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From Camille Fournier <cami...@apache.org>
Subject Re: cross DC setup - is it Ok for ZK?
Date Tue, 21 Oct 2014 21:15:36 GMT
You'll have to ask Flavio because I don't really understand what he's
saying there, tbh. You have to have (n+1)/2 nodes available and
communicating to each other to maintain a live quorum (so, in your case,
you must have 4 nodes available at all times to maintain a live quorum).

On Tue, Oct 21, 2014 at 5:08 PM, Denis Samoilov <samoilov@gmail.com> wrote:

> Camille, thank you very much! Very interesting read. I also found a thread
> where you participated three years ago and found one particular comment a
> bit confusing *"One quick comment. We do not require majority quorums in
> ZooKeeper,  and one reason we implemented this feature was exactly to
> enable more flexibility in deployments with multiple data centers*" (
>
> http://mail-archives.apache.org/mod_mbox/zookeeper-user/201109.mbox/%3C0B4CC52A-939E-4896-A269-50DC31E20AA6@yahoo-inc.com%3E
> )
> but this potentially contradicts FAQ: *"if the leader is in the non-quorum
> side of the partition, that side of the partition will recognize that it no
> longer has a quorum of the ensemble"* (
> https://cwiki.apache.org/confluence/display/ZOOKEEPER/FailureScenarios).
>
> Where is the truth? :)
>
> On Tue, Oct 21, 2014 at 12:35 PM, Camille Fournier <camille@apache.org>
> wrote:
>
> > I have a blog post on this topic:
> >
> >
> http://whilefalse.blogspot.com/2012/12/building-global-highly-available.html
> >
> > I think you will find it helpful.
> > The short answer is: the scheme you have proposed will cause the ZK to be
> > unavailable when you do maintenance on the data center with 4 quorum
> > members.
> >
> > Best,
> > C
> >
> > On Tue, Oct 21, 2014 at 3:03 PM, Denis Samoilov <samoilov@gmail.com>
> > wrote:
> >
> > > hi,
> > >
> > > Could you please help to understand the following setup: we have two
> > > datacenters and want to setup ZK cluster so it will use servers (ZK
> > servers
> > > not clients) in both: like 3 ZK servers in DC1 and 4 ZK servers in DC2.
> > We
> > > sometime do maintenance in one or other DC. So ZK will completely lose
> > > replicas in one of the DC for several hours. E.g. if DC2 is under
> > > maintenance ZK will have only 3 out of 7 nodes and these 3 nodes
> supposed
> > > to receive writes.
> > >
> > > The questions:
> > > 1) is it Ok for ZK to have such setup?
> > > 2) will ZK catch up after losing 4 Servers and getting them back in
> some
> > > time? (this will be a majority actually :) )
> > > 3) what is right number of nodes, is 5 sufficient : 2 +  3?
> > >
> > > Latency between DCs is pretty low (DCs are close to each other).
> > >
> > >
> > > Thank you for any advice.
> > >
> > > -Denis
> > >
> >
>

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