分布式锁之Zookeeper
zookeeper
zookeeper介绍
ZooKeeper是一种用于分布式应用程序的分布式开源协调服务。它公开了一组简单的原语，分布式应用程序可以构建使用这些原语，以实现更高级别的服务，如实现同步，配置维护以及组和命名。 它被设计为易于编程，并使用在熟悉的文件系统目录树结构之后设计的数据模型。
Curator介绍
Curator是ZooKeeper的一个Java客户端框架, 它将zk本身提供的原子功能进行功能性封装, 提供诸如客户端断线自动重连, 分布式锁, 选举, 服务发现等开箱即用的功能;
锁的实现代码
- Curator提供了一个跨JVM进程的可重入互斥锁实现-InterProcessMutex;
- 申请获取锁
a. 获取锁, 支持重入, 并且有超时获取时间
private boolean internalLock(long time, TimeUnit unit) throws Exception
{
/*
Note on concurrency: a given lockData instance
can be only acted on by a single thread so locking isn't necessary
*/
Thread currentThread = Thread.currentThread();
// 本地存储获得锁的线程, 在锁的元信息中保存重入标志
LockData lockData = threadData.get(currentThread);
if ( lockData != null )
{
// re-entering
lockData.lockCount.incrementAndGet();
return true;
}
// 在指定时间内获取锁
String lockPath = internals.attemptLock(time, unit, getLockNodeBytes());
if ( lockPath != null )
{
LockData newLockData = new LockData(currentThread, lockPath);
threadData.put(currentThread, newLockData);
return true;
}
return false;
}
b. 本地内存中标识当前线程没有获得锁, 那就要去zk中去获取锁了;
String attemptLock(long time, TimeUnit unit, byte[] lockNodeBytes) throws Exception
{
final long startMillis = System.currentTimeMillis();
final Long millisToWait = (unit != null) ? unit.toMillis(time) : null;
final byte[] localLockNodeBytes = (revocable.get() != null) ? new byte[0] : lockNodeBytes;
int retryCount = 0;
String ourPath = null;
boolean hasTheLock = false;
boolean isDone = false;
// 轮询获取锁
while ( !isDone )
{
isDone = true;
try
{
// 在zk指定namespace下的锁节点中创建当前线程的锁节点
ourPath = driver.createsTheLock(client, path, localLockNodeBytes);
// 在一个内部循环中获取锁
hasTheLock = internalLockLoop(startMillis, millisToWait, ourPath);
}
catch ( KeeperException.NoNodeException e )
{
// gets thrown by StandardLockInternalsDriver when it can't find the lock node
// this can happen when the session expires, etc. So, if the retry allows, just try it all again
if ( client.getZookeeperClient().getRetryPolicy().allowRetry(retryCount++, System.currentTimeMillis() - startMillis, RetryLoop.getDefaultRetrySleeper()) )
{
isDone = false;
}
else
{
throw e;
}
}
}
if ( hasTheLock )
{
return ourPath;
}
return null;
}
c. 因为上一步创建的锁节点是有序的, 所以这里设置当前线程获取锁znode监听它的前节点状态然后进行wait状态, 等待被唤醒; 如果获取锁带了超时时间就会进行带超时时间的等待状态;
private boolean internalLockLoop(long startMillis, Long millisToWait, String ourPath) throws Exception
{
boolean haveTheLock = false;
boolean doDelete = false;
try
{
if ( revocable.get() != null )
{
client.getData().usingWatcher(revocableWatcher).forPath(ourPath);
}
while ( (client.getState() == CuratorFrameworkState.STARTED) && !haveTheLock )
{
List children = getSortedChildren();
String sequenceNodeName = ourPath.substring(basePath.length() + 1); // +1 to include the slash
PredicateResults predicateResults = driver.getsTheLock(client, children, sequenceNodeName, maxLeases);
if ( predicateResults.getsTheLock() )
{
haveTheLock = true;
}
else
{
String previousSequencePath = basePath + "/" + predicateResults.getPathToWatch();
synchronized(this)
{
try
{
// use getData() instead of exists() to avoid leaving unneeded watchers which is a type of resource leak
// 监听前驱节点
client.getData().usingWatcher(watcher).forPath(previousSequencePath);
// 进入等待状态
if ( millisToWait != null )
{
millisToWait -= (System.currentTimeMillis() - startMillis);
startMillis = System.currentTimeMillis();
if ( millisToWait <= 0 )
{
doDelete = true; // timed out - delete our node
break;
}
wait(millisToWait);
}
else
{
wait();
}
}
catch ( KeeperException.NoNodeException e )
{
// it has been deleted (i.e. lock released). Try to acquire again
}
}
}
}
}
catch ( Exception e )
{
ThreadUtils.checkInterrupted(e);
doDelete = true;
throw e;
}
finally
{
if ( doDelete )
{
deleteOurPath(ourPath);
}
}
return haveTheLock;
}
d. 判断当前线程是否能够获得锁, 即判断它的锁节点znode是否在顺序第一位;
@Override
public PredicateResults getsTheLock(CuratorFramework client, List children, String sequenceNodeName, int maxLeases) throws Exception
{
int ourIndex = children.indexOf(sequenceNodeName);
validateOurIndex(sequenceNodeName, ourIndex);
// maxLeases默认为1, 如果当前线程znode的索引为0, 即表示能够获得锁
boolean getsTheLock = ourIndex < maxLeases;
String pathToWatch = getsTheLock ? null : children.get(ourIndex - maxLeases);
return new PredicateResults(pathToWatch, getsTheLock);
}
- 释放锁
a. 先处理锁重入的释放, 直接对锁重入的计数减一; 当锁重入数为0时, 那就要进行zk进行节点删除释放锁了;
public void release() throws Exception
{
/*
Note on concurrency: a given lockData instance
can be only acted on by a single thread so locking isn't necessary
*/
Thread currentThread = Thread.currentThread();
LockData lockData = threadData.get(currentThread);
if ( lockData == null )
{
throw new IllegalMonitorStateException("You do not own the lock: " + basePath);
}
int newLockCount = lockData.lockCount.decrementAndGet();
if ( newLockCount > 0 )
{
return;
}
if ( newLockCount < 0 )
{
throw new IllegalMonitorStateException("Lock count has gone negative for lock: " + basePath);
}
try
{
internals.releaseLock(lockData.lockPath);
}
finally
{
threadData.remove(currentThread);
}
}
b. 删除锁关联的znode
private void deleteOurPath(String ourPath) throws Exception
{
try
{
client.delete().guaranteed().forPath(ourPath);
}
catch ( KeeperException.NoNodeException e )
{
// ignore - already deleted (possibly expired session, etc.)
}
}
Zookeeper实现分布式锁的原理
- 借助zk集群模式, 实现高可用;
- 通过临时的Znode模式, 保证了获得锁的线程意外挂掉后, 锁可以被zk主动释放掉, 不会发生死锁;
- 使用本地进程内记录锁的信息, 实现锁重入;
- 节点监听机制, 实现锁释放被动通知;