Added log messages when leadership is gained or lost (#25053)

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

@@ -313,6 +313,8 @@ private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatCh
     cluster.settings.SelfDataCenter,
     cluster.settings.MultiDataCenter.CrossDcConnections)
 
+  var isCurrentlyLeader = false
+
   def latestGossip: Gossip = membershipState.latestGossip
 
   val statsEnabled = PublishStatsInterval.isFinite
@@ -1020,6 +1022,10 @@ private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatCh
   def leaderActions(): Unit = {
     if (membershipState.isLeader(selfUniqueAddress)) {
       // only run the leader actions if we are the LEADER of the data center
+      if (!isCurrentlyLeader) {
+        logInfo("Cluster Node [{}] dc [{}] is the new leader", selfAddress, cluster.settings.SelfDataCenter)
+        isCurrentlyLeader = true
+      }
       val firstNotice = 20
       val periodicNotice = 60
       if (membershipState.convergence(exitingConfirmed)) {
@@ -1041,6 +1047,9 @@ private[cluster] class ClusterCoreDaemon(publisher: ActorRef, joinConfigCompatCh
                 s"${m.address} ${m.status} seen=${latestGossip.seenByNode(m.uniqueAddress)}"
             }.mkString(", "))
       }
+    } else if (isCurrentlyLeader) {
+      logInfo("Cluster Node [{}] dc [{}] is no longer the leader", selfAddress, cluster.settings.SelfDataCenter)
+      isCurrentlyLeader = false
     }
     cleanupExitingConfirmed()
     shutdownSelfWhenDown()

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

@@ -0,0 +1,57 @@
+/**
+ *  Copyright (C) 2009-2018 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package akka.cluster
+
+import akka.actor.{ Address, ExtendedActorSystem }
+import akka.cluster.InternalClusterAction.LeaderActionsTick
+import akka.testkit.{ AkkaSpec, EventFilter, ImplicitSender }
+
+object ClusterLogSpec {
+  val config = """
+    akka.cluster {
+      auto-down-unreachable-after = 0s
+      publish-stats-interval = 0 s # always, when it happens
+      failure-detector.implementation-class = akka.cluster.FailureDetectorPuppet
+    }
+    akka.actor.provider = "cluster"
+    akka.remote.log-remote-lifecycle-events = off
+    akka.remote.netty.tcp.port = 0
+    akka.remote.artery.canonical.port = 0
+    akka.loglevel = "INFO"
+    akka.loggers = ["akka.testkit.TestEventListener"]
+    """
+
+}
+
+class ClusterLogSpec extends AkkaSpec(ClusterLogSpec.config) with ImplicitSender {
+
+  val selfAddress: Address = system.asInstanceOf[ExtendedActorSystem].provider.getDefaultAddress
+
+  val cluster = Cluster(system)
+  def clusterView: ClusterReadView = cluster.readView
+
+  "A Cluster" must {
+
+    "Log a message when becoming and stopping being a leader" in {
+      EventFilter
+        .info(occurrences = 1, pattern = "is the new leader")
+        .intercept {
+          cluster.join(selfAddress)
+        }
+
+      awaitCond(clusterView.isSingletonCluster)
+      clusterView.self.address should ===(selfAddress)
+      clusterView.members.map(_.address) should ===(Set(selfAddress))
+      awaitAssert(clusterView.status should ===(MemberStatus.Up))
+
+      EventFilter
+        .info(occurrences = 1, pattern = "is no longer the leader")
+        .intercept {
+          cluster.down(selfAddress)
+        }
+    }
+
+  }
+}