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()

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 <[EMAIL PROTECTED]> 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',

Interesting - this issue has come up several times with Curator users. I
ended up writing a Tech Note on it.

https://github.com/Netflix/curator/wiki/Tech-Note-1
-JZ

On 5/9/12 1:23 PM, "Patrick Hunt" <[EMAIL PROTECTED]> wrote:

>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 <[EMAIL PROTECTED]> 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

Good point Jordan, I added a jira for this:
https://issues.apache.org/jira/browse/ZOOKEEPER-1464

Patrick

On Wed, May 9, 2012 at 1:27 PM, Jordan Zimmerman <[EMAIL PROTECTED]> wrote:
> Interesting - this issue has come up several times with Curator users. I
> ended up writing a Tech Note on it.
>
> https://github.com/Netflix/curator/wiki/Tech-Note-1
>
>
> -JZ
>
> On 5/9/12 1:23 PM, "Patrick Hunt" <[EMAIL PROTECTED]> wrote:
>
>>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 <[EMAIL PROTECTED]> 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',