背景
继续上一篇文章：http://agapple.iteye.com/blog/1183972 ，项目中需要对分布式任务进行调度，那对应的分布式lock实现在所难免。
这一周，在基于BooleanMutex的基础上，实现了zookeeper的分布式锁，用于控制多进程+多线程的lock控制
算法
可以预先看一下zookeeper的官方文档：
lock操作过程：
- 首先为一个lock场景，在zookeeper中指定对应的一个根节点，用于记录资源竞争的内容
- 每个lock创建后，会lazy在zookeeper中创建一个node节点，表明对应的资源竞争标识。 (小技巧：node节点为EPHEMERAL_SEQUENTIAL，自增长的临时节点)
- 进行lock操作时，获取对应lock根节点下的所有字节点，也即处于竞争中的资源标识
- 按照Fair竞争的原则，按照对应的自增内容做排序，取出编号最小的一个节点做为lock的owner，判断自己的节点id是否就为owner id，如果是则返回，lock成功。
- 如果自己非owner id，按照排序的结果找到序号比自己前一位的id，关注它锁释放的操作(也就是exist watcher)，形成一个链式的触发过程。
unlock操作过程：
- 将自己id对应的节点删除即可，对应的下一个排队的节点就可以收到Watcher事件，从而被唤醒得到锁后退出
其中的几个关键点：
- node节点选择为EPHEMERAL_SEQUENTIAL很重要。 * 自增长的特性，可以方便构建一个基于Fair特性的锁，前一个节点唤醒后一个节点，形成一个链式的触发过程。可以有效的避免"惊群效应"(一个锁释放，所有等待的线程都被唤醒)，有针对性的唤醒，提升性能。 * 选择一个EPHEMERAL临时节点的特性。因为和zookeeper交互是一个网络操作，不可控因素过多，比如网络断了，上一个节点释放锁的操作会失败。临时节点是和对应的session挂接的，session一旦超时或者异常退出其节点就会消失，类似于ReentrantLock中等待队列Thread的被中断处理。
- 获取lock操作是一个阻塞的操作，而对应的Watcher是一个异步事件，所以需要使用信号进行通知，正好使用上一篇文章中提到的BooleanMutex，可以比较方便的解决锁重入的问题。(锁重入可以理解为多次读操作，锁释放为写抢占操作)
注意：
- 使用EPHEMERAL会引出一个风险：在非正常情况下，网络延迟比较大会出现session timeout，zookeeper就会认为该client已关闭，从而销毁其id标示，竞争资源的下一个id就可以获取锁。这时可能会有两个process同时拿到锁在跑任务，所以设置好session timeout很重要。
- 同样使用PERSISTENT同样会存在一个死锁的风险，进程异常退出后，对应的竞争资源id一直没有删除，下一个id一直无法获取到锁对象。
没有两全其美的做法，两者取其一，选择自己一个能接受的即可
代码
Java代码 
- public class DistributedLock {
- private static final byte[] data = { 0x12, 0x34 };
- private ZooKeeperx zookeeper = ZooKeeperClient.getInstance();
- private final String root; //根节点路径
- private String id;
- private LockNode idName;
- private String ownerId;
- private String lastChildId;
- private Throwable other = null;
- private KeeperException exception = null;
- private InterruptedException interrupt = null;
- public DistributedLock(String root) {
- this.root = root;
- ensureExists(root);
- }
- /**
- * 尝试获取锁操作，阻塞式可被中断
- */
- public void lock() throws InterruptedException, KeeperException {
- // 可能初始化的时候就失败了
- if (exception != null) {
- throw exception;
- }
- if (interrupt != null) {
- throw interrupt;
- }
- if (other != null) {
- throw new NestableRuntimeException(other);
- }
- if (isOwner()) {//锁重入
- return;
- }
- BooleanMutex mutex = new BooleanMutex();
- acquireLock(mutex);
- // 避免zookeeper重启后导致watcher丢失，会出现死锁使用了超时进行重试
- try {
- mutex.get(DEFAULT_TIMEOUT_PERIOD, TimeUnit.MICROSECONDS);// 阻塞等待值为true
- // mutex.get();
- } catch (TimeoutException e) {
- if (!mutex.state()) {
- lock();
- }
- }
- if (exception != null) {
- throw exception;
- }
- if (interrupt != null) {
- throw interrupt;
- }
- if (other != null) {
- throw new NestableRuntimeException(other);
- }
- }
- /**
- * 尝试获取锁对象, 不会阻塞
- *
- * @throws InterruptedException
- * @throws KeeperException
- */
- public boolean tryLock() throws KeeperException {
- // 可能初始化的时候就失败了
- if (exception != null) {
- throw exception;
- }
- if (isOwner()) {//锁重入
- return true;
- }
- acquireLock(null);
- if (exception != null) {
- throw exception;
- }
- if (interrupt != null) {
- Thread.currentThread().interrupt();
- }
- if (other != null) {
- throw new NestableRuntimeException(other);
- }
- return isOwner();
- }
- /**
- * 释放锁对象
- */
- public void unlock() throws KeeperException {
- if (id != null) {
- try {
- zookeeper.delete(root + "/" + id, -1);
- } catch (InterruptedException e) {
- Thread.currentThread().interrupt();
- } catch (KeeperException.NoNodeException e) {
- // do nothing
- } finally {
- id = null;
- }
- } else {
- //do nothing
- }
- }
- private void ensureExists(final String path) {
- try {
- Stat stat = zookeeper.exists(path, false);
- if (stat != null) {
- return;
