[Tarantool-patches] [PATCH 1/2] replication: introduce ballot.can_be_leader

[Cyrill Gorcunov]

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.