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From Alexis Midon <alexismi...@gmail.com>
Subject Watchers & error handling
Date Sun, 13 Jun 2010 05:07:26 GMT
Hi all,

I implemented queues and locks on top of ZooKeeper, and I'm pretty happy so
far. Thanks for the nice work. Tests look good. So good that we can focus on
exception/error handling and I got a couple of questions.

#1. Regarding the use of the default watcher. A ZooKeeper instance has a
default watcher, most operations can also specify a watcher. When both are
set, does the operation watcher override the default watcher?
 or will both watchers be invoked? if so in which order? Does each watcher
receive all the types of event?
I had a look at the code, and my understanding is that the default watcher
will always receive the type-NONE events, even if an "operation" watcher is
set. No guarantee on the order of invocation though. Could you confirm
and/or complete please?

#2 After a connection loss, the client will eventually reconnect to the ZK
cluster so I guess I can keep using the same client instance. But are there
cases where it is necessary to re-instantiate a ZooKeeper client? As a first
recovery-strategy, is that ok to always recreate a client so that any
ephemeral node previously owned disappear?
The case I struggle with is the following:
Let's say I've acquired a lock (i.e. an ephemeral locknode is created).
Some application logic failed due to a connection loss. At this stage I'd
like to give up/roll back. Here I would typically throw an exception, the
lock being released in a finally. But I can't release the lock since the
connection is down. Later the client eventually reconnects, the session
didn't expire so the locknode still exists. Now no one else can acquire this
lock until my session expires.

#3. could you describe the recommended actions for each exception code?

I hope my questions would make some sense to you. Thanks in advance for your


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