- }
- zookeeper.create(path, data, CreateMode.PERSISTENT);
- } catch (KeeperException e) {
- exception = e;
- } catch (InterruptedException e) {
- Thread.currentThread().interrupt();
- interrupt = e;
- }
- }
- /**
- * 返回锁对象对应的path
- */
- public String getRoot() {
- return root;
- }
- /**
- * 判断当前是不是锁的owner
- */
- public boolean isOwner() {
- return id != null && ownerId != null && id.equals(ownerId);
- }
- /**
- * 返回当前的节点id
- */
- public String getId() {
- return this.id;
- }
- // ===================== helper method =============================
- /**
- * 执行lock操作，允许传递watch变量控制是否需要阻塞lock操作
- */
- private Boolean acquireLock(final BooleanMutex mutex) {
- try {
- do {
- if (id == null) {//构建当前lock的唯一标识
- long sessionId = zookeeper.getDelegate().getSessionId();
- String prefix = "x-" + sessionId + "-";
- //如果第一次，则创建一个节点
- String path = zookeeper.create(root + "/" + prefix, data,
- CreateMode.EPHEMERAL_SEQUENTIAL);
- int index = path.lastIndexOf("/");
- id = StringUtils.substring(path, index + 1);
- idName = new LockNode(id);
- }
- if (id != null) {
- List<String> names = zookeeper.getChildren(root, false);
- if (names.isEmpty()) {
- id = null;//异常情况，重新创建一个
- } else {
- //对节点进行排序
- SortedSet<LockNode> sortedNames = new TreeSet<LockNode>();
- for (String name : names) {
- sortedNames.add(new LockNode(name));
- }
- if (sortedNames.contains(idName) == false) {
- id = null;//清空为null，重新创建一个
- continue;
- }
- //将第一个节点做为ownerId
- ownerId = sortedNames.first().getName();
- if (mutex != null && isOwner()) {
- mutex.set(true);//直接更新状态，返回
- return true;
- } else if (mutex == null) {
- return isOwner();
- }
- SortedSet<LockNode> lessThanMe = sortedNames.headSet(idName);
- if (!lessThanMe.isEmpty()) {
- //关注一下排队在自己之前的最近的一个节点
- LockNode lastChildName = lessThanMe.last();
- lastChildId = lastChildName.getName();
- //异步watcher处理
- zookeeper.exists(root + "/" + lastChildId, new AsyncWatcher() {
- public void asyncProcess(WatchedEvent event) {
- acquireLock(mutex);
- }
- });
- if (stat == null) {
- acquireLock(mutex);// 如果节点不存在，需要自己重新触发一下，watcher不会被挂上去
- }
- } else {
- if (isOwner()) {
- mutex.set(true);
- } else {
- id = null;// 可能自己的节点已超时挂了，所以id和ownerId不相同
- }
- }
- }
- }
- } while (id == null);
- } catch (KeeperException e) {
- exception = e;
- if (mutex != null) {
- mutex.set(true);
- }
- } catch (InterruptedException e) {
- interrupt = e;
- if (mutex != null) {
- mutex.set(true);
- }
- } catch (Throwable e) {
- other = e;
- if (mutex != null) {
- mutex.set(true);
- }
- }
- if (isOwner() && mutex != null) {
- mutex.set(true);
- }
- return Boolean.FALSE;
- }
- }
public class DistributedLock {
private static final byte[] data = { 0x12, 0x34 };
private ZooKeeperx zookeeper = ZooKeeperClient.getInstance();
private final String root; //根节点路径
private String id;
private LockNode idName;
private String ownerId;
private String lastChildId;
private Throwable other = null;
private KeeperException exception = null;
private InterruptedException interrupt = null;
public DistributedLock(String root) {
this.root = root;
ensureExists(root);
}
/**
* 尝试获取锁操作，阻塞式可被中断
*/
public void lock() throws InterruptedException, KeeperException {
// 可能初始化的时候就失败了
if (exception != null) {
throw exception;
}
if (interrupt != null) {
throw interrupt;
}
if (other != null) {
throw new NestableRuntimeException(other);
}
if (isOwner()) {//锁重入
return;
}
BooleanMutex mutex = new BooleanMutex();
acquireLock(mutex);
// 避免zookeeper重启后导致watcher丢失，会出现死锁使用了超时进行重试
try {
mutex.get(DEFAULT_TIMEOUT_PERIOD, TimeUnit.MICROSECONDS);// 阻塞等待值为true
// mutex.get();
