Apache Zookeeper

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

public class ZookeeperExample implements Watcher {
    private ZooKeeper zooKeeper;
    private static final String CONNECTION_STRING = "localhost:2181";
    private static final int SESSION_TIMEOUT = 3000;

    public void connect() throws Exception {
        zooKeeper = new ZooKeeper(CONNECTION_STRING, SESSION_TIMEOUT, this);

        // Wait for connection to establish
        synchronized (this) {
            wait();
        }
    }

    // Create a znode
    public void createZnode(String path, String data) throws Exception {
        zooKeeper.create(
            path,
            data.getBytes(),
            ZooDefs.Ids.OPEN_ACL_UNSAFE,
            CreateMode.PERSISTENT
        );
        System.out.println("Created znode: " + path);
    }

    // Read znode data
    public String getData(String path) throws Exception {
        Stat stat = new Stat();
        byte[] data = zooKeeper.getData(path, true, stat);
        return new String(data);
    }

    // Update znode data
    public void setData(String path, String newData) throws Exception {
        Stat stat = zooKeeper.exists(path, true);
        if (stat != null) {
            zooKeeper.setData(path, newData.getBytes(), stat.getVersion());
            System.out.println("Updated znode: " + path);
        }
    }

    // Leader election pattern
    public void electLeader(String electionPath) throws Exception {
        // Create election directory if it doesn't exist
        if (zooKeeper.exists(electionPath, false) == null) {
            zooKeeper.create(electionPath, new byte[0],
                ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        }

        // Create ephemeral sequential znode
        String myPath = zooKeeper.create(
            electionPath + "/candidate-",
            InetAddress.getLocalHost().getHostAddress().getBytes(),
            ZooDefs.Ids.OPEN_ACL_UNSAFE,
            CreateMode.EPHEMERAL_SEQUENTIAL
        );

        // Check if we're the leader (lowest sequence number)
        List<String> children = zooKeeper.getChildren(electionPath, false);
        Collections.sort(children);

        String smallestChild = children.get(0);
        if (myPath.endsWith(smallestChild)) {
            System.out.println("I am the leader!");
        } else {
            // Watch the node before us
            String watchPath = electionPath + "/" +
                children.get(children.indexOf(myPath.substring(myPath.lastIndexOf('/') + 1)) - 1);
            zooKeeper.exists(watchPath, this);
            System.out.println("Watching: " + watchPath);
        }
    }

    // Distributed lock pattern
    public boolean acquireLock(String lockPath, String clientId) throws Exception {
        String myLockPath = zooKeeper.create(
            lockPath + "/" + clientId + "-lock-",
            clientId.getBytes(),
            ZooDefs.Ids.OPEN_ACL_UNSAFE,
            CreateMode.EPHEMERAL_SEQUENTIAL
        );

        List<String> children = zooKeeper.getChildren(lockPath, false);
        Collections.sort(children);

        String mySequence = myLockPath.substring(myLockPath.lastIndexOf('/') + 1);

        if (children.get(0).equals(mySequence)) {
            System.out.println("Lock acquired by: " + clientId);
            return true;
        } else {
            // Watch the previous node
            int myIndex = children.indexOf(mySequence);
            String watchPath = lockPath + "/" + children.get(myIndex - 1);
            zooKeeper.exists(watchPath, this);
            System.out.println("Waiting for lock...");
            return false;
        }
    }

    @Override
    public void process(WatchedEvent event) {
        if (event.getState() == KeeperState.SyncConnected) {
            synchronized (this) {
                notify();
            }
        } else if (event.getType() == Event.EventType.NodeDeleted) {
            // Previous node deleted, we might be next in line
            System.out.println("Node deleted, checking for leadership/lock...");
        }
    }

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