Zookeeper客户端Curator使用详解-分布式锁（三）

转：http://throwable.coding.me/2018/12/16/zookeeper-curator-usage
提醒：

1.推荐使用ConnectionStateListener监控连接的状态，因为当连接LOST时你不再拥有锁

2.分布式的锁全局同步， 这意味着任何一个时间点不会有两个客户端都拥有相同的锁。

可重入共享锁—Shared Reentrant Lock

Shared意味着锁是全局可见的， 客户端都可以请求锁。 Reentrant和JDK的ReentrantLock类似，即可重入， 意味着同一个客户端在拥有锁的同时，可以多次获取，不会被阻塞。 它是由类InterProcessMutex来实现。 它的构造函数为：

public InterProcessMutex(CuratorFramework client, String path)

通过acquire()获得锁，并提供超时机制：

public void acquire()
Acquire the mutex - blocking until it's available. Note: the same thread can call acquire
re-entrantly. Each call to acquire must be balanced by a call to release()

public boolean acquire(long time,TimeUnit unit)
Acquire the mutex - blocks until it's available or the given time expires. Note: the same thread can call acquire re-entrantly. Each call to acquire that returns true must be balanced by a call to release()

Parameters:
time - time to wait
unit - time unit
Returns:
true if the mutex was acquired, false if not

通过release()方法释放锁。 InterProcessMutex 实例可以重用。

Revoking ZooKeeper recipes wiki定义了可协商的撤销机制。 为了撤销mutex, 调用下面的方法：

public void makeRevocable(RevocationListener<T> listener)
将锁设为可撤销的. 当别的进程或线程想让你释放锁时Listener会被调用。
Parameters:
listener - the listener

如果你请求撤销当前的锁， 调用attemptRevoke()方法,注意锁释放时RevocationListener将会回调。

public static void attemptRevoke(CuratorFramework client,String path) throws Exception
Utility to mark a lock for revocation. Assuming that the lock has been registered
with a RevocationListener, it will get called and the lock should be released. Note,
however, that revocation is cooperative.
Parameters:
client - the client
path - the path of the lock - usually from something like InterProcessMutex.getParticipantNodes()

二次提醒：错误处理 还是强烈推荐你使用ConnectionStateListener处理连接状态的改变。 当连接LOST时你不再拥有锁。

首先让我们创建一个模拟的共享资源， 这个资源期望只能单线程的访问，否则会有并发问题。

public class FakeLimitedResource {
    private final AtomicBoolean inUse = new AtomicBoolean(false);

    public void use() throws InterruptedException {
        // 真实环境中我们会在这里访问/维护一个共享的资源
        //这个例子在使用锁的情况下不会非法并发异常IllegalStateException
        //但是在无锁的情况由于sleep了一段时间，很容易抛出异常
        if (!inUse.compareAndSet(false, true)) {
            throw new IllegalStateException("Needs to be used by one client at a time");
        }
        try {
            Thread.sleep((long) (3 * Math.random()));
        } finally {
            inUse.set(false);
        }
    }
}

然后创建一个InterProcessMutexDemo类， 它负责请求锁， 使用资源，释放锁这样一个完整的访问过程。

public class InterProcessMutexDemo {

    private InterProcessMutex lock;
    private final FakeLimitedResource resource;
    private final String clientName;

    public InterProcessMutexDemo(CuratorFramework client, String lockPath, FakeLimitedResource resource, String clientName) {
        this.resource = resource;
        this.clientName = clientName;
        this.lock = new InterProcessMutex(client, lockPath);
    }

    public void doWork(long time, TimeUnit unit) throws Exception {
        if (!lock.acquire(time, unit)) {
            throw new IllegalStateException(clientName + " could not acquire the lock");
        }
        try {
            System.out.println(clientName + " get the lock");
            resource.use(); //access resource exclusively
        } finally {
            System.out.println(clientName + " releasing the lock");
            lock.release(); // always release the lock in a finally block
        }
    }

    private static final int QTY = 5;
    private static final int REPETITIONS = QTY * 10;
    private static final String PATH = "/examples/locks";

    public static void main(String[] args) throws Exception {
        final FakeLimitedResource resource = new FakeLimitedResource();
        ExecutorService service = Executors.newFixedThreadPool(QTY);
        final TestingServer server = new TestingServer();
        try {
            for (int i = 0; i < QTY; ++i) {
                final int index = i;
                Callable<Void> task = new Callable<Void>() {
                    @Override
                    public Void call() throws Exception {
                        CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
                        try {
                            client.start();
                            final InterProcessMutexDemo example = new InterProcessMutexDemo(client, PATH, resource, "Client " + index);
                            for (int j = 0; j < REPETITIONS; ++j) {
                                example.doWork(10, TimeUnit.SECONDS);
                            }
                        } catch (Throwable e) {
                            e.printStackTrace();
                        } finally {
                            CloseableUtils.closeQuietly(client);
                        }
                        return null;
                    }
                };
                service.submit(task);
            }
            service.shutdown();
            service.awaitTermination(10, TimeUnit.MINUTES);
        } finally {
            CloseableUtils.closeQuietly(server);
        }
    }
}

