[pgpool-hackers: 3384] Re: Failover consensus on even number of nodes

[Muhammad Usama]

Hi Ishii-San


On Thu, Aug 15, 2019 at 11:42 AM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:

> Hi Usama,
>
> When number of Pgpool-II nodes is even, it seems consensus based
> failover occurs if n/2 Pgpool-II agrees on the failure. For example,
> if there are 4 nodes of Pgpool-II, 2 nodes agree on the failure,
> failover occurs. Is there any reason behind this? I am asking because
> it could easily lead to split brain, because 2 nodes could agree on
> the failover while other 2 nodes disagree. Actually other HA software,
> for example etcd, requires n/2+1 vote to gain consensus.
>
>
> https://github.com/etcd-io/etcd/blob/master/Documentation/faq.md#what-is-failure-tolerance
>
> With n/2+1 vote requirements, there's no possibility of split brain.
>
>
Yes, your observation is spot on. The original motivation to consider the
exact n/2 votes for consensus rather (n/2 +1)
was to ensure the working of 2 node Pgpool-II clusters.
My understanding was that most of the users use 2 Pgpool-II nodes in their
setup, so I wanted
to make sure that in the case when one of the Pgpool-II nodes goes down (
In 2 node) cluster the consensus
should still be possible.
But your point is also valid that makes the system prone to split-brain. So
what are your suggestions on that?
I think we can introduce a new configuration parameter to enable/disable
n/2 node consensus.

Thanks
Best Regards
Muhammad Usama



> Best regards,
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese:http://www.sraoss.co.jp
>
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