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From Kaushal Shriyan <kaushalshri...@gmail.com>
Subject Re: Meaning of quorum and ensemble
Date Fri, 29 May 2015 19:04:55 GMT
Thanks Chris for the explanation. Please help me understand how would I
implement a 3 node or a 5 node ensemble in a AWS region for example
Singapore Region which has two Availability Zone 1a and 1b if either of the
zone becomes unavailable due to say for example Network Glitch or not
reachable.

Regards,

Kaushal
 On 29 May 2015 23:41, "Chris Nauroth" <cnauroth@hortonworks.com> wrote:

> Hello Kaushal,
>
> 1. "Quorum" refers to the minimum number of nodes that must agree on a
> transaction before it is considered committed.
> 2. Yes, your understanding of ceil(N/2) is correct.  It sounds like your
> formulation of it is more like floor((N+1)/2).  The two formulations are
> equivalent, so whichever one is more intuitive to you is valid.
> 3. "Ensemble" refers to the full set of peer servers in a ZooKeeper
> cluster.
>
> Tying this back to your examples, a 3-node ensemble requires a quorum of 2
> servers running to commit a transaction.  A 5-node ensemble requires a
> quorum of 3 servers running to commit a transaction.  An administrator can
> deploy a ZooKeeper cluster sized according to trade-offs between budget
> (number of servers) and desired fault tolerance (number of simultaneous
> server failures).  A more detailed discussion is available in the
> ZooKeeper Internals document:
>
> http://zookeeper.apache.org/doc/r3.4.6/zookeeperInternals.html
>
>
> I hope this helps.
>
> --Chris Nauroth
>
>
>
>
> On 5/29/15, 10:08 AM, "Kaushal Shriyan" <kaushalshriyan@gmail.com> wrote:
>
> >Hi,
> >
> >I have a question regarding quorum in context to
> >
> http://stackoverflow.com/questions/13022244/zookeeper-reliability-three-ve
> >rsus-five-nodes
> >
> >Zookeeper requires that you have a quorum of servers up, where quorum is
> >ceil(N/2). For a 3 server ensemble, that means 2 servers must be up at any
> >time, for a 5 server ensemble, 3 servers need to be up at any time.
> >
> >   1. what does quorum mean?
> >   2. so does ceil(N/2) mean 3+1/2 = 4/2 which is 2 servers in context to
> >3
> >   server ensemble and 5+1/2 = 6/2 which is 3 servers in context to 5
> >server
> >   ensemble if the ceil(N/2) is a ceiling function?
> >   3. what does ensemble mean in context to Apache zookeeper?
> >
> >I would appreciate your help.
> >
> >Regards,
> >
> >Kaushal
>
>

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