代码也很简单，生成10个client， 每个client重复执行10次 请求锁–访问资源–释放锁的过程。每个client都在独立的线程中。 结果可以看到，锁是随机的被每个实例排他性的使用。

既然是可重用的，你可以在一个线程中多次调用acquire(),在线程拥有锁时它总是返回true。

你不应该在多个线程中用同一个InterProcessMutex， 你可以在每个线程中都生成一个新的InterProcessMutex实例，它们的path都一样，这样它们可以共享同一个锁。

不可重入共享锁—Shared Lock

这个锁和上面的InterProcessMutex相比，就是少了Reentrant的功能，也就意味着它不能在同一个线程中重入。这个类是InterProcessSemaphoreMutex,使用方法和InterProcessMutex类似

public class InterProcessSemaphoreMutexDemo {

    private InterProcessSemaphoreMutex lock;
    private final FakeLimitedResource resource;
    private final String clientName;

    public InterProcessSemaphoreMutexDemo(CuratorFramework client, String lockPath, FakeLimitedResource resource, String clientName) {
        this.resource = resource;
        this.clientName = clientName;
        this.lock = new InterProcessSemaphoreMutex(client, lockPath);
    }

    public void doWork(long time, TimeUnit unit) throws Exception {
        if (!lock.acquire(time, unit))
        {
            throw new IllegalStateException(clientName + " 不能得到互斥锁");
        }
        System.out.println(clientName + " 已获取到互斥锁");
        if (!lock.acquire(time, unit))
        {
            throw new IllegalStateException(clientName + " 不能得到互斥锁");
        }
        System.out.println(clientName + " 再次获取到互斥锁");
        try {
            System.out.println(clientName + " get the lock");
            resource.use(); //access resource exclusively
        } finally {
            System.out.println(clientName + " releasing the lock");
            lock.release(); // always release the lock in a finally block
            lock.release(); // 获取锁几次 释放锁也要几次
        }
    }

    private static final int QTY = 5;
    private static final int REPETITIONS = QTY * 10;
    private static final String PATH = "/examples/locks";

    public static void main(String[] args) throws Exception {
        final FakeLimitedResource resource = new FakeLimitedResource();
        ExecutorService service = Executors.newFixedThreadPool(QTY);
        final TestingServer server = new TestingServer();
        try {
            for (int i = 0; i < QTY; ++i) {
                final int index = i;
                Callable<Void> task = new Callable<Void>() {
                    @Override
                    public Void call() throws Exception {
                        CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
                        try {
                            client.start();
                            final InterProcessSemaphoreMutexDemo example = new InterProcessSemaphoreMutexDemo(client, PATH, resource, "Client " + index);
                            for (int j = 0; j < REPETITIONS; ++j) {
                                example.doWork(10, TimeUnit.SECONDS);
                            }
                        } catch (Throwable e) {
                            e.printStackTrace();
                        } finally {
                            CloseableUtils.closeQuietly(client);
                        }
                        return null;
                    }
                };
                service.submit(task);
            }
            service.shutdown();
            service.awaitTermination(10, TimeUnit.MINUTES);
        } finally {
            CloseableUtils.closeQuietly(server);
        }
        Thread.sleep(Integer.MAX_VALUE);
    }
}

运行后发现，有且只有一个client成功获取第一个锁(第一个acquire()方法返回true)，然后它自己阻塞在第二个acquire()方法，获取第二个锁超时；其他所有的客户端都阻塞在第一个acquire()方法超时并且抛出异常。

这样也就验证了InterProcessSemaphoreMutex实现的锁是不可重入的。

可重入读写锁—Shared Reentrant Read Write Lock

类似JDK的ReentrantReadWriteLock。一个读写锁管理一对相关的锁。一个负责读操作，另外一个负责写操作。读操作在写锁没被使用时可同时由多个进程使用，而写锁在使用时不允许读(阻塞)。

