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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 14:33:51 GMT
ZooKeeper only works when a majority of the participants are up.
Since 2 out of 3 participants in your ensemble are down, ZooKeeper won't
allow you to issue any commands, including a reconfiguration.
You should have enough participants such that a situation where a majority
is simultaneously down doesn't happen.


On Wed, Jun 15, 2016 at 4:33 PM, Nomar Morado <j316services@icloud.com>
wrote:

> Corrected typos
>
> Printing e-mails wastes valuable natural resources. Please don't print
> this message unless it is absolutely necessary. Thank you for thinking
> green!
>
> Sent from my iPhone
>
> > On Jun 15, 2016, at 9:31 AM, Nomar Morado <j316services@icloud.com>
> wrote:
> >
> > This is what I have done so far:
> >
> > A,B,C are participants
> > C,D are observers
> >
> > B,C are offline (crashed)
> >
> > I am trying to:
> >
> > 1. Remove C, D
> > 2. Add C,D back as participants
> >
> > Will this work?
> >
> > At least in my testing (might be doing wrong) I am getting this error on
> the first step and hence can't get forward:
> >
> > Client could not connect to reestablished  quorum: giving up after 30+
> seconds
> >
> >
> > I am passing the original server configure string to zk's reconfig
> method.
> >
> >
> >
> > Thanks
> >
> >
> > Printing e-mails wastes valuable natural resources. Please don't print
> this message unless it is absolutely necessary. Thank you for thinking
> green!
> >
> > Sent from my iPhone
> >
> >> On Jun 14, 2016, at 10:55 PM, Alexander Shraer <shralex@gmail.com>
> wrote:
> >>
> >> 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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