[pgpool-hackers: 3399] Re: Failover consensus on even number of nodes

[Muhammad Usama]

Hi Ishii-San,

The patch looks good overall, but I have few observations.

First I don't think we needed the changes in the
get_mimimum_nodes_required_for_quorum()
function, Since the function, returns the int so the change are no-op I
believe.

Also, I think we need kind of similar changes in
compute_failover_consensus() where we are checking
if we have got enough votes for failover as we have done in
update_quorum_status() function.

So I have updated your patch a little bit. Can you see if the changes I
made looks good to you?

Secondly, I think we may go for a different configuration parameter name
for *allow_a_half_consensus*.
I am not 100 percent convinced on which name we should go with but I have a
few suggestions for that.




*1-- consensus_require_half_of_total_votes2--
resolve_consensus_on_half_of_total_votes3--
half_of_total_votes_satisfy_majority*
*4-- half_of_total_votes_are_enough_for_majority*
*5-- half_of_total_votes_are_enough_for_consensus*
Thoughts and suggestions?


Best Regards
Muhammad Usama

On Wed, Aug 28, 2019 at 1:22 PM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:

> From: Tatsuo Ishii <ishii at sraoss.co.jp>
> Subject: [pgpool-hackers: 3396] Re: Failover consensus on even number of
> nodes
> Date: Tue, 27 Aug 2019 11:11:51 +0900 (JST)
> Message-ID: <20190827.111151.2130894466144469209.t-ishii at sraoss.co.jp>
>
> >>>>> 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.
>
> If there's no objection, I will commit/push tomorrow.
>
> 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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