} catch (TimeoutException e) {
if (!mutex.state()) {
lock();
}
}
if (exception != null) {
throw exception;
}
if (interrupt != null) {
throw interrupt;
}
if (other != null) {
throw new NestableRuntimeException(other);
}
}
/**
* 尝试获取锁对象, 不会阻塞
*
* @throws InterruptedException
* @throws KeeperException
*/
public boolean tryLock() throws KeeperException {
// 可能初始化的时候就失败了
if (exception != null) {
throw exception;
}
if (isOwner()) {//锁重入
return true;
}
acquireLock(null);
if (exception != null) {
throw exception;
}
if (interrupt != null) {
Thread.currentThread().interrupt();
}
if (other != null) {
throw new NestableRuntimeException(other);
}
return isOwner();
}
/**
* 释放锁对象
*/
public void unlock() throws KeeperException {
if (id != null) {
try {
zookeeper.delete(root + "/" + id, -1);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
} catch (KeeperException.NoNodeException e) {
// do nothing
} finally {
id = null;
}
} else {
//do nothing
}
}
private void ensureExists(final String path) {
try {
Stat stat = zookeeper.exists(path, false);
if (stat != null) {
return;
}
zookeeper.create(path, data, CreateMode.PERSISTENT);
} catch (KeeperException e) {
exception = e;
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
interrupt = e;
}
}
/**
* 返回锁对象对应的path
*/
public String getRoot() {
return root;
}
/**
* 判断当前是不是锁的owner
*/
public boolean isOwner() {
return id != null && ownerId != null && id.equals(ownerId);
}
/**
* 返回当前的节点id
*/
public String getId() {
return this.id;
}
// ===================== helper method =============================
/**
* 执行lock操作，允许传递watch变量控制是否需要阻塞lock操作
*/
private Boolean acquireLock(final BooleanMutex mutex) {
try {
do {
if (id == null) {//构建当前lock的唯一标识
long sessionId = zookeeper.getDelegate().getSessionId();
String prefix = "x-" + sessionId + "-";
//如果第一次，则创建一个节点
String path = zookeeper.create(root + "/" + prefix, data,
CreateMode.EPHEMERAL_SEQUENTIAL);
int index = path.lastIndexOf("/");
id = StringUtils.substring(path, index + 1);
idName = new LockNode(id);
}
if (id != null) {
List<String> names = zookeeper.getChildren(root, false);
if (names.isEmpty()) {
id = null;//异常情况，重新创建一个
} else {
//对节点进行排序
SortedSet<LockNode> sortedNames = new TreeSet<LockNode>();
for (String name : names) {
sortedNames.add(new LockNode(name));
}
if (sortedNames.contains(idName) == false) {
id = null;//清空为null，重新创建一个
continue;
}
//将第一个节点做为ownerId
ownerId = sortedNames.first().getName();
if (mutex != null && isOwner()) {
mutex.set(true);//直接更新状态，返回
return true;
} else if (mutex == null) {
return isOwner();
}
SortedSet<LockNode> lessThanMe = sortedNames.headSet(idName);
if (!lessThanMe.isEmpty()) {
//关注一下排队在自己之前的最近的一个节点
LockNode lastChildName = lessThanMe.last();
lastChildId = lastChildName.getName();
//异步watcher处理
zookeeper.exists(root + "/" + lastChildId, new AsyncWatcher() {
public void asyncProcess(WatchedEvent event) {
acquireLock(mutex);
}
});
if (stat == null) {
acquireLock(mutex);// 如果节点不存在，需要自己重新触发一下，watcher不会被挂上去
}
} else {
if (isOwner()) {
mutex.set(true);
} else {
id = null;// 可能自己的节点已超时挂了，所以id和ownerId不相同
}
}
}
}
} while (id == null);
} catch (KeeperException e) {
exception = e;
if (mutex != null) {
mutex.set(true);
}
} catch (InterruptedException e) {
interrupt = e;
if (mutex != null) {
mutex.set(true);
}
} catch (Throwable e) {
other = e;
if (mutex != null) {
mutex.set(true);
}
}
if (isOwner() && mutex != null) {
mutex.set(true);
}
return Boolean.FALSE;
}
}
相关说明：
测试代码：
Java代码
- @Test
- public void test_lock() {
- ExecutorService exeucotr = Executors.newCachedThreadPool();
- final int count = 50;
