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Date: Wed, 23 Jan 2013 12:12:12 +0000
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Subject: 
 =?windows-1252?Q?ZooKeeper_Watch_triggered_multiple_times_for_the_sam?=
	=?windows-1252?Q?e_notification_=96_contrary_to_docs?=
From: Jonathan Smith <jonsmith@mail.com>
To: user@zookeeper.apache.org
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I have a node that has a children watch on it. Whenever the
NodeChildrenChanged event is received I obtain the children using
getChildren passing in my watcher then iterate through all children
and perform a getData() also passing in my watcher so that I am also
notified of any changes to the data on the children.

E.g.

    public void check() {
        List<String> children =3D
zkclient.getChildren().usingWatcher(myWatcher).forPath(PATH);
        for (String name : children) {
            byte[] data =3D
zkclient.getData().usingWatcher(myWatcher).forPath(getPathFromName(name));
            // do stuff
        }
    }


    CuratorWatcher myWatcher =3D new CuratorWatcher() {
        @Override
        public void process(WatchedEvent event) throws Exception {
            logger.info("received watch event: " + event);
            if (event.getType().equals(EventType.NodeChildrenChanged)
                    || event.getType().equals(EventType.NodeDataChanged)) {
                check();
            }
        }
    };

This means that whenever a child is added/removed, another getData
call will be made with the watcher.

After reading the ZooKeeper documentation I thought it would be fine
to keep setting the watcher since I should only be given the same
watch notification once:

=93A watch object, or function/context pair, will only be triggered once
for a given notification. For example, if the same watch object is
registered for an exists and a getData call for the same file and that
file is then deleted, the watch object would only be invoked once with
the deletion notification for the file.=94

http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#ch_zkWatche=
s

However I have observed that when a node is deleted I am receiving the
NodeDeleted watch event multiple times:

received watch event: WatchedEvent state:SyncConnected
type:NodeDeleted path:/a/b
received watch event: WatchedEvent state:SyncConnected
type:NodeDeleted path:/a/b
received watch event: WatchedEvent state:SyncConnected
type:NodeDeleted path:/a/b
...

This means the watch is being triggered multiple times unnecessarily.

I am using the Curator framework however I believe this just passes
the watches through and therefore this is a ZooKeeper issue.  I am
using zookeeper server version 3.4.5, zookeeper client 3.3.1,
curator-framework 1.2.2.

Is the documentation wrong / is this a bug or have I misunderstood?

Many thanks,
Jon