Re: [Zookeeper-user] [Bug?] Notification not guaranteed

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Hi Ted,
  Whenever you restablish a watch you might miss some updates on the
node. Though we do have the version numbers to track if you missed
something or not but still you would miss the data update sometimes.

Regards
Mahadev

> -----Original Message-----
> From: zoo...@li...
[mailto:zookeeper-user-
> bo...@li...] On Behalf Of Ted Dunning
> Sent: Friday, April 25, 2008 11:31 AM
> To: Bartlomiej Niechwiej; Benjamin Reed
> Cc: zoo...@li...
> Subject: Re: [Zookeeper-user] [Bug?] Notification not guaranteed
> 
> 
> Bartlomiej,
> 
> How is a watch that is always reasserted on every read different from
a
> permanent watch?  The client side implementation has the virtue that
it
> collapses multiple changes if the client goes away or gets busy.
> 
> Is it just a client API issue?
> 
> 
> On 4/25/08 10:26 AM, "Bartlomiej Niechwiej" <ba...@ya...>
wrote:
> 
> > Thanks, Ben.
> >
> > However subscriptions are significantly different from watches (they
> > provide data as well), so I don't consider them as permanent watches
per
> > se.
> >
> > Basically, there are two use cases here, (1) if you are interested
in
> > existence of a node, rather than in the data it carries on and (2)
in
> > being notified about data changes of a node. For (1) you essentially
> > need a permanent watch, as you don't care about the data, while for
(2)
> > subscriptions is a must. It'd be worth to support both mechanisms at
the
> > same time. It's conceivable that a permanent watch is much cheaper
than
> > a subscription.
> >
> > I'm not sure how the watches are implemented now, but I would be
> > surprised if supporting permanent watches would require more than a
few
> > minutes of work (instead of removing the watch after sending an
event,
> > you would have to keep it or re-register again, plus we would need a
way
> > to pass information whether a watch is permanent or one-time).
> >
> > Bart
> >
> >
> > -----Original Message-----
> > From: Benjamin Reed [mailto:br...@ya...]
> > Sent: Friday, April 25, 2008 10:16 AM
> > To: Bartlomiej Niechwiej
> > Cc: zoo...@li...
> > Subject: Re: [Zookeeper-user] [Bug?] Notification not guaranteed
> >
> > We call permantent watches subscriptions see
> > (http://zookeeper.wiki.sourceforge.net/SubscribeMethod). I really
want
> > to do
> > it and I think it would be very useful and more than a convenience
it
> > actually provides more functionality. Unfortunately, we just don't
have
> > the
> > bandwidth right now to implement it. The fortunate thing is that it
is
> > "just
> > a matter of programming" all the information is there and all the
needed
> >
> > information is already being replicated and saved in the transaction
> > log.
> >
> > ben
> >
> > On Friday 25 April 2008 10:01:40 Bartlomiej Niechwiej wrote:
> >> Ben,
> >>
> >> I think the best approach would be to support permanent watches, a
> >> feature which has been asked for since the very beginning...
> >>
> >> Not only would it cease the endless discussion of naming, but it
would
> >> also provide a powerful feature ;-)
> >>
> >> Just a thought ;-)
> >>
> >> B.
> >>
> >> -----Original Message-----
> >> From: zoo...@li...
> >> [mailto:zoo...@li...] On Behalf Of
> >> Benjamin Reed
> >> Sent: Friday, April 25, 2008 8:48 AM
> >> To: zoo...@li...
> >> Subject: Re: [Zookeeper-user] [Bug?] Notification not guaranteed
> >>
> >> Naming and the semantics it implies can be an endless argument.
Unless
> >> we call
> >> it a one time trigger I think people will think of SQL triggers
which
> >> are not
> >> one time. (I'm not saying that watch is any better.)
> >>
> >> I agree about the java doc clarifications. I've been working on it
> > since
> >> Ted
> >> brought it up, but I've been side tracked by the session loss bug.
I'm
> >> hoping
> >> to finish up today. I'll make sure the javadoc emphasizes the way
> >> watches
> >> work.
> >>
> >> ben
> >>
> >> On Friday 25 April 2008 00:14:11 Patrick Hunt wrote:
