[pgpool-general: 7556] Re: VIP with one node

[MichaelDBA]

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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