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From Alexander Shraer <shra...@yahoo-inc.com>
Subject RE: Rolling upgrades
Date Fri, 09 Mar 2012 00:15:45 GMT
I don't think they must be an odd number, but it does make sense to have an odd number
because with majority quorums the fault tolerance you get with 3 servers is the same
as the fault tolerance you get with 4 - in both cases you can only tolerate 1 failure.

Alex

> -----Original Message-----
> From: Jordan Zimmerman [mailto:jzimmerman@netflix.com]
> Sent: Thursday, March 08, 2012 4:10 PM
> To: user@zookeeper.apache.org
> Subject: Re: Rolling upgrades
> 
> Also҆
> 
> I thought that ZK ensembles need to be odd in number. How would ZK
> handle
> a temporary state where there is an even number?
> 
> -JZ
> 
> On 3/8/12 3:39 PM, "Alexander Shraer" <shralex@yahoo-inc.com> wrote:
> 
> >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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