此锁是可重入的。一个拥有写锁的线程可重入读锁，但是读锁却不能进入写锁。这也意味着写锁可以降级成读锁， 比如请求写锁 —>请求读锁—>释放读锁 —->释放写锁。从读锁升级成写锁是不行的。

可重入读写锁主要由两个类实现：InterProcessReadWriteLock、InterProcessMutex。使用时首先创建一个InterProcessReadWriteLock实例，然后再根据你的需求得到读锁或者写锁，读写锁的类型是InterProcessMutex。

public class ReentrantReadWriteLockDemo {

    private final InterProcessReadWriteLock lock;
    private final InterProcessMutex readLock;
    private final InterProcessMutex writeLock;
    private final FakeLimitedResource resource;
    private final String clientName;

    public ReentrantReadWriteLockDemo(CuratorFramework client, String lockPath, FakeLimitedResource resource, String clientName) {
        this.resource = resource;
        this.clientName = clientName;
        lock = new InterProcessReadWriteLock(client, lockPath);
        readLock = lock.readLock();
        writeLock = lock.writeLock();
    }

    public void doWork(long time, TimeUnit unit) throws Exception {
        // 注意只能先得到写锁再得到读锁，不能反过来！！！
        if (!writeLock.acquire(time, unit)) {
            throw new IllegalStateException(clientName + " 不能得到写锁");
        }
        System.out.println(clientName + " 已得到写锁");
        if (!readLock.acquire(time, unit)) {
            throw new IllegalStateException(clientName + " 不能得到读锁");
        }
        System.out.println(clientName + " 已得到读锁");
        try {
            resource.use(); // 使用资源
            Thread.sleep(1000);
        } finally {
            System.out.println(clientName + " 释放读写锁");
            readLock.release();
            writeLock.release();
        }
    }

    private static final int QTY = 5;
    private static final int REPETITIONS = QTY ;
    private static final String PATH = "/examples/locks";

    public static void main(String[] args) throws Exception {
        final FakeLimitedResource resource = new FakeLimitedResource();
        ExecutorService service = Executors.newFixedThreadPool(QTY);
        final TestingServer server = new TestingServer();
        try {
            for (int i = 0; i < QTY; ++i) {
                final int index = i;
                Callable<Void> task = new Callable<Void>() {
                    @Override
                    public Void call() throws Exception {
                        CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
                        try {
                            client.start();
                            final ReentrantReadWriteLockDemo example = new ReentrantReadWriteLockDemo(client, PATH, resource, "Client " + index);
                            for (int j = 0; j < REPETITIONS; ++j) {
                                example.doWork(10, TimeUnit.SECONDS);
                            }
                        } catch (Throwable e) {
                            e.printStackTrace();
                        } finally {
                            CloseableUtils.closeQuietly(client);
                        }
                        return null;
                    }
                };
                service.submit(task);
            }
            service.shutdown();
            service.awaitTermination(10, TimeUnit.MINUTES);
        } finally {
            CloseableUtils.closeQuietly(server);
        }
    }
}

信号量—Shared Semaphore

一个计数的信号量类似JDK的Semaphore。 JDK中Semaphore维护的一组许可(permits)，而Curator中称之为租约(Lease)。 有两种方式可以决定semaphore的最大租约数。第一种方式是用户给定path并且指定最大LeaseSize。第二种方式用户给定path并且使用SharedCountReader类。如果不使用SharedCountReader, 必须保证所有实例在多进程中使用相同的(最大)租约数量,否则有可能出现A进程中的实例持有最大租约数量为10，但是在B进程中持有的最大租约数量为20，此时租约的意义就失效了。

这次调用acquire()会返回一个租约对象。 客户端必须在finally中close这些租约对象，否则这些租约会丢失掉。 但是， 但是，如果客户端session由于某种原因比如crash丢掉， 那么这些客户端持有的租约会自动close， 这样其它客户端可以继续使用这些租约。 租约还可以通过下面的方式返还：

public void returnAll(Collection<Lease> leases)
public void returnLease(Lease lease)

注意你可以一次性请求多个租约，如果Semaphore当前的租约不够，则请求线程会被阻塞。 同时还提供了超时的重载方法。

public Lease acquire()
public Collection<Lease> acquire(int qty)
public Lease acquire(long time, TimeUnit unit)
public Collection<Lease> acquire(int qty, long time, TimeUnit unit)

