[pgpool-hackers: 3391] 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?

Attached is a proof of concept patch. GUC and doc change are not
included. With the patch, 2 watchdog node cluster will go into "quorum
absent" state if one the nodes goes down.

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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