Improved Leader Election Implementation - Avoiding Herd Effect with Smart Watching

src/main/java/LeaderElection.java

@@ -12,13 +12,14 @@ public class LeaderElection implements Watcher {
 
     private String currentZnodeName;
     private ZooKeeper zooKeeper;
+    private boolean isLeader = false;
 
 
     public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
         LeaderElection leaderElection = new LeaderElection();
         leaderElection.connectToZookeeper();
         leaderElection.volunteerForLeadership();
-        leaderElection.electLeader();
+        leaderElection.electLeaderWithSmartWatching();
         leaderElection.run();
         leaderElection.close();
         System.out.println("Successfully Closed");
@@ -32,18 +33,28 @@ public class LeaderElection implements Watcher {
         this.currentZnodeName = znodeFullPath.replace(ELECTION_NAMESPACE + "/", "");
     }
 
-    public void electLeader() throws InterruptedException, KeeperException {
-        List<String> children = zooKeeper.getChildren(ELECTION_NAMESPACE, this);
-
+    public void electLeaderWithSmartWatching() throws InterruptedException, KeeperException {
+        List<String> children = zooKeeper.getChildren(ELECTION_NAMESPACE, false);
         Collections.sort(children);
-        String smallestChild = children.get(0); //the first element
+
+        String smallestChild = children.get(0);
 
         if (smallestChild.equals(currentZnodeName)) {
-            System.out.println("I'm a leader");
+            isLeader = true;
+            System.out.println("I'm the LEADER: " + currentZnodeName);
         } else {
-            System.out.println("I'm not a leader" + smallestChild + " is a leader.");
+            isLeader = false;
+            int currentIndex = children.indexOf(currentZnodeName);
+            String previousNode = children.get(currentIndex - 1);
+
+            System.out.println("I'm a FOLLOWER. Watching previous node: " + previousNode);
+            watchPreviousNode(previousNode);
         }
+    }
 
+    private void watchPreviousNode(String previousNode) throws KeeperException, InterruptedException {
+        String previousNodePath = ELECTION_NAMESPACE + "/" + previousNode;
+        zooKeeper.exists(previousNodePath, this);
     }
 
     private void close() throws InterruptedException {
@@ -74,15 +85,14 @@ public class LeaderElection implements Watcher {
                 } else if (watchedEvent.getState() == Event.KeeperState.Closed) {
                     System.out.println("Closed Successfully");
                 }
-            case NodeChildrenChanged:
+            case NodeDeleted:
+                System.out.println("Previous node deleted - re-electing leader");
                 try {
-                    electLeader();
-                    System.out.println(ELECTION_NAMESPACE + " NodeChildrenChanged");
-                } catch (InterruptedException e) {
-                    throw new RuntimeException(e);
-                } catch (KeeperException e) {
-                    throw new RuntimeException(e);
+                    electLeaderWithSmartWatching();
+                } catch (Exception e) {
+                    e.printStackTrace();
                 }
+                break;
 
         }
     }