Initial commit: Leader Election and Watchers projects

.gitignore

+target/
+!.mvn/wrapper/maven-wrapper.jar
+!**/src/main/**/target/
+!**/src/test/**/target/
+
+### IntelliJ IDEA ###
+.idea/modules.xml
+.idea/jarRepositories.xml
+.idea/compiler.xml
+.idea/libraries/
+*.iws
+*.iml
+*.ipr
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+build/
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
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.idea/.gitignore

+# Default ignored files
+/shelf/
+/workspace.xml

.idea/encodings.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="Encoding">
+    <file url="file://$PROJECT_DIR$/src/main/java" charset="UTF-8" />
+    <file url="file://$PROJECT_DIR$/src/main/resources" charset="UTF-8" />
+  </component>
+</project>
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.idea/misc.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ExternalStorageConfigurationManager" enabled="true" />
+  <component name="MavenProjectsManager">
+    <option name="originalFiles">
+      <list>
+        <option value="$PROJECT_DIR$/pom.xml" />
+      </list>
+    </option>
+  </component>
+  <component name="ProjectRootManager" version="2" languageLevel="JDK_17" default="true" project-jdk-name="17" project-jdk-type="JavaSDK">
+    <output url="file://$PROJECT_DIR$/out" />
+  </component>
+</project>
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.idea/vcs.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>
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pom.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>org.example</groupId>
+    <artifactId>leader-election</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <maven.compiler.source>17</maven.compiler.source>
+        <maven.compiler.target>17</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>3.11.0</version>
+                <configuration>
+                    <source>17</source>
+                    <target>17</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.zookeeper</groupId>
+            <artifactId>zookeeper</artifactId>
+            <version>3.9.1</version>
+        </dependency>
+    </dependencies>
+</project>
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src/main/java/org/example/ImprovedLeaderElection.java

+package org.example;
+
+import org.apache.zookeeper.*;
+import org.apache.zookeeper.data.Stat;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.List;
+
+public class ImprovedLeaderElection implements Watcher {
+    // Make sure to update this address to match your ZooKeeper server address
+    private static final String address = "192.168.184.103:2181";
+    private static final int SESSION_TIMEOUT = 3000;
+    private static final String ELECTION_NAMESPACE = "/election";
+    private static final String ZNODE_PREFIX = ELECTION_NAMESPACE + "/c_";
+
+    private String currentZnodeName;
+    private ZooKeeper zooKeeper;
+
+    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
+        ImprovedLeaderElection leaderElection = new ImprovedLeaderElection();
+        leaderElection.connectToZookeeper();
+
+        // 1. Volunteer to create the candidacy node
+        leaderElection.volunteerForLeadership();
+
+        // 2. Apply the Sequential Watching algorithm
+        leaderElection.electLeader();
+
+        leaderElection.run();
+        leaderElection.close();
+        System.out.println("Closed successfully");
+    }
+
+    public void volunteerForLeadership() throws InterruptedException, KeeperException {
+        // Create an Ephemeral Sequential znode
+        String znodeFullPath = zooKeeper.create(ZNODE_PREFIX, new byte[]{}, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
+
+        System.out.println("Candidacy Znode created: " + znodeFullPath);
+        this.currentZnodeName = znodeFullPath.replace(ELECTION_NAMESPACE + "/", "");
+    }
+
+    /**
+     * The core function of the Sequential Watching algorithm.
+     * Each node places a Watcher only on the node immediately preceding it.
+     */
+    public void electLeader() throws InterruptedException, KeeperException {
+        // 1. Get the list of children without setting a watcher (false)
+        List<String> children = zooKeeper.getChildren(ELECTION_NAMESPACE, false);
+        Collections.sort(children);
+
+        String smallestChild = children.get(0); // Potential Leader
+        int myIndex = children.indexOf(currentZnodeName);
+
+        if (smallestChild.equals(currentZnodeName)) {
+            // I am the Leader: I monitor nothing
+            System.out.println("\n-----------------------------------------");
+            System.out.println("I am the LEADER: " + currentZnodeName);
+            System.out.println("-----------------------------------------\n");
+        } else {
+            // I am a Follower: I must monitor only the node preceding me
+            String smallerSiblingName = children.get(myIndex - 1);
+            String smallerSiblingPath = ELECTION_NAMESPACE + "/" + smallerSiblingName;
+
+            // **We only set a watcher on the previous node via exists**
+            // The watcher is 'this', and will receive a NodeDeleted notification when the previous node falls (Herd Effect broken)
+            Stat stat = zooKeeper.exists(smallerSiblingPath, this);
+
+            if (stat == null) {
+                // The previous node fell before the watcher was set, re-elect
+                System.out.println("The previous node does not exist. Re-election...");
+                electLeader();
+            } else {
+                System.out.println("I am a Follower. I monitor the previous node: " + smallerSiblingName);
+            }
+        }
+    }
+
+    private void close() throws InterruptedException {
+        this.zooKeeper.close();
+    }
+
+    public void connectToZookeeper() throws IOException {
+        this.zooKeeper = new ZooKeeper(address, SESSION_TIMEOUT, this);
+    }
+
+    public void run() throws InterruptedException {
+        synchronized (zooKeeper) {
+            zooKeeper.wait();
+        }
+    }
+
+    /**
+     * Event processing: Re-election is triggered only upon receiving a NodeDeleted notification.
+     */
+    @Override
+    public void process(WatchedEvent watchedEvent) {
+        switch (watchedEvent.getType()) {
+            case None:
+                if (watchedEvent.getState() == Event.KeeperState.SyncConnected) {
+                    System.out.println("Connected to ZooKeeper successfully");
+                } else if (watchedEvent.getState() == Event.KeeperState.Disconnected) {
+                    synchronized (zooKeeper) {
+                        System.out.println("The connection to ZooKeeper has been lost");
+                        zooKeeper.notifyAll();
+                    }
+                } else if (watchedEvent.getState() == Event.KeeperState.Closed) {
+                    System.out.println("Closed successfully");
+                }
+                break;
+
+            // The only event that should trigger the re-election process
+            case NodeDeleted:
+                try {
+                    // Only the node monitoring the deleted node will reach here,
+                    // which breaks the synchronization (Herd Effect)
+                    System.out.println("--- I was notified: The previous node has been deleted. Re-election... ---");
+                    electLeader();
+                } catch (InterruptedException e) {
+                    throw new RuntimeException(e);
+                } catch (KeeperException e) {
+                    throw new RuntimeException(e);
+                }
+                break;
+
+            default:
+                // Ignore any other events that do not serve the election algorithm
+                break;
+        }
+    }
+}
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src/main/java/org/example/WatchersAndTriggers.java

