Zookeeper（13）Zookeeper的顺序节点的作用是什么？Zookeeper 的顺序节点（Sequential

Zookeeper 的顺序节点（Sequential Node）是一种特殊类型的节点，在创建时会自动附加一个递增的序列号。顺序节点可以是持久顺序节点（Persistent Sequential）和临时顺序节点（Ephemeral Sequential）。顺序节点在某些分布式系统的设计中非常有用，主要用于确保操作的有序性和唯一性。

顺序节点的作用

唯一命名：
- 顺序节点在创建时会附加一个全局唯一的递增序列号，确保每个顺序节点的路径都是唯一的。这对于需要唯一命名的场景，如分布式锁、分布式队列等非常有用。
操作有序性：
- 顺序节点的序列号反映了它们的创建顺序，确保了操作的有序性。这对于需要按顺序处理的任务，如消息队列、任务调度等场景非常重要。
分布式锁：
- 在分布式锁的实现中，顺序节点可以用于确保锁的公平性。客户端创建顺序节点，并根据序列号的顺序来获取锁，确保锁是按照请求的顺序被获取的。
分布式队列：
- 顺序节点可以用于实现分布式队列，确保任务按照加入队列的顺序被处理。

代码示例

以下代码示例展示了如何在 Zookeeper 中创建和使用顺序节点，并演示其在分布式锁和分布式队列中的应用。

1. 添加 Maven 依赖

在 pom.xml 中添加 Zookeeper 客户端的依赖：

<dependency>
    <groupId>org.apache.zookeeper</groupId>
    <artifactId>zookeeper</artifactId>
    <version>3.6.3</version>
</dependency>

2. 创建 Zookeeper 客户端

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

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("Event received: " + event);
            }
        });
    }

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

    public ZooKeeper getZooKeeper() {
        return zooKeeper;
    }
}

3. 创建顺序节点

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;

public class ZookeeperSequentialNodeExample {
    private static final String PERSISTENT_SEQUENTIAL_PATH = "/persistent_sequential_node";
    private static final String EPHEMERAL_SEQUENTIAL_PATH = "/ephemeral_sequential_node";

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

        // 创建持久顺序节点
        String createdPersistentSequentialPath = zooKeeper.create(PERSISTENT_SEQUENTIAL_PATH, "persistent_sequential_data".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT_SEQUENTIAL);
        System.out.println("Created persistent sequential node path: " + createdPersistentSequentialPath);

        // 创建临时顺序节点
        String createdEphemeralSequentialPath = zooKeeper.create(EPHEMERAL_SEQUENTIAL_PATH, "ephemeral_sequential_data".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        System.out.println("Created ephemeral sequential node path: " + createdEphemeralSequentialPath);

        client.close();
    }
}

4. 分布式锁的实现

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;

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

public class DistributedLock {
    private static final String LOCK_ROOT_PATH = "/locks";
    private static final String LOCK_NODE_NAME = "lock_";
    private String lockPath;
    private ZooKeeper zooKeeper;

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

    public void acquireLock() throws Exception {
        lockPath = zooKeeper.create(LOCK_ROOT_PATH + "/" + LOCK_NODE_NAME, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        System.out.println("Lock path: " + lockPath);

        while (true) {
            List<String> children = zooKeeper.getChildren(LOCK_ROOT_PATH, false);
            Collections.sort(children);
            if (lockPath.equals(LOCK_ROOT_PATH + "/" + children.get(0))) {
                System.out.println("Acquired lock: " + lockPath);
                break;
            } else {
                int previousNodeIndex = Collections.binarySearch(children, lockPath.substring(LOCK_ROOT_PATH.length() + 1)) - 1;
                String previousNode = children.get(previousNodeIndex);
                final Object lock = new Object();
                Stat stat = zooKeeper.exists(LOCK_ROOT_PATH + "/" + previousNode, event -> {
                    synchronized (lock) {
                        lock.notifyAll();
                    }
                });
                if (stat != null) {
                    synchronized (lock) {
                        lock.wait();
                    }
                }
            }
        }
    }

    public void releaseLock() throws Exception {
        zooKeeper.delete(lockPath, -1);
        System.out.println("Released lock: " + lockPath);
    }

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

        DistributedLock lock = new DistributedLock(zooKeeper);
        lock.acquireLock();

        // Do some work with the lock

        lock.releaseLock();

        client.close();
    }
}

详细解释

顺序节点创建和使用：
- 在 ZookeeperSequentialNodeExample 类中，使用 CreateMode.PERSISTENT_SEQUENTIAL 和 CreateMode.EPHEMERAL_SEQUENTIAL 分别创建了持久顺序节点和临时顺序节点。
- 每次创建顺序节点时，Zookeeper 会自动附加一个递增的序列号，确保节点路径的唯一性。
分布式锁的实现：
- 在 DistributedLock 类中，通过创建临时顺序节点来实现分布式锁。
- 每个客户端请求锁时，都会在 /locks 目录下创建一个临时顺序节点。
- 客户端通过比较自身创建的节点与目录下所有节点的序列号来判断是否获得锁。
- 如果客户端创建的节点是序列号最小的节点，则获得锁。
- 如果不是最小节点，则监听比自己小的前一个节点的删除事件。当前一个节点被删除时，再次尝试获取锁。
- 获得锁后，客户端可以执行其临界区代码，完成后需主动删除自身的锁节点以释放锁。

总结

顺序节点在分布式系统中非常有用，主要用于确保操作的有序性和唯一性。通过上述代码示例，可以了解如何在 Zookeeper 中创建和使用顺序节点，以及其在分布式锁中的应用。了解顺序节点的特性和用途，有助于在实际应用中设计高效的分布式系统。