[pgpool-general: 7560] Re: VIP with one node

[Tatsuo Ishii]

Pgpool-II needs to manage failover/failoback/promote etc. by the
leader node in any clustering mode.

> 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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