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From Eric Bowman <ebow...@boboco.ie>
Subject how to handle re-add watch fails
Date Mon, 25 Jan 2010 13:05:31 GMT
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?" );
            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?


Eric Bowman
Boboco Ltd

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