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From: Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Konstantin Osipov <kostja.osipov@gmail.com>
Cc: tarantool-patches@dev.tarantool.org,
	Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Subject: Re: [Tarantool-patches] [PATCH 1/2] replication: introduce ballot.can_be_leader
Date: Wed, 21 Jul 2021 21:51:06 +0300
Message-ID: <YPhsmlDR0B1ImFOp@grain> (raw)
In-Reply-To: <20210720232057.GA85781@starling>

On Wed, Jul 21, 2021 at 02:20:57AM +0300, Konstantin Osipov wrote:
> 
> Right. That's what I am saying - the cluster is stuck even though
> the quorum (3 nodes) is present. And this is not something
> consistent, such clusters will get stuck simply based on the state
> of the voter - sometimes they will, sometimes they won't.

Which is exactly the same as it would be if node E is a follower but at
the moment when it gonna be selected as a leader the network environment
get broken (or enter a flip/flop cycle). In such case even traditional
Raft nodes (ie those which doen't have "voters" at all) will behave
the same way -- sometimes quorum will be achieved and sometimes not.
Though I see what you mean, thanks!

> > Lets assume the E's index is low, say 3
> > 
> >     A  B  C  D  E
> >    {5, 4, 4, 3, 3}
> >     *  *
> >     L  F  F  F  V
> > 
> > in this config the leader won't commit record 5 until one
> > of {C,D,E} write the new record(s) since otherwise the quorum
> > won't be reached. Assume A and B get out of the set without
> > record 4 written to C
> > 
> >      A  B  C  D  E
> >     {-, -, 4, 3, 3}
> >            F  F  V
> > 
> > Now the node E can vote for C and D because its index is LE.
> > And since C's index is bigger than others it will be elected
> > next as far as I understand, no?
> 
> You're right, assuming the voter never casts a vote for a
> candidate with a shorter log the safety is not violated. I wasn't
> sure it's the case, and presumed that the voter may have no log of
> its own. But still there are issues with liveness. Raft PHD has
> learners, so why not use them instead.

Yeah, voters are to carry logs as well and it is their critical
feature but idea about Raft learners might be even better, I think
Vlad has some thoughts about learners.

  reply	other threads:[~2021-07-21 18:51 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-15 23:49 [Tarantool-patches] [PATCH 0/2] Bootstrap voter Vladislav Shpilevoy via Tarantool-patches
2021-07-15 23:49 ` [Tarantool-patches] [PATCH 1/2] replication: introduce ballot.can_be_leader Vladislav Shpilevoy via Tarantool-patches
2021-07-16 10:59   ` Serge Petrenko via Tarantool-patches
2021-07-18 17:00     ` Vladislav Shpilevoy via Tarantool-patches
2021-07-19  9:11       ` Sergey Petrenko via Tarantool-patches
2021-07-16 14:29   ` Konstantin Osipov via Tarantool-patches
2021-07-18 17:00     ` Vladislav Shpilevoy via Tarantool-patches
2021-07-19  9:12       ` Konstantin Osipov via Tarantool-patches
2021-07-19 22:06         ` Vladislav Shpilevoy via Tarantool-patches
2021-07-20  8:49           ` Konstantin Osipov via Tarantool-patches
2021-07-20 20:02             ` Vladislav Shpilevoy via Tarantool-patches
2021-07-20 20:18               ` Konstantin Osipov via Tarantool-patches
2021-07-20 21:16         ` Cyrill Gorcunov via Tarantool-patches
2021-07-20 23:20           ` Konstantin Osipov via Tarantool-patches
2021-07-21 18:51             ` Cyrill Gorcunov via Tarantool-patches [this message]
2021-07-21 21:43             ` Vladislav Shpilevoy via Tarantool-patches
2021-07-15 23:49 ` [Tarantool-patches] [PATCH 2/2] election: during bootstrap prefer candidates Vladislav Shpilevoy via Tarantool-patches
2021-07-16 11:30   ` Serge Petrenko via Tarantool-patches
2021-07-18 17:00     ` Vladislav Shpilevoy via Tarantool-patches
2021-07-16 14:27 ` [Tarantool-patches] [PATCH 0/2] Bootstrap voter Konstantin Osipov via Tarantool-patches
2021-07-18 17:00   ` Vladislav Shpilevoy via Tarantool-patches
2021-07-19  9:13     ` Konstantin Osipov via Tarantool-patches
2021-07-19 22:04       ` Vladislav Shpilevoy via Tarantool-patches

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