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</head><body text="#000000">I think you should say that right up front
in the documentation, but also mentioning it could work in a 2 node
configuration with the "enable_consensus_with_half_votes" turned on
while also noting that you may get into split-brain scenarios without
further customizing of the configuration commands plus some other
external automatic programming to effectively fence out the previous
primary.<br>
<br>
<br>
<span>Anssi Kanninen wrote on 5/8/2021 9:08 AM:</span><br>
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cite="mid:32EDE258-F3DE-4B82-8EA1-9376C4B2C858@iki.fi">
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Isn't that the case with all primary-standby type clusters?<br>
<br>
<div class="gmail_quote">On 8 May 2021 15.39.26 EEST, MichaelDBA
<a class="moz-txt-link-rfc2396E" href="mailto:MichaelDBA@sqlexec.com"><MichaelDBA@sqlexec.com></a> wrote:<blockquote class="gmail_quote"
style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204,
204); padding-left: 1ex;"><pre class="k9mail">Wow, first I heard of that --> "We do not recommend 2-node pgpool
configuration. "
Tatsuo Ishii wrote on 5/5/2021 5:17 PM:
<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;"><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;"> Dear Tatsuo,
thank you for your answer.
I do recognize the dangers of a split-brain scenario.
With a 2-node setup, however, it is possible to configure PgPool to
have the VIP on one node while the other node is down (unreachable),
even though it may very well be a split-brain situation:
Node B loses connection to Node A. Neither of them has the quorum (in
fact there can never be a quorum in a 2-node cluster), still Node B
will have the VIP as well as Node A.
With that risk in mind, why can we not have such an option for
clusters with more than 2 nodes?
</blockquote> We do not recommend 2-node pgpool configuration. The option for 2-node
which your are referring to is enable_consensus_with_half_votes (the
default is off). It was created just for a compromise because there
have been already too many 2-node users.
<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;"> Regards,
tamas
2021. 05. 05. 14:04 keltezéssel, Tatsuo Ishii írta:
<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #8ae234; padding-left: 1ex;"><blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #fcaf3e; padding-left: 1ex;"> Dear all,
I have fairly common 3-node cluster, with each node running a PgPool
and a PostreSQL instance.
I have set up priorities so that:
- when all 3 nodes are up, the 1st node is gonna have the VIP,
- when the 1st node is down, the 2nd node is gonna have the VIP, and
- when both the 1st and the 2nd nodes are down, then the 3rd node
should get the VIP.
My problem is that when only 1 node is up, the VIP is not brought up,
because there is no quorum.
How can I get PgPool to bring up the VIP to the only remaining node,
</blockquote> There's no way to do that.
<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #fcaf3e; padding-left: 1ex;">which still could and should serve requests?
</blockquote> No, because it's dangerous. Think about a split brain scenario: node A
loses connection to node B and c. In this case node A loses the
quorum, while node B and C have the quorum and will keep the VIP. If
we allow the VIP to node A, there are 2 VIPs are up. We surely want to
avoid it.
Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
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