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From "Flavio Junqueira" <fpjunque...@yahoo.com>
Subject RE: Understanding event interleaving
Date Thu, 23 May 2013 14:04:38 GMT
Hi Scott,

The R client could receive either version 1 or version 2. In the case it receives version
1, it will receive the notification for the version 2 change once the server the client is
connected to receives the update. If the getData returns version 2 instead, it won't get the
watch triggered by the setData of steps 2 and 3 below. It should get it triggered by a future
update, though, assuming there is one.


-----Original Message-----
From: Scott Fines [mailto:scottfines@gmail.com] 
Sent: 23 May 2013 14:48
To: user@zookeeper.apache.org
Subject: Understanding event interleaving

Hello all, 

This is perhaps a silly question, but one I'm not clear on.

Suppose you have two clients (R and W), and they are interacting through znode Z. Now consider
the following situation:

0. Z at version 1
1. R begins getData(Z) (attach Watcher)
2. W begins setData(Z) 
3. W completed setData(Z) successfully --e.g. the client call returns with no errors 
4. R completes getData(Z) successfully -- e.g. the client call returns with no errors

My questions are then:

1. What version of data will R receive? Version 1, or Version 2? My assumption here is that
R would receive version 1 of the data--is that correct?

2. Does R's watch fire with the changes made by W? If so, is a WatchEvent delivered all the
way to the caller? A comment made on ZOOKEEPER-1505 (https://issues.apache.org/jira/browse/ZOOKEEPER-1505)
implies that it would not, but I'm unclear if that is in reference to the proposed changes
or not.

Thanks for your help!

Scott Fines 

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