Zookeeper（64） Zookeeper如何实现分布式协调？

Victor356
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Zookeeper 提供了一种可靠的机制来实现分布式协调。在分布式系统中，协调多个节点的状态和操作是非常关键的。Zookeeper 通过其一致性协议和分布式锁等机制，可以实现分布式系统中节点之间的协调。以下是详细的步骤和代码示例，展示如何在 Zookeeper 中实现分布式协调。

1. 分布式协调的基本原理

分布式协调可以通过 Zookeeper 的分布式锁、选主机制等特性来实现。分布式锁可以确保在同一时刻只有一个节点执行某个关键操作，而选主机制可以确保在一个集群中只有一个节点作为主节点来执行特定任务。

2. 添加依赖

首先，需要在项目中添加 Zookeeper 的依赖。以下是 Maven 和 Gradle 的依赖配置：

Maven

在 pom.xml 中添加 Zookeeper 依赖：

<dependency>
    <groupId>org.apache.zookeeper</groupId>
    <artifactId>zookeeper</artifactId>
    <version>3.7.0</version>
</dependency>

Gradle

在 build.gradle 中添加 Zookeeper 依赖：

dependencies {
    implementation 'org.apache.zookeeper:zookeeper:3.7.0'
}

3. 实现分布式锁

以下是一个完整的实现分布式锁的示例代码。

ZookeeperClient.java

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;

public class ZookeeperClient {
    private static final String ZK_ADDRESS = "localhost:2181";
    private static final int SESSION_TIMEOUT = 3000;
    private ZooKeeper zooKeeper;

    public void connect() throws Exception {
        zooKeeper = new ZooKeeper(ZK_ADDRESS, SESSION_TIMEOUT, new Watcher() {
            @Override
            public void process(WatchedEvent event) {
                System.out.println("Received event: " + event);
            }
        });
    }

    public void close() throws InterruptedException {
        if (zooKeeper != null) {
            zooKeeper.close();
        }
    }

    public ZooKeeper getZooKeeper() {
        return zooKeeper;
    }

    public static void main(String[] args) {
        ZookeeperClient client = new ZookeeperClient();
        try {
            client.connect();
            // 在这里可以测试分布式锁
            client.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

DistributedLock.java

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;

import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;

public class DistributedLock {
    private ZooKeeper zooKeeper;
    private String lockRoot = "/locks";
    private String lockName;
    private String lockPath;
    private String currentLock;
    private String waitLock;
    private CountDownLatch latch;

    public DistributedLock(ZooKeeper zooKeeper, String lockName) throws KeeperException, InterruptedException {
        this.zooKeeper = zooKeeper;
        this.lockName = lockName;
        if (zooKeeper.exists(lockRoot, false) == null) {
            zooKeeper.create(lockRoot, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        }
    }

    public void lock() throws KeeperException, InterruptedException {
        lockPath = zooKeeper.create(lockRoot + "/" + lockName + "_", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        attemptLock();
    }

    private void attemptLock() throws KeeperException, InterruptedException {
        List<String> children = zooKeeper.getChildren(lockRoot, false);
        Collections.sort(children);
        String sequenceNodeName = lockPath.substring(lockRoot.length() + 1);
        int index = children.indexOf(sequenceNodeName);

        if (index == 0) {
            currentLock = lockPath;
            return;
        } else {
            waitLock = lockRoot + "/" + children.get(index - 1);
            Stat stat = zooKeeper.exists(waitLock, new Watcher() {
                @Override
                public void process(WatchedEvent event) {
                    if (event.getType() == Event.EventType.NodeDeleted) {
                        latch.countDown();
                    }
                }
            });

            if (stat != null) {
                latch = new CountDownLatch(1);
                latch.await();
                attemptLock();
            }
        }
    }

    public void unlock() throws KeeperException, InterruptedException {
        if (currentLock != null) {
            zooKeeper.delete(currentLock, -1);
            currentLock = null;
        }
    }

    public static void main(String[] args) {
        ZookeeperClient client = new ZookeeperClient();
        try {
            client.connect();
            ZooKeeper zooKeeper = client.getZooKeeper();
            DistributedLock lock = new DistributedLock(zooKeeper, "testLock");

            // 获取锁
            lock.lock();
            System.out.println("Lock acquired!");

            // 模拟业务逻辑
            Thread.sleep(3000);

            // 释放锁
            lock.unlock();
            System.out.println("Lock released!");

            client.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

4. 代码说明

ZookeeperClient 类

ZookeeperClient 类负责连接和关闭 Zookeeper 客户端，并提供获取 ZooKeeper 实例的方法。

DistributedLock 类

DistributedLock 类负责实现分布式锁的逻辑。

  • lock() 方法：在 Zookeeper 中创建一个临时顺序节点，并尝试获取锁。
  • attemptLock() 方法：尝试获取锁，如果当前节点是最小的节点，则获取锁成功；否则，设置监视器等待前一个节点被删除。
  • unlock() 方法：释放锁，删除当前节点。

