[Tarantool-patches] [PATCH 1/2] replication: introduce ballot.can_be_leader
Cyrill Gorcunov
gorcunov at gmail.com
Wed Jul 21 21:51:06 MSK 2021
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.
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