zookeeper实现分布式锁及Leader选举原理

小灰灰 2022-09-08 06:18 211阅读 0赞

synchronized锁只能解决单个进程下多线程问题，而针对多个进程（比如多个用户参与秒杀）的情况，是无法解决的，所以需要依赖第三方独立中间件来实现分布式锁。zookeeper是负责分布式协调，所以可以实现分布式锁。

# 依赖zookeeper特性实现分布式锁： #

1.  同目录下节点唯一：可以依赖zookeeper的特性之一：同一节点下只能创建一个名字相同的节点；那么创建节点成功的客户端就抢占到了锁，没有创建成功的客户端就没有竞争到锁。但是会有惊群效应问题，一个客户端释放了锁，其他所有客户端都会再次去竞争，然而只有一个客户端可以竞争到锁，浪费性能。
2.  临时有序节点：针对惊群效应问题，可以采用zookeeper另一个特性，在同一节点下创建临时有序节点，哪个客户端创建的节点序号最小，则代表抢占到锁；抢占到锁的客户端释放了锁，会从其他客户端中筛选出序号最小的那个去获取锁，根据节点序号顺序获取锁。

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# Curator实现分布式锁 #

// 基于临时有序节点实现分布式锁
    InterProcessMutex lock = new InterProcessMutex(curatorFramework, "/lock");
    try {
        lock.acquire(); //抢占分布式锁资源，阻塞方法
        // 需要分布式锁的业务代码
        Order order = orderServie.getOne(orderId);
        order.setStock(order.getStock - 1);
        orderService.update(order);
    } finally {
        lock.release();//释放锁
    }

--------------------

# zookeeper实现分布式锁源码分析 #

zookeeper实现分布式锁，用到了容器节点和临时有序节点；创建一个容器节点作为目录，利用其特性，当容器节点下面没有子节点时会被标注并自动删除；在容器节点下创建临时有序节点，根据每个客户端进程创建的有序节点的序号来觉得抢占锁顺序，序号最小的进程优先获得锁。

public InterProcessMutex(CuratorFramework client, String path) {
        this(client, path, new StandardLockInternalsDriver());
    }
    public InterProcessMutex(CuratorFramework client, String path, LockInternalsDriver driver) {
        this(client, path, "lock-", 1, driver);
    }
    InterProcessMutex(CuratorFramework client, String path, String lockName, int maxLeases, LockInternalsDriver driver) {
        this.threadData = Maps.newConcurrentMap();
        this.basePath = PathUtils.validatePath(path); //lock目录
        this.internals = new LockInternals(client, driver, path, lockName, maxLeases);
    }

### acquire抢占锁 ###

public void acquire() throws Exception {
        if (!this.internalLock(-1L, (TimeUnit)null)) {
            throw new IOException("Lost connection while trying to acquire lock: " + this.basePath);
        }
    }
    
    private boolean internalLock(long time, TimeUnit unit) throws Exception {
        Thread currentThread = Thread.currentThread();
        // 判断当前线程是否已经获得过锁（考虑重入锁情况，避免死锁）
        InterProcessMutex.LockData lockData = (InterProcessMutex.LockData)this.threadData.get(currentThread);
        if (lockData != null) {
            lockData.lockCount.incrementAndGet();
            return true;
        } else {
            /**
             * 1、去zookeeper server 创建临时有序节点
             * 2、判断创建的节点的序号是否是所有节点中最小的，如果是，则获取锁并返回
             * 3、如果没有获取锁则等待
             */
            String lockPath = this.internals.attemptLock(time, unit, this.getLockNodeBytes());
            if (lockPath != null) {
                InterProcessMutex.LockData newLockData = new InterProcessMutex.LockData(currentThread, lockPath);
                this.threadData.put(currentThread, newLockData);
                return true;
            } else {
                return false;
            }
        }
    }

