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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 00:16:48 +0300
Message-ID: <YPc9QCmbEHUIQjOT@grain> (raw)
In-Reply-To: <20210719091248.GA4257@starling>

On Mon, Jul 19, 2021 at 12:12:48PM +0300, Konstantin Osipov wrote:
> 
> 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.

Wait, Kostya, here is a set

     A  B  C  D  E
    {5, 4, 3, 2, 5}
     *  *        *
     L  F  F  F  V

where L - leader, F - follower, V - voter, LCI is 4 (least common index),
Q(uorum) = 3, then

     A  B  C  D  E
    {-, -, 3, 2, 5}
           F  F  V

The Voter E won't be able to choose C or D because its log
is bigger and the cluster get stuck (this is guaranteed by
vclock comparision).

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? The E won't be leader but will
help C to gather the majority. So the cluster should be safe
I only I'm not missing something obvious.

  parent reply	other threads:[~2021-07-20 21:16 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 [this message]
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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