> >>> Benjamin Reed wrote:
> >>>> This is really a key point for new users to understand, so any
> >>
> >> pointers
> >>
> >>>> on how we can clarify that page would be very helpful!
> >>>
> >>> Part of the issue may be the naming itself. For whatever reason I
> >>> associate "watch" with something long lived - not a "one time
> >>> notification" which I would associate more naturally with
"trigger".
> >>>
> >>> I jumped in right way and started using the java client/server.
The
> >>
> >> java
> >>
> >>> client javadoc in particular is not clear about watches being "one
> >>
> >> time
> >>
> >>> notifications". Would def help to fix the javadoc on this point.
> >>>
> >>> Patrick
> >>>
> >>>> ----- Original Message ----
> >>>> From: Stefan Groschupf <sg...@10...>
> >>>> To: zoo...@li...
> >>>> Sent: Thursday, April 24, 2008 10:19:16 PM
> >>>> Subject: [Zookeeper-user] [Bug?] Notification not guaranteed
> >>>>
> >>>> Hi there,
> >>>> as far I understand notification is guaranteed for e.g. child
> >>
> >> creation.
> >>
> >>>> However I have a test that shows that randomly I do not get all
> >>>> notification I do expect.
> >>>> I basically create 10 child in /folder but only get 8 or 9
> >>>> notifications back.
> >>>> However in case I add a Thread.sleep(50); between my child znode
> >>>> creation I always get 10 notifications.
> >>>>
> >>>> Is that a bug or an optimization, e.g. getting one notification
> > for
> >>>> two new sub znode creations?
> >>>> Thanks for any hints.
> >>>> Stefan
> >>>>
> >>>> My code looks like this:
> >>>>
> >>>> TestMethod:
> >>>>
> >>>>   public void testNotifications() throws Exception {
> >>>>          KattaConfiguration conf = new KattaConfiguration();
> >>>>          Server server = new Server(conf);
> >>>>          ZKClient client = new ZKClient(conf);
> >>>>          MyListener listener = new MyListener();
> >>>>          String katta = "/katta";
> >>>>          if (client.exists(katta)) {
> >>>>              client.deleteRecursiv(katta);
> >>>>          }
> >>>>          client.create(katta);
> >>>>          client.subscribeChildChanges(katta, listener);
> >>>>          for (int i = 0; i < 10; i++) {
> >>>>              client.create(katta + "/" + i);
> >>>> //            Thread.sleep(50);
> >>>>          }
> >>>>          Thread.sleep(1000);
> >>>>          assertEquals(10, listener._counter);
> >>>>          server.shutdown();
> >>>>      }
> >>>>
> >>>> Watcher process Method:
> >>>>
> >>>>      public void process(WatcherEvent event) {
> >>>>          synchronized (_mutex) {
> >>>>              String path = event.getPath();
> >>>>               if (event.getType() ==
> >>>> Watcher.Event.EventNodeChildrenChanged) {
> >>>>                  HashSet<IZKEventListener> listeners =
> >>>> _childListener.get(path);
> >>>>                  if (listeners != null) {
> >>>>                      for (IZKEventListener listener : listeners)
{
> >>>>                          listener.process(event);
> >>>>                      }
> >>>>                      // re subscribe to event.
> >>>>                      try {
> >>>>                          _zk.getChildren(event.getPath(), true);
> >>>>                      } catch (Exception e) {
> >>>>                          for (IZKEventListener listener :
> > listeners)
> >>
> >> {
> >>
> >>>>                              removeListener(path, listener);
> >>>>                          }
> >>>>                          Logger.warn("unable to re subscribe to
> >>
> >> child
> >>
> >>>> change notification", e);
> >>>>                      }
> >>>>                  }
> >>>>              }
> >>>>          }
> >>>>      }
> >>>>
> >>>>
> >>>>
> >>>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >>>> 101tec Inc.
> >>>> Menlo Park, California, USA
> >>>> http://www.101tec.com
> >>
> >>
> >
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> >>
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> >>
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