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From Alan Cabrera <l...@toolazydogs.com>
Subject Re: Kazoo 'state listener' issue...
Date Mon, 24 Dec 2012 17:28:18 GMT

On Dec 21, 2012, at 10:00 PM, Matt Wise wrote:

> I just want to circle around on this issue.. The root cause was that the Kazoo thread
triggering the 'state listener' callback does so with a lock that it does not release until
after the callbacks are finished. However, if your callback tries to use this thread to make
a Zookeeper call (ie, get, set, create, delete), it waits on that lock and immediately causes
a deadlock.

Ahh, calling user callbacks while holding a lock.  That would do it.  :)

> To handle this scenario they have a 'spawn()' function in the Zookeeper thread handler
that you can use:
> 
>>            self._zk.handler.spawn(self._re_establish_registrations)
> 
> This spawns a thread and immediately returns, allowing the Zookeeper thread to finish
its callbacks and release the lock. Thanks a ton to Ben B. for helping me track that down.

Yeah, Ben is awesome.


Regards,
Alan

> On Dec 10, 2012, at 11:18 AM, Alan Cabrera <list@toolazydogs.com> wrote:
> 
>> Without really digging into this I'll toss in my initial observation.
>> 
>> Calling zk while still being inside a zk callback seems a bit dangerous.  I would
have a queue and event thread and have work from the callbacks feed this queue which would
be executed inside the event thread.
>> 
>> 
>> Regards,
>> Alan
>> 
>> On Dec 9, 2012, at 2:30 PM, Matt Wise wrote:
>> 
>>> Just to clarify, if you go and change test() into:
>>> 
>>>>  def test(self):
>>>>      # now register a node
>>>>      self.register_node('/abc/a')
>>>>      self._zk.stop()
>>>>      self._zk.start()
>>>>      self.register_node('/abc/a')
>>>>      self._zk.get_children('/abc')
>>>> 
>>> 
>>> and then remove these lines from the state_handler() method:
>>>> 
>>>>>         for node in nodes.iteritems():
>>>>>             self.register_node(node[0], data=node[1])
>>>> 
>>> 
>>> then it works perfectly.. no hang, nothing. it seems that the register_node cannot
be called from within the state handler class. Why?
>>> 
>>> On Dec 9, 2012, at 2:26 PM, Matt Wise <matt@nextdoor.com> wrote:
>>> 
>>>> Hrmm here's a cleaner way to reproduce the issue:
>>>> 
>>>> test.py:
>>>>> from kazoo.client import KazooClient
>>>>> from kazoo.client import KazooState
>>>>> from kazoo.handlers.threading import TimeoutError
>>>>> from kazoo.handlers.gevent import SequentialGeventHandler
>>>>> import logging
>>>>> 
>>>>> 
>>>>> class Test(object):
>>>>> def __init__(self):
>>>>>     self.log = logging.getLogger()
>>>>>     format = 'zk_watcher[%(name)s-%(thread)d-%(funcName)s: (%(levelname)s)
%(message)s'
>>>>>     self.log.setLevel(logging.INFO)
>>>>>     formatter = logging.Formatter(format)
>>>>>     handler = logging.StreamHandler()
>>>>>     handler.setFormatter(formatter)
>>>>>     self.log.addHandler(handler)
>>>>> 
>>>>>     self.registered_nodes = {}
>>>>> 
>>>>>     self.log.setLevel(logging.DEBUG)
>>>>> 
>>>>>     self._zk = KazooClient(hosts='localhost:2182', handler=SequentialGeventHandler())
>>>>>     self._zk.start()
>>>>>     self._zk.add_listener(self._state_listener)
>>>>>     self._state_listener(self._zk.state)
>>>>> 
>>>>> def test(self):
>>>>>     # now register a node
>>>>>     self.register_node('/abc/a')
>>>>>     self._zk.stop()
>>>>>     self._zk.start()
>>>>>     self._zk.get_children('/abc')
>>>>> 
>>>>> def register_node(self, node, data=None):
>>>>>     if node in self.registered_nodes:
>>>>>         if data == self.registered_nodes[node]:
>>>>>             self.log.debug('Already registered [%s] in data provider.'
% node)
>>>>>             return
>>>>>     self.log.debug('Registering [%s] in data provider.' % node)
>>>>>     self._zk.create(node, ephemeral=True, makepath=True)
>>>>>     self.registered_nodes[node] = data
>>>>> 
>>>>> 
>>>>> def _state_listener(self,state):
>>>>>     self.log.warning('Zookeeper connection state changed: %s' % state)
>>>>>     if state == KazooState.SUSPENDED:
>>>>>         self.CONNECTION_STATE=False