- final CountDownLatch latch = new CountDownLatch(count);
- final DistributedLock[] nodes = new DistributedLock[count];
- for (int i = 0; i < count; i++) {
- final DistributedLock node = new DistributedLock(dir);
- nodes[i] = node;
- exeucotr.submit(new Runnable() {
- public void run() {
- try {
- Thread.sleep(1000);
- node.lock(); //获取锁
- Thread.sleep(100 + RandomUtils.nextInt(100));
- System.out.println("id: " + node.getId() + " is leader: " + node.isOwner());
- } catch (InterruptedException e) {
- want.fail();
- } catch (KeeperException e) {
- want.fail();
- } finally {
- latch.countDown();
- try {
- node.unlock();
- } catch (KeeperException e) {
- want.fail();
- }
- }
- }
- });
- }
- try {
- latch.await();
- } catch (InterruptedException e) {
- want.fail();
- }
- exeucotr.shutdown();
- }
@Test
public void test_lock() {
ExecutorService exeucotr = Executors.newCachedThreadPool();
final int count = 50;
final CountDownLatch latch = new CountDownLatch(count);
final DistributedLock[] nodes = new DistributedLock[count];
for (int i = 0; i < count; i++) {
final DistributedLock node = new DistributedLock(dir);
nodes[i] = node;
exeucotr.submit(new Runnable() {
public void run() {
try {
Thread.sleep(1000);
node.lock(); //获取锁
Thread.sleep(100 + RandomUtils.nextInt(100));
System.out.println("id: " + node.getId() + " is leader: " + node.isOwner());
} catch (InterruptedException e) {
want.fail();
} catch (KeeperException e) {
want.fail();
} finally {
latch.countDown();
try {
node.unlock();
} catch (KeeperException e) {
want.fail();
}
}
}
});
}
try {
latch.await();
} catch (InterruptedException e) {
want.fail();
}
exeucotr.shutdown();
}
升级版
实现了一个分布式lock后，可以解决多进程之间的同步问题，但设计多线程+多进程的lock控制需求，单jvm中每个线程都和zookeeper进行网络交互成本就有点高了，所以基于DistributedLock，实现了一个分布式二层锁。
大致原理就是ReentrantLock 和 DistributedLock的一个结合。
- 单jvm的多线程竞争时，首先需要先拿到第一层的ReentrantLock的锁
- 拿到锁之后这个线程再去和其他JVM的线程竞争锁，最后拿到之后锁之后就开始处理任务。
锁的释放过程是一个反方向的操作，先释放DistributedLock，再释放ReentrantLock。 可以思考一下，如果先释放ReentrantLock，假如这个JVM ReentrantLock竞争度比较高，一直其他JVM的锁竞争容易被饿死。
代码：
Java代码
- public class DistributedReentrantLock extends DistributedLock {
- private static final String ID_FORMAT = "Thread[{0}] Distributed[{1}]";
- private ReentrantLock reentrantLock = new ReentrantLock();
- public DistributedReentrantLock(String root) {
- super(root);
- }
- public void lock() throws InterruptedException, KeeperException {
- reentrantLock.lock();//多线程竞争时，先拿到第一层锁
- super.lock();
- }
- public boolean tryLock() throws KeeperException {
- //多线程竞争时，先拿到第一层锁
- return reentrantLock.tryLock() && super.tryLock();
- }
- public void unlock() throws KeeperException {
- super.unlock();
- reentrantLock.unlock();//多线程竞争时，释放最外层锁
- }
- @Override
- public String getId() {
- return MessageFormat.format(ID_FORMAT, Thread.currentThread().getId(), super.getId());
- }
- @Override
- public boolean isOwner() {
- return reentrantLock.isHeldByCurrentThread() && super.isOwner();
- }
- }
public class DistributedReentrantLock extends DistributedLock {
private static final String ID_FORMAT = "Thread[{0}] Distributed[{1}]";
private ReentrantLock reentrantLock = new ReentrantLock();
public DistributedReentrantLock(String root) {
super(root);
}
public void lock() throws InterruptedException, KeeperException {
reentrantLock.lock();//多线程竞争时，先拿到第一层锁
super.lock();
}
public boolean tryLock() throws KeeperException {
//多线程竞争时，先拿到第一层锁
return reentrantLock.tryLock() && super.tryLock();
}
public void unlock() throws KeeperException {
super.unlock();
reentrantLock.unlock();//多线程竞争时，释放最外层锁
}
@Override
public String getId() {