Shared Semaphore使用的主要类包括下面几个：

• InterProcessSemaphoreV2
• Lease
• SharedCountReader

    private static final int MAX_LEASE = 10;
    private static final String PATH = "/examples/locks";

    public static void main(String[] args) throws Exception {
        FakeLimitedResource resource = new FakeLimitedResource();
        try (TestingServer server = new TestingServer()) {

            CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
            client.start();

            InterProcessSemaphoreV2 semaphore = new InterProcessSemaphoreV2(client, PATH, MAX_LEASE);
            Collection<Lease> leases = semaphore.acquire(5);
            System.out.println("get " + leases.size() + " leases");
            Lease lease = semaphore.acquire();
            System.out.println("get another lease");

            resource.use();

            Collection<Lease> leases2 = semaphore.acquire(5, 10, TimeUnit.SECONDS);
            System.out.println("Should timeout and acquire return " + leases2);

            System.out.println("return one lease");
            semaphore.returnLease(lease);
            System.out.println("return another 5 leases");
            semaphore.returnAll(leases);
        }
    }
}

首先我们先获得了5个租约， 最后我们把它还给了semaphore。 接着请求了一个租约，因为semaphore还有5个租约，所以请求可以满足，返回一个租约，还剩4个租约。 然后再请求一个租约，因为租约不够，阻塞到超时，还是没能满足，返回结果为null(租约不足会阻塞到超时，然后返回null，不会主动抛出异常；如果不设置超时时间，会一致阻塞)。

上面说讲的锁都是公平锁(fair)。 总ZooKeeper的角度看， 每个客户端都按照请求的顺序获得锁，不存在非公平的抢占的情况。

多共享锁对象 —Multi Shared Lock

Multi Shared Lock是一个锁的容器。 当调用acquire()， 所有的锁都会被acquire()，如果请求失败，所有的锁都会被release。 同样调用release时所有的锁都被release(失败被忽略)。 基本上，它就是组锁的代表，在它上面的请求释放操作都会传递给它包含的所有的锁。

主要涉及两个类：

• InterProcessMultiLock
• InterProcessLock

它的构造函数需要包含的锁的集合，或者一组ZooKeeper的path。

public InterProcessMultiLock(List<InterProcessLock> locks)
public InterProcessMultiLock(CuratorFramework client, List<String> paths)

用法和Shared Lock相同。

public class MultiSharedLockDemo {

    private static final String PATH1 = "/examples/locks1";
    private static final String PATH2 = "/examples/locks2";

    public static void main(String[] args) throws Exception {
        FakeLimitedResource resource = new FakeLimitedResource();
        try (TestingServer server = new TestingServer()) {
            CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
            client.start();

            InterProcessLock lock1 = new InterProcessMutex(client, PATH1);
            InterProcessLock lock2 = new InterProcessSemaphoreMutex(client, PATH2);

            InterProcessMultiLock lock = new InterProcessMultiLock(Arrays.asList(lock1, lock2));

            if (!lock.acquire(10, TimeUnit.SECONDS)) {
                throw new IllegalStateException("could not acquire the lock");
            }
            System.out.println("has got all lock");

            System.out.println("has got lock1: " + lock1.isAcquiredInThisProcess());
            System.out.println("has got lock2: " + lock2.isAcquiredInThisProcess());

            try {
                resource.use(); //access resource exclusively
            } finally {
                System.out.println("releasing the lock");
                lock.release(); // always release the lock in a finally block
            }
            System.out.println("has got lock1: " + lock1.isAcquiredInThisProcess());
            System.out.println("has got lock2: " + lock2.isAcquiredInThisProcess());
        }
    }
}

新建一个InterProcessMultiLock， 包含一个重入锁和一个非重入锁。 调用acquire()后可以看到线程同时拥有了这两个锁。 调用release()看到这两个锁都被释放了。

最后再重申一次， 强烈推荐使用ConnectionStateListener监控连接的状态，当连接状态为LOST，锁将会丢失。

分布式计数器

顾名思义，计数器是用来计数的, 利用ZooKeeper可以实现一个集群共享的计数器。 只要使用相同的path就可以得到最新的计数器值， 这是由ZooKeeper的一致性保证的。Curator有两个计数器， 一个是用int来计数(SharedCount)，一个用long来计数(DistributedAtomicLong)。

