分佈式鎖與實現（二）基於ZooKeeper實現

時間 2019-11-06

原文原文鏈接

引言

ZooKeeper是一個分佈式的，開放源碼的分佈式應用程序協調服務，是Google的Chubby一個開源的實現，是Hadoop和Hbase的重要組件。它是一個爲分佈式應用提供一致性服務的軟件，提供的功能包括：配置維護、域名服務、分佈式同步、組服務等。java

ZooKeeper的架構經過冗餘服務實現高可用性。所以，若是第一次無應答，客戶端就能夠詢問另外一臺ZooKeeper主機。ZooKeeper節點將它們的數據存儲於一個分層的命名空間，很是相似於一個文件系統或一個前綴樹結構。客戶端能夠在節點讀寫，從而以這種方式擁有一個共享的配置服務。更新是全序的。node

基於ZooKeeper分佈式鎖的流程

在zookeeper指定節點（locks）下建立臨時順序節點node_n
獲取locks下全部子節點children
對子節點按節點自增序號從小到大排序
判斷本節點是否是第一個子節點，如果，則獲取鎖；若不是，則監聽比該節點小的那個節點的刪除事件
若監聽事件生效，則回到第二步從新進行判斷，直到獲取到鎖

具體實現

下面就具體使用java和zookeeper實現分佈式鎖，操做zookeeper使用的是apache提供的zookeeper的包。apache

經過實現Watch接口，實現process(WatchedEvent event)方法來實施監控，使CountDownLatch來完成監控，在等待鎖的時候使用CountDownLatch來計數，等到後進行countDown，中止等待，繼續運行。
如下總體流程基本與上述描述流程一致，只是在監聽的時候使用的是CountDownLatch來監聽前一個節點。

分佈式鎖session

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

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;

/**
 * Created by liuyang on 2017/4/20.
 */
public class DistributedLock implements Lock, Watcher {
    private ZooKeeper zk = null;
    // 根節點
    private String ROOT_LOCK = "/locks";
    // 競爭的資源
    private String lockName;
    // 等待的前一個鎖
    private String WAIT_LOCK;
    // 當前鎖
    private String CURRENT_LOCK;
    // 計數器
    private CountDownLatch countDownLatch;
    private int sessionTimeout = 30000;
    private List<Exception> exceptionList = new ArrayList<Exception>();

    /**
     * 配置分佈式鎖
     * @param config 鏈接的url
     * @param lockName 競爭資源
     */
    public DistributedLock(String config, String lockName) {
        this.lockName = lockName;
        try {
            // 鏈接zookeeper
            zk = new ZooKeeper(config, sessionTimeout, this);
            Stat stat = zk.exists(ROOT_LOCK, false);
            if (stat == null) {
                // 若是根節點不存在，則建立根節點
                zk.create(ROOT_LOCK, new byte[0], ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
    }

    // 節點監視器
    public void process(WatchedEvent event) {
        if (this.countDownLatch != null) {
            this.countDownLatch.countDown();
        }
    }

    public void lock() {
        if (exceptionList.size() > 0) {
            throw new LockException(exceptionList.get(0));
        }
        try {
            if (this.tryLock()) {
                System.out.println(Thread.currentThread().getName() + " " + lockName + "得到了鎖");
                return;
            } else {
                // 等待鎖
                waitForLock(WAIT_LOCK, sessionTimeout);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
    }

    public boolean tryLock() {
        try {
            String splitStr = "_lock_";
            if (lockName.contains(splitStr)) {
                throw new LockException("鎖名有誤");
            }
            // 建立臨時有序節點
            CURRENT_LOCK = zk.create(ROOT_LOCK + "/" + lockName + splitStr, new byte[0],
                    ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
            System.out.println(CURRENT_LOCK + " 已經建立");
            // 取全部子節點
            List<String> subNodes = zk.getChildren(ROOT_LOCK, false);
            // 取出全部lockName的鎖
            List<String> lockObjects = new ArrayList<String>();
            for (String node : subNodes) {
                String _node = node.split(splitStr)[0];
                if (_node.equals(lockName)) {
                    lockObjects.add(node);
                }
            }
            Collections.sort(lockObjects);
            System.out.println(Thread.currentThread().getName() + " 的鎖是 " + CURRENT_LOCK);
            // 若當前節點爲最小節點，則獲取鎖成功
            if (CURRENT_LOCK.equals(ROOT_LOCK + "/" + lockObjects.get(0))) {
                return true;
            }

            // 若不是最小節點，則找到本身的前一個節點
            String prevNode = CURRENT_LOCK.substring(CURRENT_LOCK.lastIndexOf("/") + 1);
            WAIT_LOCK = lockObjects.get(Collections.binarySearch(lockObjects, prevNode) - 1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
        return false;
    }

    public boolean tryLock(long timeout, TimeUnit unit) {
        try {
            if (this.tryLock()) {
                return true;
            }
            return waitForLock(WAIT_LOCK, timeout);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return false;
    }

    // 等待鎖
    private boolean waitForLock(String prev, long waitTime) throws KeeperException, InterruptedException {
        Stat stat = zk.exists(ROOT_LOCK + "/" + prev, true);

        if (stat != null) {
            System.out.println(Thread.currentThread().getName() + "等待鎖 " + ROOT_LOCK + "/" + prev);
            this.countDownLatch = new CountDownLatch(1);
            // 計數等待，若等到前一個節點消失，則precess中進行countDown，中止等待，獲取鎖
            this.countDownLatch.await(waitTime, TimeUnit.MILLISECONDS);
            this.countDownLatch = null;
            System.out.println(Thread.currentThread().getName() + " 等到了鎖");
        }
        return true;
    }

    public void unlock() {
        try {
            System.out.println("釋放鎖 " + CURRENT_LOCK);
            zk.delete(CURRENT_LOCK, -1);
            CURRENT_LOCK = null;
            zk.close();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (KeeperException e) {
            e.printStackTrace();
        }
    }

    public Condition newCondition() {
        return null;
    }

    public void lockInterruptibly() throws InterruptedException {
        this.lock();
    }


    public class LockException extends RuntimeException {
        private static final long serialVersionUID = 1L;
        public LockException(String e){
            super(e);
        }
        public LockException(Exception e){
            super(e);
        }
    }
}

測試代碼架構

public class Test {
    static int n = 500;

    public static void secskill() {
        System.out.println(--n);
    }

    public static void main(String[] args) {
        
        Runnable runnable = new Runnable() {
            public void run() {
                DistributedLock lock = null;
                try {
                    lock = new DistributedLock("127.0.0.1:2181", "test1");
                    lock.lock();
                    secskill();
                    System.out.println(Thread.currentThread().getName() + "正在運行");
                } finally {
                    if (lock != null) {
                        lock.unlock();
                    }
                }
            }
        };

        for (int i = 0; i < 10; i++) {
            Thread t = new Thread(runnable);
            t.start();
        }
    }
}

運行結果：
分佈式