zookeeper适用场景：分布式锁实现

xuxiaohui9216

　　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\n"%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