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From: Mahadev Konar <mahadev@yahoo-inc.com>
To: "user@zookeeper.apache.org" <user@zookeeper.apache.org>
Date: Thu, 9 Dec 2010 13:42:00 -0800
Subject: Re: Failure scenarios and consequences
Thread-Topic: Failure scenarios and consequences
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I just read through the wiki. It seems fine to me. Please feel free to add =
documentation to that wiki and get it reviewed by folks on the list.

Thanks
mahadev


On 12/9/10 1:29 PM, "Jeremy Hanna" <jeremy.hanna1234@gmail.com> wrote:

I created a link off of the main wiki and the page itself:
http://wiki.apache.org/hadoop/ZooKeeper/FailureScenarios

Would someone please review it?  Specifically, I am curious to know about t=
his:
"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. T=
he leader will be demoted to being a regular ZooKeeper server and those nod=
es will no longer accept reads or writes."
I just wanted to clarify - in the time for the non-quorum side to recognize=
 it is no longer a quorum, will there ever be writes that get through?  Is =
it guaranteed that it won't accept writes after the partition?  I don't thi=
nk that guarantee can exist, but wondered how to handle that.

On Dec 9, 2010, at 2:04 PM, Mahadev Konar wrote:

> Hi Jeremy,
>   Responses in line below:
>
> On 12/9/10 11:53 AM, "Jeremy Hanna" <jeremy.hanna1234@gmail.com> wrote:
>
> I looked around on the wiki and in the user list archives and couldn't fi=
nd something definitive about certain failure scenarios.
>
> A partition splits the ensemble where a quorum is on one side of the part=
ition
> -- if the leader is on the quorum side of the partition, what happens to =
reads/writes that go to the non-quorum side?  I assume writes return errors=
 because it can't get to the leader.  Reads?
>
>> The reads will also fail on all the quorum nodes until a new quorum is e=
lected.
>
> -- if the leader is on the non-quorum side of the partition, I would assu=
me that the quorum side of the partition would elect a new leader for those=
 clients on its side of the partition.  However, is there the possibility f=
or the leader on the non-quorum side to accept writes before it realizes th=
at there's no longer a quorum?  Just wondering about the possibility of cor=
ruption and then when the cluster syncs back up how the cluster would handl=
e that data.
>
>> No there isnt. The leader relinquishes its right as a leader as soon as =
it realizes a quorum isnt committing the changes it proposed.
>
> (I would be happy to create a wiki page for failure scenarios if one does=
n't exist that people could add to, but maybe this is just common knowledge=
.)
>
>> Please do!
>
> thanks
> mahadev


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