[pgpool-general: 7558] Re: VIP with one node

[MichaelDBA]

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.


Anssi Kanninen wrote on 5/8/2021 9:08 AM:
> Isn't that the case with all primary-standby type clusters?
>
> On 8 May 2021 15.39.26 EEST, MichaelDBA <MichaelDBA at sqlexec.com> wrote:
>
>     Wow, first I heard of that --> "We do not recommend 2-node pgpool
>     configuration. "
>
>
>
>     Tatsuo Ishii wrote on 5/5/2021 5:17 PM:
>
>             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? 
>
>         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.
>
>             Regards, tamas 2021. 05. 05. 14:04 keltezéssel, Tatsuo
>             Ishii írta:
>
>                     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, 
>
>                 There's no way to do that.
>
>                     which still could and should serve requests? 
>
>                 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 English:
>                 http://www.sraoss.co.jp/index_en.php
>                 Japanese:http://www.sraoss.co.jp
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