分布式int计数器—SharedCount

这个类使用int类型来计数。 主要涉及三个类。

• SharedCount
• SharedCountReader
• SharedCountListener

SharedCount代表计数器， 可以为它增加一个SharedCountListener，当计数器改变时此Listener可以监听到改变的事件，而SharedCountReader可以读取到最新的值， 包括字面值和带版本信息的值VersionedValue。

public class SharedCounterDemo implements SharedCountListener {

    private static final int QTY = 5;
    private static final String PATH = "/examples/counter";

    public static void main(String[] args) throws IOException, Exception {
        final Random rand = new Random();
        SharedCounterDemo example = new SharedCounterDemo();
        try (TestingServer server = new TestingServer()) {
            CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
            client.start();

            SharedCount baseCount = new SharedCount(client, PATH, 0);
            baseCount.addListener(example);
            baseCount.start();

            List<SharedCount> examples = Lists.newArrayList();
            ExecutorService service = Executors.newFixedThreadPool(QTY);
            for (int i = 0; i < QTY; ++i) {
                final SharedCount count = new SharedCount(client, PATH, 0);
                examples.add(count);
                Callable<Void> task = () -> {
                    count.start();
                    Thread.sleep(rand.nextInt(10000));
                    System.out.println("Increment:" + count.trySetCount(count.getVersionedValue(), count.getCount() + rand.nextInt(10)));
                    return null;
                };
                service.submit(task);
            }

            service.shutdown();
            service.awaitTermination(10, TimeUnit.MINUTES);

            for (int i = 0; i < QTY; ++i) {
                examples.get(i).close();
            }
            baseCount.close();
        }
        Thread.sleep(Integer.MAX_VALUE);
    }

    @Override
    public void stateChanged(CuratorFramework arg0, ConnectionState arg1) {
        System.out.println("State changed: " + arg1.toString());
    }

    @Override
    public void countHasChanged(SharedCountReader sharedCount, int newCount) throws Exception {
        System.out.println("Counter's value is changed to " + newCount);
    }
}

这里我们使用trySetCount去设置计数器。 第一个参数提供当前的VersionedValue,如果期间其它client更新了此计数值， 你的更新可能不成功， 但是这时你的client更新了最新的值，所以失败了你可以尝试再更新一次。 而setCount是强制更新计数器的值。

注意计数器必须start,使用完之后必须调用close关闭它。

强烈推荐使用ConnectionStateListener。 在本例中SharedCountListener扩展ConnectionStateListener。

分布式long计数器—DistributedAtomicLong

再看一个Long类型的计数器。 除了计数的范围比SharedCount大了之外， 它首先尝试使用乐观锁的方式设置计数器， 如果不成功(比如期间计数器已经被其它client更新了)， 它使用InterProcessMutex方式来更新计数值。

可以从它的内部实现DistributedAtomicValue.trySet()中看出：

AtomicValue<byte[]>   trySet(MakeValue makeValue) throws Exception
 {
     MutableAtomicValue<byte[]>  result = new MutableAtomicValue<byte[]>(null, null, false);

     tryOptimistic(result, makeValue);
     if ( !result.succeeded() && (mutex != null) )
     {
         tryWithMutex(result, makeValue);
     }

     return result;
 }

此计数器有一系列的操作：

• get(): 获取当前值
• increment()： 加一
• decrement(): 减一
• add()： 增加特定的值
• subtract(): 减去特定的值
• trySet(): 尝试设置计数值
• forceSet(): 强制设置计数值

你必须检查返回结果的succeeded()， 它代表此操作是否成功。 如果操作成功， preValue()代表操作前的值， postValue()代表操作后的值。

public class DistributedAtomicLongDemo {

    private static final int QTY = 5;
    private static final String PATH = "/examples/counter";

    public static void main(String[] args) throws IOException, Exception {
        List<DistributedAtomicLong> examples = Lists.newArrayList();
        try (TestingServer server = new TestingServer()) {
            CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
            client.start();
            ExecutorService service = Executors.newFixedThreadPool(QTY);
            for (int i = 0; i < QTY; ++i) {
                final DistributedAtomicLong count = new DistributedAtomicLong(client, PATH, new RetryNTimes(10, 10));

                examples.add(count);
                Callable<Void> task = () -> {
                    try {
                        AtomicValue<Long> value = count.increment();
                        System.out.println("succeed: " + value.succeeded());
                        if (value.succeeded())
                            System.out.println("Increment: from " + value.preValue() + " to " + value.postValue());
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    return null;
                };
                service.submit(task);
            }

            service.shutdown();
            service.awaitTermination(10, TimeUnit.MINUTES);
            Thread.sleep(Integer.MAX_VALUE);
        }
    }
}