### attemptLock试图获得锁 ###

String attemptLock(long time, TimeUnit unit, byte[] lockNodeBytes) throws Exception {
        long startMillis = System.currentTimeMillis();
        Long millisToWait = unit != null ? unit.toMillis(time) : null;
        byte[] localLockNodeBytes = this.revocable.get() != null ? new byte[0] : lockNodeBytes;
        int retryCount = 0;
        String ourPath = null;
        boolean hasTheLock = false;
        boolean isDone = false;
    
        while(!isDone) {
            isDone = true;
    
            try {
                // 创建一个临时有序节点，返回节点名称
                ourPath = this.driver.createsTheLock(this.client, this.path, localLockNodeBytes);
                // 通过循环方式获取锁
                hasTheLock = this.internalLockLoop(startMillis, millisToWait, ourPath);
            } catch (KeeperException.NoNodeException var14) {
                if (!this.client.getZookeeperClient().getRetryPolicy().allowRetry(retryCount++, System.currentTimeMillis() - startMillis, RetryLoop.getDefaultRetrySleeper())) {
                    throw var14;
                }
    
                isDone = false;
            }
        }
    
        return hasTheLock ? ourPath : null;
    }

### createsTheLock创建父级容器节点和临时有序节点 ###

public String createsTheLock(CuratorFramework client, String path, byte[] lockNodeBytes) throws Exception {
        String ourPath;
        // 创建父级容器节点，然后创建临时有序节点
        if (lockNodeBytes != null) {
            ourPath = (String)((ACLBackgroundPathAndBytesable)client.create().creatingParentContainersIfNeeded().withProtection().withMode(CreateMode.EPHEMERAL_SEQUENTIAL)).forPath(path, lockNodeBytes);
        } else {
            ourPath = (String)((ACLBackgroundPathAndBytesable)client.create().creatingParentContainersIfNeeded().withProtection().withMode(CreateMode.EPHEMERAL_SEQUENTIAL)).forPath(path);
        }
        return ourPath;
    }

### internalLockLoop内部循环获得锁 ###

private boolean internalLockLoop(long startMillis, Long millisToWait, String ourPath) throws Exception {
        boolean haveTheLock = false;
        boolean doDelete = false;
    
        try {
            if (this.revocable.get() != null) {
                ((BackgroundPathable)this.client.getData().usingWatcher(this.revocableWatcher)).forPath(ourPath);
            }
            // 如果连接是启动状态，并且没有获得锁，就不断循环去获取锁
            while(this.client.getState() == CuratorFrameworkState.STARTED && !haveTheLock) {
                // 对节点列表进行排序
                List<String> children = this.getSortedChildren();
                // 截取创建节点的序列号
                String sequenceNodeName = ourPath.substring(this.basePath.length() + 1);
                // 比较序列号是否是最小的
                PredicateResults predicateResults = this.driver.getsTheLock(this.client, children, sequenceNodeName, this.maxLeases);
                if (predicateResults.getsTheLock()) {
                    haveTheLock = true;
                } else {
                    String previousSequencePath = this.basePath + "/" + predicateResults.getPathToWatch();
                    synchronized(this) {
                        try {
                            ((BackgroundPathable)this.client.getData().usingWatcher(this.watcher)).forPath(previousSequencePath);
                            if (millisToWait == null) {
                                this.wait();
                            } else {
                                millisToWait = millisToWait - (System.currentTimeMillis() - startMillis);
                                startMillis = System.currentTimeMillis();
                                if (millisToWait > 0L) {
                                    this.wait(millisToWait);
                                } else {
                                    doDelete = true;
                                    break;
                                }
                            }
                        } catch (KeeperException.NoNodeException var19) {
                        }
                    }
                }
            }
        } catch (Exception var21) {
            ThreadUtils.checkInterrupted(var21);
            doDelete = true;
            throw var21;
        } finally {
            if (doDelete) {
                this.deleteOurPath(ourPath);
            }
    