return MessageFormat.format(ID_FORMAT, Thread.currentThread().getId(), super.getId());
}
@Override
public boolean isOwner() {
return reentrantLock.isHeldByCurrentThread() && super.isOwner();
}
}
测试代码：
Java代码
- @Test
- public void test_lock() {
- ExecutorService exeucotr = Executors.newCachedThreadPool();
- final int count = 50;
- final CountDownLatch latch = new CountDownLatch(count);
- final DistributedReentrantLock lock = new DistributedReentrantLock(dir); //单个锁
- for (int i = 0; i < count; i++) {
- exeucotr.submit(new Runnable() {
- public void run() {
- try {
- Thread.sleep(1000);
- lock.lock();
- Thread.sleep(100 + RandomUtils.nextInt(100));
- System.out.println("id: " + lock.getId() + " is leader: " + lock.isOwner());
- } catch (InterruptedException e) {
- want.fail();
- } catch (KeeperException e) {
- want.fail();
- } finally {
- latch.countDown();
- try {
- lock.unlock();
- } catch (KeeperException e) {
- want.fail();
- }
- }
- }
- });
- }
- try {
- latch.await();
- } catch (InterruptedException e) {
- want.fail();
- }
- exeucotr.shutdown();
- }
@Test
public void test_lock() {
ExecutorService exeucotr = Executors.newCachedThreadPool();
final int count = 50;
final CountDownLatch latch = new CountDownLatch(count);
final DistributedReentrantLock lock = new DistributedReentrantLock(dir); //单个锁
for (int i = 0; i < count; i++) {
exeucotr.submit(new Runnable() {
public void run() {
try {
Thread.sleep(1000);
lock.lock();
Thread.sleep(100 + RandomUtils.nextInt(100));
System.out.println("id: " + lock.getId() + " is leader: " + lock.isOwner());
} catch (InterruptedException e) {
want.fail();
} catch (KeeperException e) {
want.fail();
} finally {
latch.countDown();
try {
lock.unlock();
} catch (KeeperException e) {
want.fail();
}
}
}
});
}
try {
latch.await();
} catch (InterruptedException e) {
want.fail();
}
exeucotr.shutdown();
}
最后
其实再可以发散一下，实现一个分布式的read/write lock，也差不多就是这个理了。项目结束后，有时间可以写一下
大致思路：
- 竞争资源标示： read_自增id , write_自增id
- 首先按照自增id进行排序，如果队列的前边都是read标识，对应的所有read都获得锁。如果队列的前边是write标识，第一个write节点获取锁
- watcher监听： read监听距离自己最近的一个write节点的exist，write监听距离自己最近的一个节点(read或者write节点)
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public class LockNode implements Comparable<LockNode> { private final String name; private String prefix; private int sequence = -1; public LockNode(String name){ Assert.notNull(name, "id cannot be null"); this.name = name; this.prefix = name; int idx = name.lastIndexOf('-'); if (idx >= 0) { this.prefix = name.substring(0, idx); try { this.sequence = Integer.parseInt(name.substring(idx + 1)); } catch (Exception e) { // ignore } } } public int compareTo(LockNode that) { int s1 = this.sequence; int s2 = that.sequence; if (s1 == -1 && s2 == -1) { return this.name.compareTo(that.name); } if (s1 == -1) { return -1; } else if (s2 == -1) { return 1; } else { return s1 - s2; } } public String getName() { return name; } public int getSequence() { return sequence; } public String getPrefix() { return prefix; } public String toString() { return name.toString(); } // ==================== hashcode & equals方法======================= @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((name == null) ? 0 : name.hashCode()); return result; } @Override public boolean equals(Object obj) { if (this == obj) { return true; } if (obj == null) { return false; } if (getClass() != obj.getClass()) { return false; } LockNode other = (LockNode) obj; if (name == null) { if (other.name != null) { return false; } } else if (!name.equals(other.name)) { return false; } return true; } }