[pgpool-hackers: 3390] Re: Failover consensus on even number of nodes

[Tatsuo Ishii]

> Hi Ishii-San,
> 
> On Sat, Aug 17, 2019 at 1:00 PM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
> 
>> > Hi Ishii-San
>> >
>> >
>> > On Thu, Aug 15, 2019 at 11:42 AM Tatsuo Ishii <ishii at sraoss.co.jp>
>> wrote:
>> >
>> >> Hi Usama,
>> >>
>> >> When number of Pgpool-II nodes is even, it seems consensus based
>> >> failover occurs if n/2 Pgpool-II agrees on the failure. For example,
>> >> if there are 4 nodes of Pgpool-II, 2 nodes agree on the failure,
>> >> failover occurs. Is there any reason behind this? I am asking because
>> >> it could easily lead to split brain, because 2 nodes could agree on
>> >> the failover while other 2 nodes disagree. Actually other HA software,
>> >> for example etcd, requires n/2+1 vote to gain consensus.
>> >>
>> >>
>> >>
>> https://github.com/etcd-io/etcd/blob/master/Documentation/faq.md#what-is-failure-tolerance
>> >>
>> >> With n/2+1 vote requirements, there's no possibility of split brain.
>> >>
>> >>
>> > Yes, your observation is spot on. The original motivation to consider the
>> > exact n/2 votes for consensus rather (n/2 +1)
>> > was to ensure the working of 2 node Pgpool-II clusters.
>> > My understanding was that most of the users use 2 Pgpool-II nodes in
>> their
>> > setup, so I wanted
>> > to make sure that in the case when one of the Pgpool-II nodes goes down (
>> > In 2 node) cluster the consensus
>> > should still be possible.
>> > But your point is also valid that makes the system prone to split-brain.
>> So
>> > what are your suggestions on that?
>> > I think we can introduce a new configuration parameter to enable/disable
>> > n/2 node consensus.
>>
>> If my understanding is correct, current behavior for 2 node Pgpool-II
>> clusters there's no difference whether failover_when_quorum_exists is
>> on or off. That means for 2 node Pgpool-II clusters even if we change
>> n/2 node consensus to n/2+1 consensus, 2 node users could keep the
>> existing behavior by turning off failover_when_quorum_exists. If this
>> is correct, we don't need to introduce the new switch for 4.1, just
>> change n/2 node consensus to n/2+1 consensus. What do you think?
>>
> 
> Yes, that's true, turning off the failover_when_quorum_exists will
> effectively give us the
> same behaviour for 2 nodes cluster.
> 
> 
>> The only concern is 4 node Pgpool-II clusters. I doubt there's 4 node
>> users in the field though.
>>
> 
> Yes, you are right there wouldn't be many users who would deploy 4 nodes
> cluster. But somehow we need
> to keep the behaviour and configurations consistent for all possible
> scenarios.
> 
> Also, the decision of considering either n/2 or (n/2 +1) as a valid
> consensus for voting is not only limited to
> the backend node failover. Pgpool-II also considers the valid consensus
> with n/2 votes when deciding the
> watchdog master. And currently, the behaviour of watchdog master elections
> and backend node failover consensus
> building is consistent. So If we want to revisit this we might need to
> consider the behaviour in both cases.

Ok, it seems creating new parameter for switching n/2 or n/2+1 could
be safer, I agree. Usama, would like to implement this for 4.1?

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
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