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From shay shimony <shays...@gmail.com>
Subject Re: ZooKeeper Leader Election Question
Date Sun, 27 Oct 2019 23:39:49 GMT
Hi Eric,

I checked the Leader Elections section in the ZooKeeper book of Junqueira
and Reed, and found that you have a mistake in the rules that you quoted -
the '>' sign should be '<':

1. If (voteZxid *<* myZxid) or (voteZxid = myZxid and voteId *<* mySid),
keep the current vote.
2. Otherwise, change my vote by assigning myZxid to voteZxid and mySid to
vote zxid.

When z3 compares votes from z1(1, 6) and its current vote (3, 5), it
changes its vote to (1, 6).
Next, z3 compares (2, 5) and its current vote (1, 6), and keeps its current
vote (1, 6).

After these 2 iterations z3 will send again its vote to z1 and z2 - this
time (1, 6).
Next, z3 and z1 form a quorum of 2, with z1 as leader and z3 as its
follower.

Hopefully z2 will be on the same page, similarly will join the quorum, and
set itself as follower of z1 as well (normally it will).

Hope it helps,
Shay

On Sun, Oct 27, 2019 at 8:11 AM Lee Eric <openlinuxsource@gmail.com> wrote:

> Hi,
>
> I have a dumb question about the way how zookeeper does leader election.
>
> There is a quorum zk1(1,6), zk2(2,5) and zk3(3,5). The number in
> parentheses is sid and latest zxid. So my question is, when each
> server sends out the vote to other 2, say, zk1 and zk2 send the votes
> to zk3. So zk3 has 2 votes (1,6) and (2,5). Now, based on the rules:
>
> 1. If (voteZxid > myZxid) or (voteZxid = myZxid and voteId > mySid),
> keep the current vote.
> 2. Otherwise, change my vote by assigning myZxid to voteZxid and mySid to
> vote
> Zxid.
>
> So what zk3 vote would be? I'm confused because to compare zk1 and
> zk3, the vote on zk3 is (1,6); but to compare zk2 and zk3, zk3 should
> keep its vote.
>
> So in this case, how the vote should be settled?
>
> Thanks.
>
> Eric
>

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