Cluster Management, Registration and Discovery

.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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README.md

+# Cluster Management, Registration and Discovery 
+
+*topics*
+* [Logback vs SLF4J vs Log4J2 - what is the difference?](https://www.youtube.com/watch?v=SWHYrCXIL38&ab_channel=JavaBrains)
+* Introduction to Service Registry & Service Discovery
+* Implement Service Registry with Zookeeper
+* Integrate into our leader-worker architecture 
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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>Service-Registration-and-Discovery</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/Application.java

+import org.apache.zookeeper.KeeperException;
+import org.apache.zookeeper.WatchedEvent;
+import org.apache.zookeeper.Watcher;
+import org.apache.zookeeper.ZooKeeper;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+
+public class Application implements Watcher {
+
+    private static final String address = "172.29.3.101:2181";
+    private static final int SESSION_TIMEOUT = 3000; //dead client
+    private static final int DEFAULT_PORT = 8080;
+    private ZooKeeper zooKeeper;
+
+    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
+
+        int currentServerPort = args.length == 1 ? Integer.parseInt(args[0]) : DEFAULT_PORT;
+        Application application = new Application();
+        ZooKeeper zooKeeper = application.connectToZookeeper();
+
+        ServiceRegistry serviceRegistry = new ServiceRegistry(zooKeeper);
+
+        OnElectionAction onElectionAction = new OnElectionAction(serviceRegistry, currentServerPort);
+
+        LeaderElection leaderElection = new LeaderElection(zooKeeper, onElectionAction);
+        leaderElection.volunteerForLeadership();
+        leaderElection.reelectLeader();
+
+        application.run();
+        application.close();
+
+    }
+
+    public ZooKeeper connectToZookeeper() throws IOException {
+        this.zooKeeper = new ZooKeeper(address, SESSION_TIMEOUT, this);
+        return zooKeeper;
+    }
+
+    public void run() throws InterruptedException {
+        synchronized (zooKeeper) {
+            zooKeeper.wait();
+        }
+    }
+
+    private void close() throws InterruptedException {
+        this.zooKeeper.close();
+    }
+
+    @Override
+    public void process(WatchedEvent watchedEvent) {
+        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;
+        }
+    }
+}

src/main/java/LeaderElection.java

+import org.apache.zookeeper.*;
+import org.apache.zookeeper.data.Stat;
+
+import java.util.Collections;
+import java.util.List;
+
+public class LeaderElection implements Watcher {
+
+    private static final String ELECTION_NAMESPACE = "/election";
+
+    private String currentZnodeName;
+    private ZooKeeper zooKeeper;
+
+    private OnElectionCallback onElectionCallback;
+
+    public LeaderElection(ZooKeeper zooKeeper, OnElectionCallback onElectionCallback) {
+        this.zooKeeper = zooKeeper;
+        this.onElectionCallback = onElectionCallback;
+
+    }
+
+    public void volunteerForLeadership() throws InterruptedException, KeeperException {
+        String znodePrefix = ELECTION_NAMESPACE + "/c_";
+        String znodeFullPath = zooKeeper.create(znodePrefix, new byte[]{}, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
+
+        System.out.println(znodeFullPath);
+        this.currentZnodeName = znodeFullPath.replace(ELECTION_NAMESPACE + "/", "");
+    }
+
+    public void reelectLeader() throws InterruptedException, KeeperException {
+        String predecessorName = "";
+        Stat predecessorStat = null;
+
+        //this while to guarantee get predecessor even if it deleted just before zookeeper.exist
+        while (predecessorStat == null) {
+            List<String> children = zooKeeper.getChildren(ELECTION_NAMESPACE, false);
+            Collections.sort(children);
+
+            String smallestChild = children.get(0); //the first element
+            if (smallestChild.equals(currentZnodeName)) {
+                System.out.println("I'm a leader");
+                onElectionCallback.onElectedToBeLeader();
+
+                return;
+            } else {
+                System.out.println("I'm not a leader");
+                int predecessorIndex = children.indexOf(currentZnodeName) - 1;
+                predecessorName = children.get(predecessorIndex);
+                predecessorStat = zooKeeper.exists(ELECTION_NAMESPACE + "/" + predecessorName, this);
+            }
+        }
+        onElectionCallback.onWorker();
+        System.out.println("Watching znode " + predecessorName);
+        System.out.println();
+
+    }
+
+    @Override
+    public void process(WatchedEvent watchedEvent) {
+        switch (watchedEvent.getType()) {
+            case NodeDeleted:
+                try {
+                    reelectLeader();
+                } catch (InterruptedException e) {
+                    throw new RuntimeException(e);
+                } catch (KeeperException e) {
+                    throw new RuntimeException(e);
+                }
+                break;
+
+        }
+    }
+}

