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From Alexander Shraer <shra...@gmail.com>
Subject Re: observer changing to participant when there is no quorum
Date Wed, 15 Jun 2016 02:55:12 GMT
Right, a quorum of participants from the old config is required to process
any command, including reconfig,
and a quorum of participants from the new config is required for the
reconfig to even start. If there's no such connected
quorum an error NewConfigNoQuorum will be thrown.

But there is one slightly confusing case where the error is thrown, which
is explained in the doc: when you are
converting an observer to a participant and there is no quorum in the new
config without counting that "future" participant.
So the server is connected, but since its not a participant we get the
error above.  In that case, one first needs to
convert the observer to remove the observer and then add it back. The
detailed explanation is in the doc, look for
"Changing an observer into a follower".

On Wed, Jun 15, 2016 at 1:17 AM, Camille Fournier <camille@apache.org>
wrote:

> I'm finding the documentation quite confusing. I was under the impression
> that quorum of some sort was needed to do a reconfig. Can you reconfig when
> there is no quorum?
>
> *Progress guarantees:* Up to the invocation of the reconfig operation, a
> quorum of the old configuration is required to be available and connected
> for ZooKeeper to be able to make progress. Once reconfig is invoked, a
> quorum of both the old and of the new configurations must be available.
>
> *Adding servers:* Before a reconfiguration is invoked, the administrator
> must make sure that a quorum (majority) of participants from the new
> configuration are already connected and synced with the current leader.
>
>
>
> On Tue, Jun 14, 2016 at 5:35 PM, Alexander Shraer <shralex@gmail.com>
> wrote:
>
> > This is needed only in case the target config doesn't have a quorum which
> > are already followers in the old config
> > and are up. We need agreement from a quorum of the target config, but
> > observers aren't participating
> > in the voting protocol.
> >
> > On Tue, Jun 14, 2016 at 7:35 PM, Michael Han <hanm@cloudera.com> wrote:
> >
> > > This might help:
> > > https://zookeeper.apache.org/doc/trunk/zookeeperReconfig.html section
> > > '*Changing
> > > an observer into a follower:'*
> > > "first invoke a reconfig to remove D from the configuration and then
> > invoke
> > > a second command to add it back as a participant (follower)."
> > >
> > >
> > > On Tue, Jun 14, 2016 at 8:53 AM, Nomar Morado <nomar.morado@gmail.com>
> > > wrote:
> > >
> > > > Hi
> > > >
> > > > I was trying to promote an observer into participant when ZK loses
> > > quorum -
> > > > but it seems that it does not allow to.
> > > >
> > > > Would you know how this can be accomplished without having to recycle
> > ZK?
> > > >
> > > > I am using 3.5.0-alpha
> > > >
> > > >
> > > > Thanks.
> > > >
> > >
> > >
> > >
> > > --
> > > Cheers
> > > Michael.
> > >
> >
>

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