5. 测试分布式锁

在 DistributedLock 类的 main 方法中，创建 ZookeeperClient 实例并连接 Zookeeper，然后创建 DistributedLock 实例并尝试获取和释放锁。可以通过运行多个实例来测试分布式锁的功能。

public static void main(String[] args) {
    ZookeeperClient client = new ZookeeperClient();
    try {
        client.connect();
        ZooKeeper zooKeeper = client.getZooKeeper();
        DistributedLock lock = new DistributedLock(zooKeeper, "testLock");

        // 获取锁
        lock.lock();
        System.out.println("Lock acquired!");

        // 模拟业务逻辑
        Thread.sleep(3000);

        // 释放锁
        lock.unlock();
        System.out.println("Lock released!");

        client.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

6. 实现选主机制

除了分布式锁，选主机制也是分布式协调的重要部分。以下是一个实现选主机制的示例代码。

LeaderElection.java

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;

import java.util.Collections;
import java.util.List;

public class LeaderElection implements Watcher {
    private ZooKeeper zooKeeper;
    private String electionPath = "/election";
    private String currentZNodeName;

    public LeaderElection(ZooKeeper zooKeeper) {
        this.zooKeeper = zooKeeper;
    }

    public void volunteerForLeadership() throws KeeperException, InterruptedException {
        String znodePrefix = electionPath + "/c_";
        String znodeFullPath = zooKeeper.create(znodePrefix, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        this.currentZNodeName = znodeFullPath.replace(electionPath + "/", "");
    }

    public void reelectLeader() throws KeeperException, InterruptedException {
        List<String> children = zooKeeper.getChildren(electionPath, false);
        Collections.sort(children);
        String smallestChild = children.get(0);

        if (smallestChild.equals(currentZNodeName)) {
            System.out.println("I am the leader");
        } else {
            System.out.println("I am not the leader. " + smallestChild + " is the leader.");
        }
    }

    @Override
    public void process(WatchedEvent event) {
        switch (event.getType()) {
            case NodeChildrenChanged:
                try {
                    reelectLeader();
                } catch (KeeperException | InterruptedException e) {
                    e.printStackTrace();
                }
                break;
        }
    }

    public static void main(String[] args) {
        ZookeeperClient client = new ZookeeperClient();
        try {
            client.connect();
            ZooKeeper zooKeeper = client.getZooKeeper();
            LeaderElection leaderElection = new LeaderElection(zooKeeper);

            // 创建选举节点
            if (zooKeeper.exists("/election", false) == null) {
                zooKeeper.create("/election", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
            }

            // 参与选举
            leaderElection.volunteerForLeadership();
            leaderElection.reelectLeader();

            // 保持连接
            Thread.sleep(Long.MAX_VALUE);

            client.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

7. 代码说明

LeaderElection 类

LeaderElection 类负责实现选主机制的逻辑。

  • volunteerForLeadership() 方法：在 Zookeeper 中创建一个临时顺序节点，表示参与选举。
  • reelectLeader() 方法：读取选举节点列表，判断当前节点是否是最小节点，如果是，则成为主节点；否则，打印当前主节点的信息。

8. 测试选主机制

在 LeaderElection 类的 main 方法中，创建 ZookeeperClient 实例并连接 Zookeeper，然后创建 LeaderElection 实例并尝试参与选举和重新选举。可以通过运行多个实例来测试选主机制的功能。

public static void main(String[] args) {
    ZookeeperClient client = new ZookeeperClient();
    try {
        client.connect();
        ZooKeeper zooKeeper = client.getZooKeeper();
        LeaderElection leaderElection = new LeaderElection(zooKeeper);

        // 创建选举节点
        if (zooKeeper.exists("/election", false) == null) {
            zooKeeper.create("/election", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        }

        // 参与选举
        leaderElection.volunteerForLeadership();
        leaderElection.reelectLeader();

        // 保持连接
        Thread.sleep(Long.MAX_VALUE);

        client.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

总结

  1. 添加依赖：在项目中添加 Zookeeper 的依赖。
  2. 实现 ZookeeperClient 类：负责连接和关闭 Zookeeper 客户端，并提供获取 ZooKeeper 实例的方法。
  3. 实现分布式锁：通过 DistributedLock 类实现分布式锁的逻辑。
  4. 实现选主机制：通过 LeaderElection 类实现选主机制的逻辑。
  5. 测试分布式协调：通过运行多个实例来测试分布式锁和选主机制的功能。

通过以上方法，可以在 Zookeeper 中实现分布式协调，确保其高效稳定地运行。根据实际情况和需求，选择适合你的实现方法并进行实施。

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