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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Update of "ZooKeeper/FAQ" by ToddLipcon
Date Mon, 26 Mar 2012 21:11:30 GMT
Dear Wiki user,

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The "ZooKeeper/FAQ" page has been changed by ToddLipcon:
http://wiki.apache.org/hadoop/ZooKeeper/FAQ?action=diff&rev1=15&rev2=16

Comment:
Added FAQ about session expiration behavior after a cluster downtime

  In conclusion, DNS RR works as good as a list of ensemble IP arguments except cluster reconfiguration
case.
  It turns out that there is a minor problem with DNS RR. If you are using a tool such as
zktop.py, it does not take care of a list of host IP returned by a DNS server. 
  
+ <<BR>>
+ <<Anchor(9)>>
+ '''9. [[#9|What happens to ZK sessions while the cluster is down?]]'''
+ 
+ Imagine that a client is connected to ZK with a 5 second session timeout, and the administrator
brings the entire ZK cluster down for an upgrade. The cluster is down for several minutes,
and then is restarted.
+ 
+ In this scenario, the client is able to reconnect and refresh its session. Because session
timeouts are tracked by the leader, the session starts counting down again with a fresh timeout
when the cluster is restarted. So, as long as the client connects within the first 5 seconds
after a leader is elected, it will reconnect without an expiration, and any ephemeral nodes
it had prior to the downtime will be maintained.
+ 
+ The same behavior is exhibited when the leader crashes and a new one is elected. In the
limit, if the leader is flip-flopping back and forth quickly, sessions will never expire since
their timers are getting constantly reset.
+ 

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