No problem, we really appreciate your feedback -- keep it coming.

Patrick

Stu Hood wrote:
Thanks a bunch Patrick! I really appreciate your work on ZooKeeper... you have 
made a tremendous impact on the project.

I will try the ZOOKEEPER-221 patch.

Thanks,
Stu


-----Original Message-----
From: "Patrick Hunt" <[EMAIL PROTECTED]>
Sent: Wednesday, November 12, 2008 6:37pm
To: zookeeper-user@hadoop.apache.org
Subject: Re: Exists Watch Triggered by Delete

Stu Hood wrote:
The comment you referenced in your original email is true - that
code should never execute as the existsWatches list only contains
watches for NONODE watch registrations (which obv couldn't be
deleted since it doesn't exist).
So I am experiencing a bug then?

Wow, this is a dumb mistake. The log message you pointed to is being output _every time_ this code is called. There needs to be a conditional on the log message to check the result of the remove call.

I have entered a JIRA and have already submitted a patch:
https://issues.apache.org/jira/browse/ZOOKEEPER-221

So unfortunately we can't tell if this is the source of the issue you are seeing. Can you run with the patch attached to ZOOKEEPER-221?

It may be better not to let the programmer know about those two
lists, and to see if the abstraction can be improved instead.
Sometimes I feel that I have to know too much about the internal
working of ZooKeeper to use its API.

That's a good point. We're working to improve the docs & code, I've made a note of it. Perhaps we should move that to "internals" doc and rework this section of the guide...
https://issues.apache.org/jira/browse/ZOOKEEPER-220

Patrick


-----Original Message----- From: "Patrick Hunt" <[EMAIL PROTECTED]> Sent: Wednesday, November 12, 2008 2:11pm To:
zookeeper-user@hadoop.apache.org Subject: Re: Exists Watch Triggered
by Delete

Hi Stu,

The zk server maintains 2 lists of watches, data and child watches: http://hadoop.apache.org/zookeeper/docs/r3.0.0/zookeeperProgrammers.html#ch_zkWatches
 (after reviewing this doc I've entered a jira to clarify that the
server is maintaining 2 lists being referenced). From the server
perspective if you register a watch on a node by calling getData &
exists, only a single watch, a data watch, is stored by the server.

The client is maintaining lists of watches as well. This is
essentially to enable the auto watch reset and multi-watcher features
added in v3.

Take a look at class ExistsWatchRegistration, it will register client
 side dataWatches for exists calls -- unless the result code is not 0
(ie NONODE), in which case it will register using existsWatches
(again, client side).

The comment you referenced in your original email is true - that code
 should never execute as the existsWatches list only contains watches
for NONODE watch registrations (which obv couldn't be deleted since
it doesn't exist).

Hope this helps,

Patrick

Stu Hood wrote:
I'm running the 3.0.0 release, and I'm receiving a warning thrown
by this block of code:
case NodeDeleted: synchronized (dataWatches) { addTo(dataWatches.remove(path), result); } // XXX This shouldn't be needed, but just in case synchronized (existWatches) { addTo(existWatches.remove(path), result); LOG.warn("We are triggering an exists watch for delete! Shouldn't happen!"); } synchronized (childWatches) { addTo(childWatches.remove(path),
result); } break;
--

Why shouldn't an exists watch be triggered by a node being deleted?
That is a really common use case in my code, so I want to rule it
out as the cause of a bug I'm hunting for.

Thanks,

Stu Hood Architecture Software Developer Mailtrust, a Division of
Rackspace




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