Cara Membuat Leader Re-Election pada Zookeeper

Source Code :

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;

import java.io.IOException;
import java.util.Collections;
import java.util.List;

public class LeaderElection implements Watcher {
  private static final String ZOOKEEPER_ADDRESS = "localhost:2181";
  private static final int SESSION_TIMEOUT = 3000;
  private static final String ELECTION_NAMESPACE = "/election";
  private ZooKeeper zooKeeper;
  private String currentZnodeName;

  public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
    LeaderElection leaderElection = new LeaderElection();

    leaderElection.connectToZookeeper();
    leaderElection.volunteerForLeadership();
    leaderElection.reelectLeader();
    leaderElection.run();
    leaderElection.close();
    System.out.println("Disconnected from Zookeeper, exiting application");
  }

  public void volunteerForLeadership() throws KeeperException, InterruptedException {
    String znodePrefix = ELECTION_NAMESPACE + "/c_";
    String znodeFullPath = zooKeeper.create(znodePrefix, new byte[] {}, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);

    System.out.println("znode name " + znodeFullPath);
    this.currentZnodeName = znodeFullPath.replace(ELECTION_NAMESPACE + "/", "");
  }

  public void reelectLeader() throws KeeperException, InterruptedException {
    Stat predecessorStat = null;
    String predecessorZnodeName = "";
    while (predecessorStat == null) {
      List < String > children = zooKeeper.getChildren(ELECTION_NAMESPACE, false);

      Collections.sort(children);
      String smallestChild = children.get(0);

      if (smallestChild.equals(currentZnodeName)) {
        System.out.println("I am the leader");
        return;
      } else {
        System.out.println("I am not the leader");
        int predecessorIndex = Collections.binarySearch(children, currentZnodeName) - 1;
        predecessorZnodeName = children.get(predecessorIndex);
        predecessorStat = zooKeeper.exists(ELECTION_NAMESPACE + "/" + predecessorZnodeName, this);
      }
    }

    System.out.println("Watching znode " + predecessorZnodeName);
    System.out.println();
  }

  public void connectToZookeeper() throws IOException {
    this.zooKeeper = new ZooKeeper(ZOOKEEPER_ADDRESS, SESSION_TIMEOUT, this);
  }

  public void run() throws InterruptedException {
    synchronized(zooKeeper) {
      zooKeeper.wait();
    }
  }

  public void close() throws InterruptedException {
    zooKeeper.close();
  }

  @Override
  public void process(WatchedEvent event) {
    switch (event.getType()) {
    case None:
      if (event.getState() == Event.KeeperState.SyncConnected) {
        System.out.println("Successfully connected to Zookeeper");
      } else {
        synchronized(zooKeeper) {
          System.out.println("Disconnected from Zookeeper event");
          zooKeeper.notifyAll();
        }
      }
      break;
    case NodeDeleted:
      try {
        reelectLeader();
      } catch (InterruptedException e) {} catch (KeeperException e) {}
    }
  }
}