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From Mahadev Konar <maha...@yahoo-inc.com>
Subject Re: Failure scenarios and consequences
Date Thu, 09 Dec 2010 21:42:00 GMT
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 this:
"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. The leader will be demoted to being
a regular ZooKeeper server and those nodes 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 think 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 find something
definitive about certain failure scenarios.
>
> A partition splits the ensemble where a quorum is on one side of the partition
> -- 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 elected.
>
> -- if the leader is on the non-quorum side of the partition, I would assume 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 for the leader on the non-quorum side to accept
writes before it realizes that there's no longer a quorum?  Just wondering about the possibility
of corruption and then when the cluster syncs back up how the cluster would handle 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 doesn't exist that
people could add to, but maybe this is just common knowledge.)
>
>> Please do!
>
> thanks
> mahadev



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