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From Patrick Hunt <ph...@apache.org>
Subject Re: Watch not sent immediately?
Date Wed, 09 May 2012 20:23:25 GMT
I believe the issue is that there is a single thread updating
watchers. If you block that thread then the event can't be delivered.

Patrick

On Fri, May 4, 2012 at 4:06 AM, guru singh <grsingh750@gmail.com> wrote:
> Hi,
>
> Sorry if the subject is not appropriately titled.
>
> I'm trying to implement a redis-failover solution using zookeeper.
> I've been working with the python binding for zk
> Basically, I have a znode called /master, a watch is set on this so
> that, whenever master changes, self.master is upated
> There is another znode called /errors, a watch is set on this via
> get_children to errors_watcher function.
> My code is supposed to continuously loop and create a childe znode on
> /errors, whenever an error is detected.
> The function errors_watcher, counts the number of children for znode
> /errors, if it exceeds a certain length, it writes a new master
> 'ip:port' to the znode /master, this calls the master watcher and
> updates self.master. I use python's threading.Condition() to block for
> certain operations, for instance initially when znode /master is
> created, I wait() for master_watcher to be called which updates
> self.master and releases the lock. This works as expected, however the
> problem is that when znode /master is changed from within
> errors_watcher, if I wait() for master_watcher to be called, updating
> self.master and then releasing the lock. The code just keeps waiting,
> the master_watcher is never called. However, if I don't wait after
> setting znode /master from within errors_watcher, master_watcher is
> called and it updates self.master.
>
> It'll be really helpful if somebody could point out what's wrong? Is
> it zk or is my understanding of threading.Condition() incorrect?
> Or both :)
> Thanks for your help
>
> This code snippet below, simulates the problem.
>
> class ZKtest:
>
>    def __init__(self,zk_server):
>        zk.set_log_stream(open('zk.log','w'))
>        self.master = None
>        self.zk_server = zk_server
>        self.connected = False
>        self.conn_cv = threading.Condition()
>
>    def global_watcher(self,handle,event,state,path):
>        self.conn_cv.acquire()
>        print 'global watcher called'
>        self.connected = True
>        self.conn_cv.notifyAll()
>        self.conn_cv.release()
>
>    def master_watcher(self,handle,event,state,path):
>        self.conn_cv.acquire()
>        print 'master watcher called'
>        master = zk.get(self.handle,path,self.master_watcher)[0]
>        self.master = master
>        print 'Master is %s' %(master)
>        self.conn_cv.notifyAll()
>        self.conn_cv.release()
>
>    def errors_watcher(self,handle,event,state,path):
>        self.conn_cv.acquire()
>        print 'error watcher called'
>        errors = len(zk.get_children(self.handle,'/errors',self.errors_watcher))
>        print 'Current errors %d' %(errors)
>        if errors > 5 :
>            print 'Set new master, update znode /master'
>            zk.set(self.handle,'/master','127.0.0.1:6380')
>            #self.conn_cv.wait() <-- Why doesn't this return??
>        self.conn_cv.notifyAll()
>        self.conn_cv.release()
>
>
>    def create_znodes(self):
>        self.conn_cv.acquire()
>        master = zk.exists(self.handle,'/master',self.master_watcher)
>        if not master:
>            print 'Creating znode /master'
>            zk.create(self.handle,'/master','127.0.0.1:6379',
>                      [ZOO_OPEN_ACL_UNSAFE])
>        else :
>            print 'Updating znode /master'
>            zk.set(self.handle,'/master','127.0.0.1:6379',master['version'])
>        self.conn_cv.wait() # wait until master_watcher has updated
> self.master, this returns after master_watcher is called
>        print self.master # should not be None, since master_watcher updates it
>        errors = zk.exists(self.handle,'/errors')
>        if not errors:
>            print 'Creating znode /errors'
>            zk.create(self.handle,'/errors','Errors follow',
>                      [ZOO_OPEN_ACL_UNSAFE])
>        else :
>            print 'Purge previous errors'
>            for err in zk.get_children(self.handle,'/errors'):
>                zk.delete(self.handle,'/errors/'+err)
>        err = zk.get_children(self.handle,'/errors',self.errors_watcher)
>    # set a watch for children of znode /errors
>        self.conn_cv.release()
>
>
>   def run(self):
>        self.conn_cv.acquire()
>        self.handle = zk.init(self.zk_server,self.global_watcher)
>        if not self.connected:
>            while not self.connected :
>                print 'Not Connected, retry in 5'
>                self.conn_cv.wait(5)
>                self.handle = zk.init(self.zk_server)
>        self.create_znodes()
>        while self.master != '127.0.0.1:6380':
>            print 'Current Master %s' %(self.master)
>            # simulate errors, until master is not 127.0.0.1:6380
>            zk.create(self.handle,'/errors/','Error!',[ZOO_OPEN_ACL_UNSAFE],
>                      zk.SEQUENCE)
>            self.conn_cv.wait()
>        self.conn_cv.release()
>
>
> if __name__ == '__main__' :
>    zkt = ZKtest('127.0.0.1:2181')
>    zkt.run()

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