<div dir="ltr"><div>Hi Ishii-San</div><div><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Aug 15, 2019 at 11:42 AM Tatsuo Ishii <<a href="mailto:ishii@sraoss.co.jp">ishii@sraoss.co.jp</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi Usama,<br>
<br>
When number of Pgpool-II nodes is even, it seems consensus based<br>
failover occurs if n/2 Pgpool-II agrees on the failure. For example,<br>
if there are 4 nodes of Pgpool-II, 2 nodes agree on the failure,<br>
failover occurs. Is there any reason behind this? I am asking because<br>
it could easily lead to split brain, because 2 nodes could agree on<br>
the failover while other 2 nodes disagree. Actually other HA software,<br>
for example etcd, requires n/2+1 vote to gain consensus.<br>
<br>
<a href="https://github.com/etcd-io/etcd/blob/master/Documentation/faq.md#what-is-failure-tolerance" rel="noreferrer" target="_blank">https://github.com/etcd-io/etcd/blob/master/Documentation/faq.md#what-is-failure-tolerance</a><br>
<br>
With n/2+1 vote requirements, there's no possibility of split brain.<br>
<br></blockquote><div><br></div><div>Yes, your observation is spot on. The original motivation to consider the exact n/2 votes for consensus rather (n/2 +1)</div><div>was to ensure the working of 2 node Pgpool-II clusters.</div><div>My understanding was that most of the users use 2 Pgpool-II nodes in their setup, so I wanted</div><div>to make sure that in the case when one of the Pgpool-II nodes goes down ( In 2 node) cluster the consensus</div><div>should still be possible.</div><div>But your point is also valid that makes the system prone to split-brain. So what are your suggestions on that?</div><div>I think we can introduce a new configuration parameter to enable/disable n/2 node consensus.</div><div><br></div><div>Thanks</div><div>Best Regards</div><div>Muhammad Usama</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
Best regards,<br>
--<br>
Tatsuo Ishii<br>
SRA OSS, Inc. Japan<br>
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</blockquote></div></div>