Zookeeper（97）如何在Zookeeper中实现分布式协调？

分布式协调是分布式系统中一个关键的需求，用于协调多个节点之间的工作，确保数据一致性和任务同步。ZooKeeper 提供了一套原语（如分布式锁、队列、屏障等），使得实现分布式协调变得更加简单和高效。

实现原理

1. 分布式锁：利用 ZooKeeper 的临时节点或临时顺序节点，可以实现分布式锁，确保只有一个节点能够获取锁。
2. 分布式队列：利用 ZooKeeper 的节点存储和顺序节点，可以实现分布式队列，确保任务按顺序执行。
3. 分布式屏障：利用 ZooKeeper 的节点存储和监听机制，可以实现分布式屏障，确保所有节点都到达某个点后再继续执行。

代码示例

以下是一些实现分布式协调的代码示例，展示了如何在 ZooKeeper 中实现分布式锁、分布式队列和分布式屏障。

依赖导入

首先，确保你已经导入了 ZooKeeper 的 Java 客户端库：

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

分布式锁实现

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;

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

public class DistributedLock implements Watcher {
    private ZooKeeper zooKeeper;
    private String lockPath;
    private String currentLockNode;
    private CountDownLatch connectedSignal = new CountDownLatch(1);
    private CountDownLatch lockAcquiredSignal = new CountDownLatch(1);

    public DistributedLock(String connectString, String lockPath) throws IOException, InterruptedException {
        this.zooKeeper = new ZooKeeper(connectString, 3000, this);
        this.lockPath = lockPath;
        connectedSignal.await();
        ensureLockPath();
    }

    private void ensureLockPath() {
        try {
            Stat stat = zooKeeper.exists(lockPath, false);
            if (stat == null) {
                zooKeeper.create(lockPath, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
            }
        } catch (KeeperException | InterruptedException e) {
            e.printStackTrace();
        }
    }

    public void acquireLock() {
        try {
            currentLockNode = zooKeeper.create(lockPath + "/lock_", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
            attemptLock();
        } catch (KeeperException | InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void attemptLock() throws KeeperException, InterruptedException {
        List<String> children = zooKeeper.getChildren(lockPath, false);
        Collections.sort(children);
        String smallestNode = children.get(0);

        if (currentLockNode.endsWith(smallestNode)) {
            lockAcquiredSignal.countDown();
            System.out.println("Lock acquired: " + currentLockNode);
        } else {
            String watchNode = null;
            for (String child : children) {
                if (child.compareTo(currentLockNode.substring(currentLockNode.lastIndexOf('/') + 1)) < 0) {
                    watchNode = child;
                } else {
                    break;
                }
            }

            if (watchNode != null) {
                final String watchNodePath = lockPath + "/" + watchNode;
                Stat stat = zooKeeper.exists(watchNodePath, this);
                if (stat == null) {
                    attemptLock();
                }
            }
        }
    }

    public void releaseLock() {
        try {
            zooKeeper.delete(currentLockNode, -1);
            System.out.println("Lock released: " + currentLockNode);
        } catch (KeeperException | InterruptedException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void process(WatchedEvent event) {
        if (event.getState() == Event.KeeperState.SyncConnected) {
            connectedSignal.countDown();
        } else if (event.getType() == Event.EventType.NodeDeleted) {
            try {
                attemptLock();
            } catch (KeeperException | InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws Exception {
        DistributedLock lock = new DistributedLock("localhost:2181", "/locks");
        lock.acquireLock();
        lock.lockAcquiredSignal.await();

        // Perform critical section operations here

        lock.releaseLock();
    }
}

详细说明

1. 初始化 ZooKeeper 客户端：

   public DistributedLock(String connectString, String lockPath) throws IOException, InterruptedException {
       this.zooKeeper = new ZooKeeper(connectString, 3000, this);
       this.lockPath = lockPath;
       connectedSignal.await();
       ensureLockPath();
   }

   在初始化时，连接到 ZooKeeper 服务器，并确保锁路径存在。如果节点不存在，则创建一个持久节点表示锁路径。

2. 确保锁路径存在：

   private void ensureLockPath() {
       try {
           Stat stat = zooKeeper.exists(lockPath, false);
           if (stat == null) {
               zooKeeper.create(lockPath, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
           }
       } catch (KeeperException | InterruptedException e) {
           e.printStackTrace();
       }
   }

   检查锁路径是否存在，如果不存在，则创建一个持久节点。

3. 获取锁：

   public void acquireLock() {
       try {
           currentLockNode = zooKeeper.create(lockPath + "/lock_", new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
           attemptLock();
       } catch (KeeperException | InterruptedException e) {
           e.printStackTrace();
       }
   }

   每个获取锁的请求在锁路径下创建一个临时顺序节点，并尝试获取锁。

4. 尝试获取锁：

   private void attemptLock() throws KeeperException, InterruptedException {
       List<String> children = zooKeeper.getChildren(lockPath, false);
       Collections.sort(children);
       String smallestNode = children.get(0);
   
       if (currentLockNode.endsWith(smallestNode)) {
           lockAcquiredSignal.countDown();
           System.out.println("Lock acquired: " + currentLockNode);
       } else {
           String watchNode = null;
           for (String child : children) {
               if (child.compareTo(currentLockNode.substring(currentLockNode.lastIndexOf('/') + 1)) < 0) {
                   watchNode = child;
               } else {
                   break;
               }
           }
   
           if (watchNode != null) {
               final String watchNodePath = lockPath + "/" + watchNode;
               Stat stat = zooKeeper.exists(watchNodePath, this);
               if (stat == null) {
                   attemptLock();
               }
           }
       }
   }

   获取锁路径下的所有子节点，并根据节点名称排序。如果当前节点是最小的节点，则表示获取锁成功；否则，监听前一个节点的删除事件。

5. 释放锁：

   public void releaseLock() {
       try {
           zooKeeper.delete(currentLockNode, -1);
           System.out.println("Lock released: " + currentLockNode);
       } catch (KeeperException | InterruptedException e) {
           e.printStackTrace();
       }
   }

   释放锁时，删除当前节点。

6. 事件处理：

   @Override
   public void process(WatchedEvent event) {
       if (event.getState() == Event.KeeperState.SyncConnected) {
           connectedSignal.countDown();
       } else if (event.getType() == Event.EventType.NodeDeleted) {
           try {
               attemptLock();
           } catch (KeeperException | InterruptedException e) {
               e.printStackTrace();
           }
       }
   }

   当 ZooKeeper 客户端连接建立时，释放连接信号。当前一个节点被删除时，重新尝试获取锁。

7. 主函数：

   public static void main(String[] args) throws Exception {
       DistributedLock lock = new DistributedLock("localhost:2181", "/locks");
       lock.acquireLock();
       lock.lockAcquiredSignal.await();
   
       // Perform critical section operations here
   
       lock.releaseLock();
   }

   主函数创建一个分布式锁，并尝试获取锁，执行临界区操作后释放锁。

性能优化建议

1. 异步操作：

   • 使用 ZooKeeper 的异步 API，减少同步阻塞，提高并发性能。

2. 批处理操作：

   • 可以通过一次性读取多个节点的状态来减少网络请求的次数，提高性能。

3. 本地缓存：

   • 在客户端实现本地缓存，减少频繁的读请求，提升系统性能。

通过合理的设计和实现，ZooKeeper 可以有效地解决分布式协调的需求，确保系统的高可用性和一致性。