>>>>>     elif state == KazooState.LOST:
>>>>>         self.CONNECTION_STATE=False
>>>>>     else:
>>>>>         self.CONNECTION_STATE=True
>>>>>         nodes = {}
>>>>>         print self.registered_nodes
>>>>>         try:
>>>>>             nodes = self.registered_nodes
>>>>>         except:
>>>>>             pass
>>>>>         self.registered_nodes = {}
>>>>>         for node in nodes.iteritems():
>>>>>             self.register_node(node[0], data=node[1])
>>>> 
>>>> python
>>>>>>> import test
>>>>>>> k = test.Test()
>>>>>>> k.test()
>>>> 
>>>> (watch it hang ... )
>>>> 
>>>> 
>>>> On Dec 9, 2012, at 1:22 PM, Matt Wise <matt@nextdoor.com> wrote:
>>>> 
>>>>> I've got a weird connection issue playing around with Kazoo... If I do
something simple like:
>>>>> 
>>>>>> k = KazooClient()
>>>>>> k.start()
>>>>>> k.create('/foo')
>>>>>> k.stop()
>>>>>> k.start()
>>>>>> k.create('/foo')
>>>>> 
>>>>> it works fine... the node is re-created, all is happy.
>>>>> 
>>>>> However, if i try to use a state_listener callback to automatically re-register
any paths that we had registered on our first connection, it fails. In fact, it doesn't really
fail .. it hangs. This only happens if we try to do re-register the paths from within the
state listener. If we do it outside of that callback (manually) it works fine. Silly code
snippet that will cause the problem:
>>>>> 
>>>>>> from kazoo.client import KazooClient
>>>>>> from kazoo.client import KazooState
>>>>>> import logging
>>>>>> log = logging.getLogger()
>>>>>> format = '[%(name)s-%(thread)d-%(funcName)s: (%(levelname)s) %(message)s'
>>>>>> log.setLevel(logging.DEBUG)
>>>>>> formatter = logging.Formatter(format)
>>>>>> handler = logging.StreamHandler()
>>>>>> handler.setFormatter(formatter)
>>>>>> log.addHandler(handler)
>>>>>> 
>>>>>> registered_nodes = {}
>>>>>> 
>>>>>> def register_node(node, data=None):
>>>>>> if node in registered_nodes:
>>>>>>    if data == registered_nodes[node]:
>>>>>>        log.debug('Already registered [%s] in data provider.' % node)
>>>>>>        return
>>>>>> log.debug('Registering [%s] in data provider.' % node)
>>>>>> _zk.create(node, ephemeral=True, makepath=True)
>>>>>> registered_nodes[node] = data
>>>>>> 
>>>>>> def _re_register_nodes(nodes):
>>>>>> for node in nodes.iteritems():
>>>>>>    register_node(node[0], data=node[1])
>>>>>> 
>>>>>> def _state_listener(state):
>>>>>> log.warning('Zookeeper connection state changed: %s' % state)
>>>>>> if state == KazooState.SUSPENDED:
>>>>>>    CONNECTION_STATE=False
>>>>>> elif state == KazooState.LOST:
>>>>>>    CONNECTION_STATE=False
>>>>>> else:
>>>>>>    CONNECTION_STATE=True
>>>>>>    try:
>>>>>>        nodes = registered_nodes
>>>>>>    except:
>>>>>>        # no local nodes
>>>>>>    registered_nodes = {}
>>>>>>    _re_register_nodes(nodes)
>>>>>> 
>>>>>> log.setLevel(logging.DEBUG) 
>>>>>> 
>>>>>> registered_nodes = {}
>>>>>> 
>>>>>> _zk = KazooClient(hosts='localhost:2182')
>>>>>> _zk.start()
>>>>>> _zk.add_listener(_state_listener)
>>>>>> _state_listener(_zk.state)
>>>>>> 
>>>>>> # now register a node
>>>>>> register_node('/abc/a')
>>>>>> _zk.stop()
>>>>>> _zk.start()
>>>>> 
>>>>> 
>>>>> If you run this in a python shell, after the _zk.start(), your path will
not reregister... instead, the _state_listener() method will basically hang when it calls
_re_register_nodes(). The _re_register_nodes() method hangs on the first attempt to call register_node(),
which will hang at the _zk.create. Obviously the above code is a bastardized stripped down
version of what we're working with, but it replicates the problem. This could just be a problem
with my understanding of how the add_listener callback works.. but I'm a bit confused here.
>>>>> 
>>>>> Long term, the goal is to have our object able to handle a disconnect
and gracefully re-generate any paths that had been disconnected during the connection loss.
Ironically Kazoo handles this already with its 'watcher' recipe. It just doesn't have the
same kind of thing for any paths we create with KazooClient.create().
>>>>> 
>>>>> --Matt
>>>> 
>>> 
>> 
> 


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