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From Henry Robinson <he...@cloudera.com>
Subject Re: What roles do "even" nodes play in the ensamble
Date Wed, 25 Aug 2010 22:18:08 GMT
Dave is correct - if you have N nodes you need  (N/2) + 1 votes (i.e. a
majority) in the standard case to get a vote to pass.

Adding a fourth voting node to a three node cluster will cause the size of a
majority to jump from 2 to 3. The number of nodes that need to fail before
you can no longer get a majority is 2 in both cases - so you don't get any
reliability for adding a new voting node to a odd-numbered cluster.

The new node will always act as a voter unless you explicitly configure it
as an observer.

Henry

On 25 August 2010 15:11, Dave Wright <wrightd@gmail.com> wrote:

> I'm not an expert on voting, so there may be a better answer, but from my
> understanding all 4 nodes participate in the voting and you need a majority
> of 3 to elect a leader.
>
> -Dave
>
> On Wed, Aug 25, 2010 at 6:09 PM, Todd Nine <todd@spidertracks.co.nz>
> wrote:
>
> >  Thanks for that Dave.  If I do not configure it as an observer just a
> > normal member, what will the last even node to join do?
> >
> >
> > 1. Will it participate as a voter on startup?  (I'm assuming not, just
> read
> > only)
> >
> > 2. If one of the voter nodes 1 through 3 dies, does it become a voter?
> >
> >
> >    todd
> > SENIOR SOFTWARE ENGINEER
> >
> > todd nine| spidertracks ltd |  117a the square
> > po box 5203 | palmerston north 4441 | new zealand
> > P: +64 6 353 3395 | M: +64 210 255 8576
> > E: todd@spidertracks.co.nz W: www.spidertracks.com
> >
> >
> >
> >
> >
> >   On Wed, 2010-08-25 at 17:57 -0400, Dave Wright wrote:
> >
> > >
> > > 1. When the 4th ZK node joins the cluster, does it take on the observer
> > > role since a quorum cannot be reached with the new node?  Can I still
> > > connect my clients to it and create/remove nodes and receive events?
> >
> > No, it joins as a normal member unless you've configured it as an
> > observer. Note that with 4 nodes you now need 3 running to get a
> > majority, which is why even numbers aren't recommended.
> >
> > >
> > >
> > > 2. In the event 1 of the 3 voting nodes fails, will this 4th node
> become
> > > a voting member of the ensemble?
> >
> > If configured as an observer it remains an observer.
> >
> > >
> > > 3. When a new node comes online, it may have a different ip than the
> > > previous node.  Do I need to update all node configurations and perform
> > > a rolling restart, or will simply connecting the new node to the
> > > existing ensemble make all nodes aware it is running?
> >
> > Unfortunately ZK doesn't have any kind of dynamic configuration like
> > that currently. You need to update all the config files and restart
> > the ensemble.
> >
> > -Dave Wright
> >
> >
>



-- 
Henry Robinson
Software Engineer
Cloudera
415-994-6679

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