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From Benjamin Reed <br...@yahoo-inc.com>
Subject Re: zookeeper.getChildren asynchronous callback
Date Thu, 11 Jun 2009 14:09:52 GMT
just to clarify i believe you are talking about callbacks on the watch 
object you are passing in the asynchronous call rather than the 
asynchronous completion callback. (Henry is making the same assumption.) 
when you say you are getting the callback 10 times, i believe your are 
talking about 10 different watch objects getting called back once each. 

it turns out that the zookeeper client does know what you are watching, 
and the zookeeper server will only register one watch. the thing that is 
missing is the clearWatches call that Henry refers to. the thing that 
complicates things a bit, perhaps not for you, is the scenario where we 
have different modules sharing the same zookeeper handle. if different 
modules are interested in watching the same object, you don't want one 
module to simply clear a the watches for a path because one module may 
mess up the other.

we have talked about adding this ability to clear watches for a while. i 
think the auto-watch reregistration patch made the issue slightly more 
pressing since it means that watches can survive for the entire lifetime 
of a session not just for the duration of a connection to a specific 
server. i've created ZOOKEEPER-442 to track this issue.


Henry Robinson wrote:
> Hi Satish -
> As you've found out, you can set multiple identical watches per znode - the
> zookeeper client will not detect identical watches in case you really meant
> to call them several times. There's no way currently, as far as I know, to
> clear the watches once they've been set. So your options are either to avoid
> repeatedly setting them by detecting whether getChildren is a repeat call,
> or by dealing with multiple invocations on the callback path and not doing
> anything once you've established you're no longer interested.
> It might well make sense to add a clearWatches(path) call to the API, which
> would be useful particularly for clients where callbacks are expensive and
> require a context switch (which I think is true for all clients right now!).
> Henry
> On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <cthd2001@gmail.com> wrote:
>> I am using the asynchronous (callback) version of zookeeper.getChildren().
>>  That call returns immediately, I then wait for a certain time interval for
>> nodes to appear, and if not I exit the method that made the
>> zookeeper.getChildren()
>> call.  Later on, a node gets added under that node and I see in my logfile
>> that the Watcher.process() callback that I set above gets called.  Now if I
>> make 10 failed attempts to get a node using the above technique, and at
>> some
>> later time a node does get added, I see in the logfile that the
>> Watcher.process() ends up being called 10 times!  Of course by this time I
>> have totally lost interest in those callbacks.  Question:  Is there a way
>> to
>> remove that asynchronous callback?  i.e. If I make a asynchronous
>> zookeeper.getChildren()
>> call, wait time t, give up, at that point can I remove the async callback?
>> Satish

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