src/main/java/OnElectionAction.java

+import org.apache.zookeeper.KeeperException;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+
+public class OnElectionAction implements OnElectionCallback {
+
+
+    private final ServiceRegistry serviceRegistry;
+    private final int port;
+
+    public OnElectionAction(ServiceRegistry serviceRegistry, int port) {
+        this.serviceRegistry = serviceRegistry;
+        this.port = port;
+    }
+
+    @Override
+    public void onElectedToBeLeader() {
+        serviceRegistry.unregisterFromCluster();
+        serviceRegistry.registerForUpdates();
+    }
+
+    @Override
+    public void onWorker() {
+        try {
+            String currentServerAddress =
+                    String.format("http://%s:%d", InetAddress.getLocalHost().getCanonicalHostName(), port);
+
+            serviceRegistry.registerToCluster(currentServerAddress);
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        } catch (UnknownHostException e) {
+            e.printStackTrace();
+        } catch (KeeperException e) {
+            e.printStackTrace();
+        }
+
+    }
+}

src/main/java/OnElectionCallback.java

+public interface OnElectionCallback {
+
+    void onElectedToBeLeader();
+
+    void onWorker();
+}

src/main/java/ServiceRegistry.java

+import org.apache.zookeeper.*;
+import org.apache.zookeeper.data.Stat;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+public class ServiceRegistry implements Watcher {
+    private static final String REGISTRY_ZNODE = "/service_registry";
+    private final ZooKeeper zooKeeper;
+
+    private String currentZnode = null;
+    private List<String> allServiceAddresses = null;
+
+    public ServiceRegistry(ZooKeeper zooKeeper) {
+        this.zooKeeper = zooKeeper;
+        createServiceRegistryZnode();
+    }
+
+    private void createServiceRegistryZnode() {
+        try {
+            if (zooKeeper.exists(REGISTRY_ZNODE, false) == null) {
+                zooKeeper.create(REGISTRY_ZNODE, new byte[]{}, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+            }
+        } catch (KeeperException e) {
+            e.printStackTrace();
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+    }
+
+    public void registerToCluster(String metadata) throws KeeperException, InterruptedException {
+        if (this.currentZnode != null) {
+            System.out.println("Already registered to service registry");
+            return;
+        }
+        this.currentZnode = zooKeeper.create(REGISTRY_ZNODE + "/n_", metadata.getBytes(),
+                ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
+        System.out.println("Registered to service registry");
+    }
+
+    public void registerForUpdates() {
+        try {
+            updateAddresses();
+        } catch (KeeperException e) {
+            e.printStackTrace();
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+    }
+
+    public void unregisterFromCluster() {
+        try {
+            if (currentZnode != null && zooKeeper.exists(currentZnode, false) != null) {
+                zooKeeper.delete(currentZnode, -1);
+            }
+        } catch (KeeperException e) {
+            e.printStackTrace();
+        } catch (InterruptedException e) {
+            e.printStackTrace();
+        }
+    }
+
+    private synchronized void updateAddresses() throws KeeperException, InterruptedException {
+        List<String> workerZnodes = zooKeeper.getChildren(REGISTRY_ZNODE, this);
+
+        List<String> addresses = new ArrayList<>(workerZnodes.size());
+
+        for (String workerZnode : workerZnodes) {
+            String workerFullPath = REGISTRY_ZNODE + "/" + workerZnode;
+            Stat stat = zooKeeper.exists(workerFullPath, false);
+            if (stat == null) {
+                continue;
+            }
+
+            byte[] addressBytes = zooKeeper.getData(workerFullPath, false, stat);
+            String address = new String(addressBytes);
+            addresses.add(address);
+        }
+
+        this.allServiceAddresses = Collections.unmodifiableList(addresses);
+        System.out.println("The cluster addresses are: " + this.allServiceAddresses);
+    }
+
+    @Override
+    public void process(WatchedEvent watchedEvent) {
+        try {
+            updateAddresses();
+        } catch (KeeperException e) {
+            throw new RuntimeException(e);
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

src/main/resources/logback.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<Configuration status="WARN">
+    <Appenders>
+        <Console name="Console" target="SYSTEM_OUT">
+            <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
+        </Console>
+    </Appenders>
+    <Loggers>
+        <Root level="WARN">
+            <AppenderRef ref="Console"/>
+        </Root>
+    </Loggers>
+</Configuration>
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