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From "Tony Jackson" <tjackso...@mail.com>
Subject Re: Watcher latency question
Date Mon, 03 Nov 2014 05:40:56 GMT
I have a client (or several clients responsible for different services) watching a particular
znode, and will notify other services that newly registered machines are available now. The
registered machines may be up to e.g. 4k~5k. So effect like diff would be preferred if possible.
Otherwise clients responsible for watching znodes need to cache and diff by itself. That seems
to be an overhead for clients because it needs to hold many information related to the registered
nodes and perform diff function with old one. 

However I am naive about this, so I appreciate any advises.  

Thanks 
 

>Sent: Monday, November 03, 2014 at 1:11 PM
>From: "Cameron McKenzie" <mckenzie.cam@gmail.com>
>To: user@curator.apache.org
>Subject: Re: Watcher latency question

>hey Tony,
>The way that watches work in ZK is that once they fire they need to be added back again.
If the data >in ZK changes before the watch is reset, then the client will not find out
about this change in data.
 
>i.e You're watching the data in node /test
>-Data changes to state 'A'
>-Watch fires
>-Data changes to state 'B'
>-Data changes to state 'A'
>-Reset watch
>-Data changes to state 'C'
>-Watch fires
 
>You're going to miss the intermediate state where the data transitioned from state A to
state B and >back to state A again. This is just a limitation of ZK, there's a window of
opportunity for these >events to be missed. I don't think that the PathCache in Curator
is going to solve this problem.
 
>Do you have a particular use case where missing these transitions is an issue?
 
>cheers
>Cam
 
>>On Mon, Nov 3, 2014 at 4:04 PM, Tony Jackson <tjacksonfr@mail.com> wrote:I read
some articles mentioning that Zookeeper watcher has latency issue. Something like before the
next watcher is placed and after the previous watcher is triggered, it is possible that the
client may not receive notification within that period.
 
>>http://www.quora.com/Does-Zookeeper-clients-keep-open-lots-of-TCP-connections-if-so-how-scalable-is-it-Any-limits[http://www.quora.com/Does-Zookeeper-clients-keep-open-lots-of-TCP-connections-if-so-how-scalable-is-it-Any-limits]
 
>>Then on the internet some people recommend using curator pathcache, which helps watch
child znodes being added, updated, etc. operations. So my question - is it the right recipe
to use if I want to avoid watcher latency problem? Otherwise which recipe should I use instead
or how to avoid such problem with curator?
 
>>http://curator.apache.org/curator-recipes/path-cache.html[http://curator.apache.org/curator-recipes/path-cache.html]
 
>>Thanks

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