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Date: Mon, 01 Feb 2010 10:21:41 +0000
From: Eric Bowman <ebowman@boboco.ie>
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To: zookeeper-user@hadoop.apache.org
Subject: Re: how to handle re-add watch fails
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I was surprised to not get a response to this ... is this a no-brainer? 
Too hard to solve?  Did I not express it clearly?  Am I doing something
dumb? :)

Thanks,
Eric

On 01/25/2010 01:05 PM, Eric Bowman wrote:
> I'm curious, what is the "best practice" for how to handle the case
> where re-adding a watch inside a Watcher.process callback fails?
>
> I keep stumbling upon the same kind of thing, and the possibility of
> race conditions or undefined behavior keep troubling me.  Maybe I'm
> missing something.
>
> Suppose I have a callback like:
>
>     public void process( WatchedEvent watchedEvent )
>     {
>         if ( watchedEvent.getType() ==
> Event.EventType.NodeChildrenChanged ) {
>             try {
>                 ... do stuff ...
>             }
>             catch ( Throwable e ) {
>                 log.error( "Could not do stuff!", e );
>             }
>             try {
>                 zooKeeperManager.watchChildren( zPath, this );
>             }
>             catch ( InterruptedException e ) {
>                 log.info( "Interrupted adding watch -- shutting down?" );
>                 return;
>             }
>             catch ( KeeperException e ) {
>                 // oh crap, now what?
>             }
>         }
>     }
>
> (In this cases, watchChildren is just calling getChildren and passing
> the watcher in.)
>
> It occurs to me I could get more and more complicated here:  I could
> wrap watchChildren in a while loop until it succeeds, but that seems
> kind of rude to the caller.  Plus what if I get a
> KeeperException.SessionExpiredException or a
> KeeperException.ConnectionLossException?  How to handle that in this
> loop?  Or I could send some other thread a message that it needs to keep
> trying until the watch has been re-added ... but ... yuck.
>
> I would very much like to just setup this watch once, and have ZooKeeper
> make sure it keeps firing until I tear down ZooKeeper -- this logic
> seems tricky for clients, and quite error prone and full of race conditions.
>
> Any thoughts?
>
> Thanks,
> Eric
>
>   


-- 
Eric Bowman
Boboco Ltd
ebowman@boboco.ie
http://www.boboco.ie/ebowman/pubkey.pgp
+35318394189/+353872801532