Distributed lock

.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
\ No newline at end of file

.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>
\ No newline at end of file

.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>
\ No newline at end of file

.idea/vcs.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>
\ No newline at end of file

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.ds</groupId>
+    <artifactId>DLock</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>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.zookeeper</groupId>
+            <artifactId>zookeeper</artifactId>
+            <version>3.9.1</version>
+        </dependency>
+
+    </dependencies>
+</project>
\ No newline at end of file

src/main/java/org/ds/Locks.java

+package org.ds;
+
+
+import org.apache.zookeeper.*;
+import org.apache.zookeeper.data.Stat;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.List;
+
+public class Locks implements Watcher {
+
+    private static final String ADDRESS = "172.29.3.101:2181";
+    private static final int SESSION_TIMEOUT = 3000;
+    private static final String LOCKS = "/locks";
+    private String myCandidate;
+    private ZooKeeper zooKeeper;
+
+    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
+
+        Locks locks = new Locks();
+        locks.connectToZookeeper();
+
+        locks.CheckLock();
+        locks.getLockOrWait();
+
+        locks.run();
+        locks.close();
+    }
+
+    private void CheckLock() throws KeeperException, InterruptedException {
+        if (zooKeeper.exists(LOCKS, false) == null) {
+            zooKeeper.create(LOCKS, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+        }
+        String path = zooKeeper.create(LOCKS + "/" + "lock-", new byte[]{}, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
+        myCandidate = path.replace(LOCKS + "/", "");
+    }
+
+    private void getLockOrWait() throws KeeperException, InterruptedException {
+        CheckLock();
+
+        Stat stat = null;
+        String predecessor = "";
+        while (stat == null) {
+            List<String> children = zooKeeper.getChildren(LOCKS, false);
+//        System.out.println(children);
+            Collections.sort(children);
+            if (children.get(0).equals(myCandidate)) {
+                System.out.println("I acquired a lock :). will leave it in 10 seconds");
+                for (int i = 0; i < 10; i++) {
+                    System.out.println("leaving in " + i + "seconds");
+                    Thread.sleep(1000);
+                }
+                zooKeeper.delete(LOCKS + "/" + children.get(0), -1);
+            } else {
+                System.out.println("i could not acquire a lock. So will wait");
+                int predecessorIndex = Collections.binarySearch(children, myCandidate) - 1;
+                predecessor = children.get(predecessorIndex);
+                    stat = zooKeeper.exists(LOCKS + "/" + predecessor, this);
+            }
+        }
+    }
+
+    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) {
+        if (watchedEvent.getType() == Event.EventType.NodeChildrenChanged) {
+            try {
+                getLockOrWait();
+            } catch (KeeperException e) {
+                throw new RuntimeException(e);
+            } catch (InterruptedException e) {
+                throw new RuntimeException(e);
+            }
+        }
+    }
+}
+