[pgpool-hackers: 3396] 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.
> 
> Attached is ready for review patch. GUC and English manual included.

In additon, attached is a patch against 004.watchdog test. Without
this, the test fails.

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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