: Map[Address, Deadline] = Map.empty) {
+
+  // FIXME can be disabled as optimization
+  assertInvariants
+
+  private def assertInvariants: Unit = {
+    val currentAndEnding = current.intersect(ending.keySet)
+    require(currentAndEnding.isEmpty,
+      "Same nodes in current and ending not allowed, got [%s]" format currentAndEnding)
+    val joinInProgressAndAll = joinInProgress.keySet.intersect(all)
+    require(joinInProgressAndAll.isEmpty,
+      "Same nodes in joinInProgress and all not allowed, got [%s]" format joinInProgressAndAll)
+    val currentNotInAll = current -- all
+    require(currentNotInAll.isEmpty,
+      "Nodes in current but not in all not allowed, got [%s]" format currentNotInAll)
+    require(all.isEmpty == consistentHash.isEmpty, "ConsistentHash doesn't correspond to all nodes [%s]"
+      format all)
+  }
+
+  val active: Set[Address] = current ++ joinInProgress.keySet
+
+  def reset(nodes: Set[Address]): ClusterHeartbeatSenderState =
+    ClusterHeartbeatSenderState(this, consistentHash = ConsistentHash(nodes, consistentHash.virtualNodesFactor),
+      all = nodes)
+
+  def addMember(a: Address): ClusterHeartbeatSenderState =
+    ClusterHeartbeatSenderState(this, all = all + a, consistentHash = consistentHash :+ a)
+
+  def removeMember(a: Address): ClusterHeartbeatSenderState =
+    ClusterHeartbeatSenderState(this, all = all - a, consistentHash = consistentHash :- a)
+
+  def removeJoinInProgress(address: Address): ClusterHeartbeatSenderState = {
+    if (joinInProgress contains address)
+      copy(joinInProgress = joinInProgress - address, ending = ending + (address -> 0))
+    else this
+  }
+
+  def addJoinInProgress(address: Address, deadline: Deadline): ClusterHeartbeatSenderState = {
+    if (all contains address) this
+    else copy(joinInProgress = joinInProgress + (address -> deadline))
   }
 
   /**
    * Cleanup overdue joinInProgress, in case a joining node never
    * became member, for some reason.
    */
-  def removeOverdueJoinInProgress(): Unit = {
+  def removeOverdueJoinInProgress(): ClusterHeartbeatSenderState = {
     val overdue = joinInProgress collect { case (address, deadline) if deadline.isOverdue ⇒ address }
-    if (overdue.nonEmpty) {
-      log.info("Overdue join in progress [{}]", overdue.mkString(", "))
-      ending ++= overdue.map(_ -> 0)
-      joinInProgress --= overdue
-    }
+    if (overdue.isEmpty) this
+    else
+      copy(ending = ending ++ overdue.map(_ -> 0), joinInProgress = joinInProgress -- overdue)
   }
 
+  def removeEnding(a: Address): ClusterHeartbeatSenderState = copy(ending = ending - a)
+
+  def increaseEndingCount(a: Address): ClusterHeartbeatSenderState = copy(ending = ending + (a -> (ending(a) + 1)))
+
 }
 
 /**
diff --git a/akka-cluster/src/test/scala/akka/cluster/ClusterHeartbeatSenderStateSpec.scala b/akka-cluster/src/test/scala/akka/cluster/ClusterHeartbeatSenderStateSpec.scala
new file mode 100644
index 0000000000..bd378ed0fe
--- /dev/null
+++ b/akka-cluster/src/test/scala/akka/cluster/ClusterHeartbeatSenderStateSpec.scala
@@ -0,0 +1,88 @@
+/**
+ *  Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.cluster
+
+import org.scalatest.WordSpec
+import org.scalatest.matchers.MustMatchers
+import akka.actor.Address
+import akka.routing.ConsistentHash
+import scala.concurrent.util.Deadline
+import scala.concurrent.util.duration._
+
+@org.junit.runner.RunWith(classOf[org.scalatest.junit.JUnitRunner])
+class ClusterHeartbeatSenderStateSpec extends WordSpec with MustMatchers {
+
+  val selfAddress = Address("akka", "sys", "myself", 2552)
+  val aa = Address("akka", "sys", "aa", 2552)
+  val bb = Address("akka", "sys", "bb", 2552)
+  val cc = Address("akka", "sys", "cc", 2552)
+  val dd = Address("akka", "sys", "dd", 2552)
+  val ee = Address("akka", "sys", "ee", 2552)
+
+  val emptyState = ClusterHeartbeatSenderState.empty(ConsistentHash(Seq.empty[Address], 10),
+    selfAddress.toString, 3)
+
+  "A ClusterHeartbeatSenderState" must {
+
+    "return empty active set when no nodes" in {
+      emptyState.active.isEmpty must be(true)
+    }
+
+    "include joinInProgress in active set" in {
+      val s = emptyState.addJoinInProgress(aa, Deadline.now + 30.seconds)
+      s.joinInProgress.keySet must be(Set(aa))
+      s.active must be(Set(aa))
+    }
+
+    "remove joinInProgress from active set after removeOverdueJoinInProgress" in {
+      val s = emptyState.addJoinInProgress(aa, Deadline.now - 30.seconds).removeOverdueJoinInProgress()
+      s.joinInProgress must be(Map.empty)
+      s.active must be(Set.empty)
+    }
+
+    "remove joinInProgress after reset" in {
+      val s = emptyState.addJoinInProgress(aa, Deadline.now - 30.seconds).reset(Set(aa, bb))
+      s.joinInProgress must be(Map.empty)
+    }
+
+    "include nodes from reset in active set" in {
+      val nodes = Set(aa, bb, cc)
+      val s = emptyState.reset(nodes)
+      s.all must be(nodes)
+      s.current must be(nodes)
+      s.ending must be(Map.empty)
+      s.active must be(nodes)
+    }
+
+    "limit current nodes to monitoredByNrOfMembers when adding members" in {
+      val nodes = Set(aa, bb, cc, dd)
+      val s = nodes.foldLeft(emptyState) { _ addMember _ }
+      s.all must be(nodes)
+      s.current.size must be(3)
+      s.addMember(ee).current.size must be(3)
+    }
+
+    "move meber to ending set when removing member" in {
+      val nodes = Set(aa, bb, cc, dd, ee)
+      val s = emptyState.reset(nodes)
+      s.ending must be(Map.empty)
+      val included = s.current.head
+      val s2 = s.removeMember(included)
+      s2.ending must be(Map(included -> 0))
+      s2.current must not contain (included)
+      val s3 = s2.addMember(included)
+      s3.current must contain(included)
+      s3.ending.keySet must not contain (included)
+    }
+
+    "increase ending count correctly" in {
+      val s = emptyState.reset(Set(aa)).removeMember(aa)
+      s.ending must be(Map(aa -> 0))
+      val s2 = s.increaseEndingCount(aa).increaseEndingCount(aa)
+      s2.ending must be(Map(aa -> 2))
+    }
+
+  }
+}