+package org.example;
+
+import org.apache.zookeeper.KeeperException;
+import org.apache.zookeeper.WatchedEvent;
+import org.apache.zookeeper.Watcher;
+import org.apache.zookeeper.ZooKeeper;
+import org.apache.zookeeper.data.Stat;
+
+import java.io.IOException;
+import java.util.List;
+
+public class WatchersAndTriggers implements Watcher {
+
+    private static final String address = "192.168.184.101:2181";
+
+    private static final int SESSION_TIMEOUT = 3000; // Dead client timeout
+    private static final String TargetZnode = "/target_znode";
+
+    private ZooKeeper zooKeeper;
+
+    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
+        WatchersAndTriggers watchersAndTriggers = new WatchersAndTriggers();
+
+        watchersAndTriggers.connectToZookeeper();
+        watchersAndTriggers.initialSetupAndWatchers();
+
+        watchersAndTriggers.run();
+        watchersAndTriggers.close();
+        System.out.println("Closed Successfully");
+    }
+
+    private void close() throws InterruptedException {
+        this.zooKeeper.close();
+    }
+
+    private void run() throws InterruptedException {
+        synchronized (zooKeeper) {
+            this.zooKeeper.wait();
+        }
+    }
+
+    private void connectToZookeeper() throws IOException {
+        this.zooKeeper = new ZooKeeper(address, SESSION_TIMEOUT, this);
+    }
+
+    /**
+     * Function to set initial watchers (Initial Setup).
+     */
+    public void initialSetupAndWatchers() throws InterruptedException, KeeperException {
+        // 1. Exist Watch: using "exists"
+        Stat stat = zooKeeper.exists(TargetZnode, this);
+
+        if (stat == null) {
+            System.out.println(TargetZnode + " not exist. Waiting for creation.");
+            return;
+        }
+
+        // 2. Data Watch: using "getData"
+        byte[] data = zooKeeper.getData(TargetZnode, this, stat);
+
+        // 3. Children Watch: using "getChildren"
+        List<String> children = zooKeeper.getChildren(TargetZnode, this);
+
+        System.out.println("Data: " + new String(data) + ", Children: " + children);
+    }
+
+    /**
+     * The modified event processing function (PROCESS).
+     * Selectively re-arms only the triggered watcher to avoid the race condition.
+     */
+    @Override
+    public void process(WatchedEvent watchedEvent) {
+        try {
+            switch (watchedEvent.getType()) {
+                case None:
+                    if (watchedEvent.getState() == Event.KeeperState.SyncConnected) {
+                        System.out.println("Successfully connected to Zookeeper");
+                    } else if (watchedEvent.getState() == Event.KeeperState.Disconnected) {
+                        synchronized (zooKeeper) {
+                            System.out.println("Disconnected from Zookeeper");
+                            zooKeeper.notifyAll();
+                        }
+                    } else if (watchedEvent.getState() == Event.KeeperState.Closed) {
+                        System.out.println("Closed Successfully");
+                    }
+                    break;
+
+                case NodeCreated:
+                    System.out.println(TargetZnode + " Created");
+                    // On creation: re-arm all watchers (Data, Children, and Exist)
+                    // and fetch current data/children
+                    rearmAllWatchersAfterCreation();
+                    break;
+
+                case NodeDeleted:
+                    System.out.println(TargetZnode + " Deleted");
+                    // On deletion: re-arm only the Exist Watch (in case the znode is recreated)
+                    zooKeeper.exists(TargetZnode, this);
+                    break;
+
+                case NodeDataChanged:
+                    System.out.println(TargetZnode + " DataChanged");
+                    // Re-arm only the Data Watch
+                    zooKeeper.getData(TargetZnode, this, new Stat());
+                    break;
+
+                case NodeChildrenChanged:
+                    System.out.println(TargetZnode + " ChildrenChanged");
+                    // Re-arm only the Children Watch
+                    zooKeeper.getChildren(TargetZnode, this);
+                    break;
+            }
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        } catch (KeeperException e) {
+            // Handle exceptions
+            System.err.println("KeeperException during re-arming watchers: " + e.getMessage());
+        }
+    }
+
+    /**
+     * Helper function to get data and re-arm all watchers after node creation,
+     * ensuring the latest state is captured.
+     */
+    private void rearmAllWatchersAfterCreation() throws InterruptedException, KeeperException {
+        Stat stat = zooKeeper.exists(TargetZnode, this); // Re-arm Exist Watch
+        if (stat != null) {
+            byte[] data = zooKeeper.getData(TargetZnode, this, stat); // Re-arm Data Watch
+            List<String> children = zooKeeper.getChildren(TargetZnode, this); // Re-arm Children Watch
+            System.out.println("New Data: " + new String(data) + ", New Children: " + children);
+        }
+    }
+}
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