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From: Benjamin Reed <breed@yahoo-inc.com>
To: "zookeeper-user@hadoop.apache.org" <zookeeper-user@hadoop.apache.org>
Date: Mon, 5 Jan 2009 14:16:49 -0800
Subject: RE: Not performing work in the zookeeper even thread.
Thread-Topic: Not performing work in the zookeeper even thread.
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I'm not sure what your Watcher:ZooKeeper notation means, but just to be cle=
ar: each ZooKeeper object has an event thread with a corresponding event qu=
eue. Whenever zookeeper needs to do a callback it queues the callback to th=
e event thread to handle the invocation.

ben
________________________________________
From: burtonator@gmail.com [burtonator@gmail.com] On Behalf Of Kevin Burton=
 [burton@spinn3r.com]
Sent: Monday, January 05, 2009 2:08 PM
To: zookeeper-user@hadoop.apache.org
Subject: Re: Not performing work in the zookeeper even thread.

On Mon, Jan 5, 2009 at 1:23 PM, Benjamin Reed <breed@yahoo-inc.com> wrote:

> the event thread is the client's own thread. zookeeper queues events to t=
he
> event thread so that it doesn't have to worry about a backlog.
>

So each Watcher:ZooKeeper shares one thread (I was thinking you might be
doing this)?  Wouldn't this be problematic when pooling ZooKeeper objects
across threads?

I implemented a new API which I think is a bit easier to use than the ZK AP=
I
(I'll publish the source in a bit).

I had it implement a watchNode() method which registers an event.  Then you=
r
thread calls poll() which handles all events from ZK...

the API then blocks and waits for ZK to add events to its own internal
queue.

This way you can have a fully event driven application but you don't have t=
o
worry about object corruption or threading issues.


>
> even if an application is slow in processing callbacks from the event
> thread, the rest of zookeeper (the heartbeats, the synchronous calls, etc=
.)
> will not be adversely affected.
>

OK... so even in multithreaded applications if you block you can't kill ZK
(which would be bad).
Kevin

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