跟着实例学习ZooKeeper的用法： 分布式锁
锁
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。
可重入锁Shared Reentrant Lock
首先我们先看一个全局可重入的锁。 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， 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 listener) 将锁设为可撤销的. 当别的进程或线程想让你释放锁是Listener会被调用。
Parameters:
listener - the listener 如果你请求撤销当前的锁， 调用Revoker方法。
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() 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。
package com.colobu.zkrecipe.lock;import java.util.concurrent.atomic.AtomicBoolean;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ExampleClientThatLocks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。
package com.colobu.zkrecipe.lock;import java.util.concurrent.TimeUnit;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.recipes.locks.InterProcessMutex;public class ExampleClientThatLocks { private final InterProcessMutex lock; private final FakeLimitedResource resource; private final String clientName; public ExampleClientThatLocks(CuratorFramework client, String lockPath, FakeLimitedResource resource, String clientName) { this.resource = resource; this.clientName = clientName;
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 + " has 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
}
}
}最后创建主程序来测试。
package com.colobu.zkrecipe.lock;import java.util.concurrent.Callable;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.TimeUnit;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.test.TestingServer;import org.apache.curator.utils.CloseableUtils;public class InterProcessMutexExample { 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 task = new Callable() { @Override
public Void call() throws Exception {
CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3)); try {
client.start(); final ExampleClientThatLocks example = new ExampleClientThatLocks(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。
不可重入锁Shared Lock
这个锁和上面的相比，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。 使用方法和上面的类类似。
首先我们将上面的例子修改一下，测试一下它的重入。 修改ExampleClientThatLocks.doWork,连续两次acquire:
public void doWork(long time, TimeUnit unit) throws Exception { if (!lock.acquire(time, unit)) { throw new IllegalStateException(clientName + " could not acquire the lock");
}
System.out.println(clientName + " has the lock"); if (!lock.acquire(time, unit)) { throw new IllegalStateException(clientName + " could not acquire the lock");
}
System.out.println(clientName + " has the lock again"); try {
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(); // always release the lock in a finally block
}
} 注意我们也需要调用release两次。这和JDK的ReentrantLock用法一致。如果少调用一次release，则此线程依然拥有锁。 上面的代码没有问题，我们可以多次调用acquire，后续的acquire也不会阻塞。 将上面的InterProcessMutex换成不可重入锁InterProcessSemaphoreMutex,如果再运行上面的代码，结果就会发现线程被阻塞再第二个acquire上。 也就是此锁不是可重入的。
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 —>读锁 —->释放写锁。 从读锁升级成写锁是不成的。
主要由两个类实现：
-
InterProcessReadWriteLock
-
InterProcessLock
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InterProcessLock。
public InterProcessLock readLock()public InterProcessLock writeLock()例子和上面的类似。
package com.colobu.zkrecipe.lock;import java.util.concurrent.TimeUnit;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.recipes.locks.InterProcessMutex;import org.apache.curator.framework.recipes.locks.InterProcessReadWriteLock;import org.apache.curator.framework.recipes.locks.InterProcessSemaphoreMutex;public class ExampleClientReadWriteLocks { private final InterProcessReadWriteLock lock; private final InterProcessMutex readLock; private final InterProcessMutex writeLock; private final FakeLimitedResource resource; private final String clientName; public ExampleClientReadWriteLocks(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 { if (!writeLock.acquire(time, unit)) { throw new IllegalStateException(clientName + " could not acquire the writeLock");
}
System.out.println(clientName + " has the writeLock"); if (!readLock.acquire(time, unit)) { throw new IllegalStateException(clientName + " could not acquire the readLock");
}
System.out.println(clientName + " has the readLock too"); try {
resource.use(); //access resource exclusively
} finally {
System.out.println(clientName + " releasing the lock");
readLock.release(); // always release the lock in a finally block
writeLock.release(); // always release the lock in a finally block
}
}
}在这个类中我们首先请求了一个写锁， 然后降级成读锁。 执行业务处理，然后释放读写锁。
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Cubator中称之为租约(Lease)。 有两种方式可以决定semaphore的最大租约数。第一种方式是有用户给定的path决定。第二种方式使用SharedCountReader类。 如果不使用SharedCountReader, 没有内部代码检查进程是否假定有10个租约而进程B假定有20个租约。 所以所有的实例必须使用相同的numberOfLeases值.
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 leases) public void returnLease(Lease lease)注意一次你可以请求多个租约，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 acquire(int qty)public Lease acquire(long time, TimeUnit unit)public Collection acquire(int qty, long time, TimeUnit unit) 主要类有:
-
InterProcessSemaphoreV2
-
Lease
-
SharedCountReader
下面是使用的例子：
package com.colobu.zkrecipe.lock;import java.util.Collection;import java.util.concurrent.TimeUnit;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.locks.InterProcessSemaphoreV2;import org.apache.curator.framework.recipes.locks.Lease;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.test.TestingServer;import org.apache.curator.utils.CloseableUtils;public class InterProcessSemaphoreExample { 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 leases = semaphore.acquire(5);
System.out.println("get " + leases.size() + " leases");
Lease lease = semaphore.acquire();
System.out.println("get another lease");
resource.use();
Collection 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。
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。 相当公平。
多锁对象 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 locks) public InterProcessMultiLock(CuratorFramework client, List paths) 用法和Shared Lock相同。
例子如下：
package com.colobu.zkrecipe.lock;import java.util.Arrays;import java.util.concurrent.TimeUnit;import org.apache.curator.framework.CuratorFramework;import org.apache.curator.framework.CuratorFrameworkFactory;import org.apache.curator.framework.recipes.locks.InterProcessLock;import org.apache.curator.framework.recipes.locks.InterProcessMultiLock;import org.apache.curator.framework.recipes.locks.InterProcessMutex;import org.apache.curator.framework.recipes.locks.InterProcessSemaphoreMutex;import org.apache.curator.retry.ExponentialBackoffRetry;import org.apache.curator.test.TestingServer;public class InterProcessMultiLockExample { 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 the lock");
System.out.println("has the lock1: " + lock1.isAcquiredInThisProcess());
System.out.println("has the 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 the lock1: " + lock1.isAcquiredInThisProcess());
System.out.println("has the 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。
再重申以便， 强烈推荐使用ConnectionStateListener监控连接的状态。
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