[pgpool-general: 7554] Re: VIP with one node

[Tatsuo Ishii]

> 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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