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From Jack Luo <j...@rim.com>
Subject Data change notification is lost during failover
Date Tue, 24 Jul 2012 13:08:48 GMT
Hi All,

I am using Zookeeper3.3.5 for a distributed project. During the test, a
watch related issue is found. Our monitor program places 100 watches on 100
different paths (e.g. /goo1 …. /goo100) for monitoring the data change, and
another writer program updates one of paths at a specified interval. We
found sometimes some data change notification messages are lost when the
monitor program is switched to a new server due to the failure of current

I check the “watch management” section in current release notes
http://zookeeper.apache.org/doc/trunk/releasenotes.html and find a statement
“In this release the client library tracks watches that a client has
registered and reregisters the watches when a connection is made to a new
server.” So based on the information, look like during server failover it is
expected behavior to lose data change notifications before watches are
successfully re-registered in a new server.

The solution that I figure out to this issue is to query all 100 paths to
check if there is any data change after the monitor program is connected to
a new server.

However if we need to monitor 1000 or 10K paths, this solution may not be
good. Can anyone suggest a better solution to this issue?

Furthermore, can ZK service is enhanced to replicate the watches on each ZK
server to solve this issue forever?

Thanks for your time and help!


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