1、写在前面

以前写过一篇文章（《拜托，面试请不要再问我Redis分布式锁的实现原理》），给你们说了一下Redisson这个开源框架是如何实现Redis分布式锁原理的，这篇文章再给你们聊一下ZooKeeper实现分布式锁的原理。性能优化

同理，我是直接基于比较经常使用的Curator这个开源框架，聊一下这个框架对ZooKeeper（如下简称zk）分布式锁的实现。微信

通常除了大公司是自行封装分布式锁框架以外，建议你们用这些开源框架封装好的分布式锁实现，这是一个比较快捷省事儿的方式。markdown

2、ZooKeeper分布式锁机制

接下来咱们一块儿来看看，多客户端获取及释放zk分布式锁的整个流程及背后的原理。架构

首先你们看看下面的图，若是如今有两个客户端一块儿要争抢zk上的一把分布式锁，会是个什么场景？并发

若是你们对zk还不太了解的话，建议先自行百度一下，简单了解点基本概念，好比zk有哪些节点类型等等。app

参见上图。zk里有一把锁，这个锁就是zk上的一个节点。而后呢，两个客户端都要来获取这个锁，具体是怎么来获取呢？框架

我们就假设客户端A抢先一步，对zk发起了加分布式锁的请求，这个加锁请求是用到了zk中的一个特殊的概念，叫作**“临时顺序节点”。**

简单来讲，就是直接在"my_lock"这个锁节点下，建立一个顺序节点，这个顺序节点有zk内部自行维护的一个节点序号。

好比说，第一个客户端来搞一个顺序节点，zk内部会给起个名字叫作：xxx-000001。而后第二个客户端来搞一个顺序节点，zk可能会起个名字叫作：xxx-000002。你们注意一下，最后一个数字都是依次递增的，从1开始逐次递增。zk会维护这个顺序。

因此这个时候，假如说客户端A先发起请求，就会搞出来一个顺序节点，你们看下面的图，Curator框架大概会弄成以下的样子：

你们看，客户端A发起一个加锁请求，先会在你要加锁的node下搞一个临时顺序节点，这一大坨长长的名字都是Curator框架本身生成出来的。

而后，那个最后一个数字是"1"。你们注意一下，由于客户端A是第一个发起请求的，因此给他搞出来的顺序节点的序号是"1"。

接着客户端A建立完一个顺序节点。还没完，他会查一下"my_lock"这个锁节点下的全部子节点，而且这些子节点是按照序号排序的，这个时候他大概会拿到这么一个集合：

接着客户端A会走一个关键性的判断，就是说：唉！兄弟，这个集合里，我建立的那个顺序节点，是否是排在第一个啊？

若是是的话，那我就能够加锁了啊！由于明明我就是第一个来建立顺序节点的人，因此我就是第一个尝试加分布式锁的人啊！

bingo！加锁成功！你们看下面的图，再来直观的感觉一下整个过程。

接着假如说，客户端A都加完锁了，客户端B过来想要加锁了，这个时候他会干同样的事儿：先是在"my_lock"这个锁节点下建立一个临时顺序节点，此时名字会变成相似于：

你们看看下面的图：

客户端B由于是第二个来建立顺序节点的，因此zk内部会维护序号为"2"。

接着客户端B会走加锁判断逻辑，查询"my_lock"锁节点下的全部子节点，按序号顺序排列，此时他看到的相似于：

同时检查本身建立的顺序节点，是否是集合中的第一个？

明显不是啊，此时第一个是客户端A建立的那个顺序节点，序号为"01"的那个。因此加锁失败！

加锁失败了之后，客户端B就会经过ZK的API对他的顺序节点的**上一个顺序节点加一个监听器。**zk自然就能够实现对某个节点的监听。

若是你们还不知道zk的基本用法，能够百度查阅，很是的简单。客户端B的顺序节点是：

他的上一个顺序节点，不就是下面这个吗？

即客户端A建立的那个顺序节点！

因此，客户端B会对：

这个节点加一个监听器，监听这个节点是否被删除等变化！你们看下面的图。

接着，客户端A加锁以后，可能处理了一些代码逻辑，而后就会释放锁。那么，释放锁是个什么过程呢？

其实很简单，就是把本身在zk里建立的那个顺序节点，也就是：

这个节点给删除。

删除了那个节点以后，zk会负责通知监听这个节点的监听器，也就是客户端B以前加的那个监听器，说：兄弟，你监听的那个节点被删除了，有人释放了锁。

此时客户端B的监听器感知到了上一个顺序节点被删除，也就是排在他以前的某个客户端释放了锁。

此时，就会通知客户端B从新尝试去获取锁，也就是获取"my_lock"节点下的子节点集合，此时为：

集合里此时只有客户端B建立的惟一的一个顺序节点了！

而后呢，客户端B判断本身竟然是集合中的第一个顺序节点，bingo！能够加锁了！直接完成加锁，运行后续的业务代码便可，运行完了以后再次释放锁。

3、总结

其实若是有客户端C、客户端D等N个客户端争抢一个zk分布式锁，原理都是相似的。

你们都是上来直接建立一个锁节点下的一个接一个的临时顺序节点
若是本身不是第一个节点，就对本身上一个节点加监听器
只要上一个节点释放锁，本身就排到前面去了，至关因而一个排队机制。

并且用临时顺序节点的另一个用意就是，若是某个客户端建立临时顺序节点以后，不当心本身宕机了也不要紧，zk感知到那个客户端宕机，会自动删除对应的临时顺序节点，至关于自动释放锁，或者是自动取消本身的排队。

最后，我们来看下用Curator框架进行加锁和释放锁的一个过程：

其实用开源框架就是这点好，方便。这个Curator框架的zk分布式锁的加锁和释放锁的实现原理，其实就是上面咱们说的那样子。

可是若是你要手动实现一套那个代码的话。仍是有点麻烦的，要考虑到各类细节，异常处理等等。因此你们若是考虑用zk分布式锁，能够参考下本文的思路。

END

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七张图完全讲清楚ZooKeeper分布式锁的实现原理【石杉的架构笔记】

1、写在前面

2、ZooKeeper分布式锁机制

3、总结