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From: Konstantin Osipov via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Cc: tarantool-patches@dev.tarantool.org
Subject: Re: [Tarantool-patches] [PATCH 1/2] replication: introduce ballot.can_be_leader
Date: Mon, 19 Jul 2021 12:12:48 +0300
Message-ID: <20210719091248.GA4257@starling> (raw)
In-Reply-To: <a69f62eb-f6f8-e754-1390-6c295f2f8022@tarantool.org>

* Vladislav Shpilevoy <v.shpilevoy@tarantool.org> [21/07/18 20:03]:
> >> The new field during bootstrap will help to avoid selecting a
> >> 'voter' as a master. Voters can't write, they are unable to boot
> >> themselves nor register others.
> >>
> >> @TarantoolBot document
> >> Title: New field - IPROTO_BALLOT_CAN_BE_LEADER
> >> It is sent as a part of `IPROTO_BALLOT (0x29)`. The field is a
> >> boolean flag which is true if the sender has `election_mode` in
> >> its config `'manual'` or `'candidate'`.
> >>
> >> During bootstrap the nodes able to become a leader are preferred
> >> over the nodes configured as `'voter'`.
> > 
> > Curious why did you add this feature in the first place, I mean
> > "eligibility"? Each voter has to be able to become a leader,
> > otherwise raft liveness guarantees are violated. Raft has
> > learners, but learners neither vote nor can become leaders.
> 
> Voters are nodes which an admin does not want to be a leader. For
> instance, they are too far away physically. As voters, they might
> help to elect a leader, for example, if there are just 3 nodes one
> of which is a voter.
> 
> Another application is when you specifically start 1 node as a
> voter and 2 candidates. The voter might skip all the replication
> data and work on a slow small machine.
> 
> It can help to form a majority. We are planning to make this
> feature even easier to use by adding dataless nodes just for
> voting.
> 
> As for Raft, it should not bring any problems. In Raft you can
> say that all nodes are candidates, but some of them are so slow,
> that they can never vote for themselves in time. Raft still works,
> and you essentially have 'voters'.

Imagine there are nodes A, B, C, D, E.
A is a leader, E is a voter which can not become a leader.

Imagine A's log index is 5, B = 4, C = 3, D = 2, E = 5.

The majority's log index is 4, so entry 4 is committed. A dies, B
is partitioned away. The cluster is stuck, because neither C nor B
can get a quorum (3 votes).

Worse yet, if E's (voter) commit index is low, not high, it can vote for a
node which doesn't have a committed entry. In that case you can
lose a committed entry.

-- 
Konstantin Osipov, Moscow, Russia

  reply	other threads:[~2021-07-19  9:12 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 [this message]
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
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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