        }
    
        return haveTheLock;
    }

### getsTheLock比较当前节点序列号是否最小 ###

public PredicateResults getsTheLock(CuratorFramework client, List<String> children, String sequenceNodeName, int maxLeases) throws Exception {
        int ourIndex = children.indexOf(sequenceNodeName); //根据节点名称获取索引
        validateOurIndex(sequenceNodeName, ourIndex); //判断索引的有效性
        boolean getsTheLock = ourIndex < maxLeases; //判断节点索引是否最小，比如当前节点索引为0，则是最小，maxLeases表示最大可出租多少锁，在这里是1
        String pathToWatch = getsTheLock ? null : (String)children.get(ourIndex - maxLeases);
        return new PredicateResults(pathToWatch, getsTheLock);
    }

### release释放锁 ###

public void release() throws Exception {
        Thread currentThread = Thread.currentThread();
        InterProcessMutex.LockData lockData = (InterProcessMutex.LockData)this.threadData.get(currentThread);
        if (lockData == null) {
            throw new IllegalMonitorStateException("You do not own the lock: " + this.basePath);
        } else {
            int newLockCount = lockData.lockCount.decrementAndGet(); //针对重入锁，减少重入次数
            if (newLockCount <= 0) {
                if (newLockCount < 0) {
                    throw new IllegalMonitorStateException("Lock count has gone negative for lock: " + this.basePath);
                } else { //重入锁次数变成0，释放锁
                    try {
                        this.internals.releaseLock(lockData.lockPath);
                    } finally {
                        this.threadData.remove(currentThread);
                    }
                }
            }
        }
    }

### releaseLock ###

final void releaseLock(String lockPath) throws Exception {
        this.client.removeWatchers();
        this.revocable.set((Object)null);
        this.deleteOurPath(lockPath);
    }
    // 删除节点
    private void deleteOurPath(String ourPath) throws Exception {
        try {
            ((ChildrenDeletable)this.client.delete().guaranteed()).forPath(ourPath);
        } catch (KeeperException.NoNodeException var3) {}
    }

--------------------

# zookeeper实现Leader选举 #

也可以利用本文开头的zookeeper节点特性去实现leader选举，创建一个容器节点(如/leader)，然后在容器节点下创建临时有序节点，谁的节点序列号最小，那么就是leader节点

zookeeper基于临时有序节点特性封装了API：

*  LeaderLatch
 *  LeaderSelector

## LeaderLatch使用 ##

### maven依赖 ###

<dependency>
        <groupId>org.apache.shardingsphere</groupId>
        <artifactId>shardingsphere-jdbc-core</artifactId>
        <version>5.0.0-beta</version>
    </dependency>
    <dependency>
        <groupId>org.apache.curator</groupId>
        <artifactId>curator-framework</artifactId>
        <version>5.2.0</version>
    </dependency>
    <dependency>
        <groupId>org.apache.curator</groupId>
        <artifactId>curator-recipes</artifactId>
        <version>5.2.0</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-quartz</artifactId>
        <version>2.5.3</version>
    </dependency>

### springboot项目代码 ###

@SpringBootApplication
    public class ZkLeaderApplication {
        public static void main(String[] args) {
            SpringApplication.run(ZkLeaderApplication.class);
        }
    }
    
    // 实现定时任务的触发器
    public class ZkSchedulerFactoryBean extends SchedulerFactoryBean {
        private LeaderLatch leaderLatch;
        private final String LEADER_PATH = "/leader";
    
        private CuratorFramework getClient() {
            CuratorFramework curatorFramework = CuratorFrameworkFactory.builder()
                    .connectString("192.168.221.128:2181").connectionTimeoutMs(2000)
                    .retryPolicy(new ExponentialBackoffRetry(1000, 3))
                    .sessionTimeoutMs(20000).build();
            curatorFramework.start();//启动
            return curatorFramework;
        }
    
        public ZkSchedulerFactoryBean() throws Exception {
            this.setAutoStartup(false); //引用启动时不会自动开启定时任务
            leaderLatch = new LeaderLatch(getClient(), LEADER_PATH);
            // 当Leader节点改变时，触发监听
            leaderLatch.addListener(new LeaderLatchListener() {
                @Override
                public void isLeader() {
                    System.out.println(Thread.currentThread().getName() + "成为了leader节点");
                    setAutoStartup(true);
                    start();
                }
                @Override
                public void notLeader() {
                    System.out.println(Thread.currentThread().getName() + "竞选leader失败");
                    setAutoStartup(false);
                    stop();
                }
            });
            leaderLatch.start();
        }
    
