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From Alexander Shraer <shra...@yahoo-inc.com>
Subject RE: Rolling upgrades
Date Thu, 08 Mar 2012 23:39:29 GMT
I don't think there is a problem if you do it as you say, or even if you just change the config
files of all servers at once and restart them, because a majority of the new config
necessarily intersects with a majority of the old one, so a server who has the latest state
will be elected leader.

Alex

> -----Original Message-----
> From: Jordan Zimmerman [mailto:jzimmerman@netflix.com]
> Sent: Thursday, March 08, 2012 3:31 PM
> To: user@zookeeper.apache.org
> Subject: Rolling upgrades
> 
> I've been reading the archives regarding rolling upgrades. Here's the
> scenario, given a stable ensemble:
> 
> ZK1 <-> ZK2 <-> ZK3
> 
> In the above, the zoo.cfg for each server looks like this (pseudo):
> server.1=ZK1
> server.2=ZK2
> server.3=ZK3
> 
> I want to add a new server, ZK4. If I understand this correctly, I'd
> bring up ZK4 with this config:
> server.1=ZK1
> server.2=ZK2
> server.3=ZK3
> server.4=ZK4
> 
> At this point, though, the configs don't match in the ensemble. How do
> the ZK instances handle this?
> 
> Continuing...
> 
> Once ZK4 is up, ZK1 would get the new config and get restarted. Once
> ZK1 is up, ZK2 gets new config, etc.
> 
> At each point of config change, the cluster is in a confused state
> about the config. Is there code in ZK to handle this?
> 
> -JZ


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