zookeeper适用场景：分布式锁实现

问题导读：
1.zookeeper如何实现分布式锁？
2.什么是羊群效应？
3.zookeeper如何释放锁？






在zookeeper应用场景有关于分布式集群配置文件同步问题的描述，设想一下如果有100台机器同时对同一台机器上某个文件进行修改，如何才能保证文本不会被写乱，这就是最简单的分布式锁，本文介绍利用zk实现分布式锁。下面是写锁的实现步骤

分布式写锁
create一个PERSISTENT类型的znode，/Locks/write_lock

• 客户端创建SEQUENCE|EPHEMERAL类型的znode，名字是lockid开头，创建的znode是/Locks/write_lock/lockid0000000001
• 调用getChildren()不要设置Watcher获取/Locks/write_lock下的znode列表
• 判断自己步骤2创建znode是不是znode列表中最小的一个，如果是就代表获得了锁，如果不是往下走
• 调用exists()判断步骤2自己创建的节点编号小1的znode节点(也就是获取的znode节点列表中最小的znode)，并且设置Watcher，如果exists()返回false，执行步骤3
• 如果exists()返回true,那么等待zk通知，从而在回掉函数里返回执行步骤3

释放锁就是删除znode节点或者断开连接就行

*注意：上面步骤2中getChildren()不设置Watcher的原因是，防止羊群效应，如果getChildren()设置了Watcher，那么集群一抖动都会收到通知。在整个分布式锁的竞争过程中，大量重复运行，并且绝大多数的运行结果都是判断出自己并非是序号最小的节点，从而继续等待下一次通知—，这个显然看起来不怎么科学。客户端无端的接受到过多的和自己不相关的事件通知，这如果在集群规模大的时候，会对Server造成很大的性能影响，并且如果一旦同一时间有多个节点的客户端断开连接，这个时候，服务器就会像其余客户端发送大量的事件通知——这就是所谓的羊群效应。
下面是代码实现

import sys

class GJZookeeper(object):

    ZK_HOST = "localhost:2181"
    ROOT = "/Locks"
    WORKERS_PATH = join(ROOT,"write_lock")
    MASTERS_NUM = 1
    TIMEOUT = 10000

    def __init__(self, verbose = True):
        self.VERBOSE = verbose
        self.masters = []
        self.is_master = False
        self.path = None

        self.zk = ZKClient(self.ZK_HOST, timeout = self.TIMEOUT)
        self.say("login ok!")
        # init
        self.__init_zk()
        # register
        self.register()

    def __init_zk(self):
        """
        create the zookeeper node if not exist
        |--Locks
                |--write_lock
        """
        nodes = (self.ROOT, self.WORKERS_PATH)
        for node in nodes: 
            if not self.zk.exists(node):
                try:
                    self.zk.create(node, "")
                except:
                    pass

    def register(self):
        """
        register a node for this worker
        |--Locks
                |--write_lock
                            |--lockid000000000x ==> hostname
        """

        import socket
        hostname = socket.gethostname()
        self.path = self.zk.create(self.WORKERS_PATH + "/lockid", hostname, flags=zookeeper.EPHEMERAL | zookeeper.SEQUENCE)
        self.lockid = basename(self.path)
        self.say("register ok! I'm %s" % self.path)
        # check who is the master
        self.get_lock()

    def get_lock(self):
        """
        get children znode try to get lock
        """
        @watchmethod
        def watcher(event):
            self.say("child changed, try to get lock again.")
            self.get_lock()

        children = self.zk.get_children(self.WORKERS_PATH)
        children.sort()
        min_lock_id = children[0]
        self.say("%s's children: %s" % (self.WORKERS_PATH, children)) 
        if cmp(self.lockid,min_lock_id) == 0:
            self.get_lock_success()
            return True
        elif cmp(self.lockid,min_lock_id) > 0:
            index = children.index(self.lockid)
            new_lockid_watch = join(self.WORKERS_PATH,children[index-1])
            self.say("Add watch on %s"%new_lockid_watch)
            res = self.zk.exists(new_lockid_watch,watcher)
            if not res :
                """代表没有存在之前小的锁，但是这并不意味着能拿到锁了，因为还可能有比当前还小的锁，还没轮到，要重新执行一遍"""
#               self.get_lock_success()
                return False
            else :
                """现在的锁有人在使用，等他释放了再抢"""
                self.say("I can not get the lock this time,wait for the next time")
                return False

    def get_lock_success(self):
        self.say("I get the lock !!!")
        self.write_file()
        self.zk.delete(join(self.WORKERS_PATH,self.lockid))
        self.say("I release the lock !!!")
        sys.exit(1)

    def write_file(self):
        fd = open("lock.log",'a')
        fd.write("%s
"%self.lockid)
        fd.close()

    def say(self, msg):
        """
        print messages to screen
        """
        if self.VERBOSE:
            if self.path:
                log.info("[ %s(%s) ] %s" % (self.path, "master" if self.is_master else "slave", msg))
            else:
                log.info(msg)

def start_get_lock():
    gj_zookeeper = GJZookeeper()

def main():
    th1 = threading.Thread(target = start_get_lock, name = "thread_1", args = ())
    th1.start()
    th1.join()
    
if __name__ == "__main__":
    main()
    time.sleep(1000)

　　文章转自：http://www.aboutyun.com/forum.php?mod=viewthread&tid=9267&ctid=16