        @Override
        protected void startScheduler(Scheduler scheduler, int startupDelay) throws SchedulerException {
            if (this.isAutoStartup()) {
                super.startScheduler(scheduler, startupDelay);
            }
        }
    
        @Override
        public void destroy() throws SchedulerException {
            CloseableUtils.closeQuietly(leaderLatch);
            super.destroy();
        }
    }
    
    public class MyQuartzJobBean extends QuartzJobBean {
        @Override
        protected void executeInternal(JobExecutionContext jobExecutionContext) throws JobExecutionException {
            System.out.println("开始执行任务");
            System.out.println("当前执行的系统时间：" + new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date()));
        }
    }
    
    @Configuration
    public class QuartzConfiguration {
        @Bean
        public ZkSchedulerFactoryBean schedulerFactoryBean(JobDetail jobDetail, Trigger trigger) throws Exception {
            ZkSchedulerFactoryBean zkSchedulerFactoryBean = new ZkSchedulerFactoryBean();
            zkSchedulerFactoryBean.setJobDetails(jobDetail);
            zkSchedulerFactoryBean.setTriggers(trigger);
            return zkSchedulerFactoryBean;
        }
        @Bean
        public JobDetail jobDetail() {
            return JobBuilder.newJob(MyQuartzJobBean.class).storeDurably().build();
        }
        @Bean
        public Trigger trigger() {
            SimpleScheduleBuilder simpleScheduleBuilder = SimpleScheduleBuilder.simpleSchedule()
                    //一秒执行一次，重复执行
                    .withIntervalInSeconds(1).repeatForever();
            return TriggerBuilder.newTrigger().forJob(jobDetail()).withSchedule(simpleScheduleBuilder).build();
        }
    }

### springboot项目配置 ###

启动两个项目，代表两个节点，两个应用同时启动，哪个应用在zookeeper目录下生成的节点序列号小，那么就抢占到锁，代表leader节点，执行定时任务。当一个应用停止，另一个应用会自动切换成leader节点，定时任务会自动执行。

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## LeaderSelector使用 ##

底层使用InterProcessMutex，与LeaderLatch的区别是，leader释放领导权之后，还可以继续参与竞争。

public class LeaderSelectorDemo extends LeaderSelectorListenerAdapter implements Closeable {
        private final String LEADER_PATH = "/leader";
        private LeaderSelector leaderSelector;
        private String nodeName;
    
        private CuratorFramework getClient() {
            CuratorFramework curatorFramework = CuratorFrameworkFactory.builder()
                    .connectString("192.168.221.128:2181").connectionTimeoutMs(2000)
                    .retryPolicy(new ExponentialBackoffRetry(1000, 3))
                    .sessionTimeoutMs(20000).build();
            curatorFramework.start();//启动
            return curatorFramework;
        }
    
        public LeaderSelectorDemo(String nodeName) {
            leaderSelector = new LeaderSelector(getClient(), LEADER_PATH, this);
            leaderSelector.autoRequeue(); //自动发起选举，重新参与竞争
            this.nodeName = nodeName;
        }
    
        @Override
        public void close() throws IOException {
            leaderSelector.close();
        }
        public void start() {
            leaderSelector.start();
        }
    
        @Override
        public void takeLeadership(CuratorFramework curatorFramework) throws Exception {
            System.out.println(nodeName + "竞争成为leader");
        }
    
        public static void main(String[] args) {
            for (int i = 1; i <= 10; i++) {
                LeaderSelectorDemo leaderSelectorDemo = new LeaderSelectorDemo("node-" + i);
                leaderSelectorDemo.start();
            }
        }
    }

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