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 <[email protected]> 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/%[email protected]%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 <[email protected]> > 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 <[email protected]> > > 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 > > > > > >
