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. right?

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!).


On Wed, Jun 10, 2009 at 8:05 PM, Satish Bhatti <cthd2...@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
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
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
remove that asynchronous callback?  i.e. If I make a asynchronous
call, wait time t, give up, at that point can I remove the async callback?

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