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From Michael Han <h...@apache.org>
Subject Re: How to deliberately cause a split brain?
Date Thu, 18 Jun 2020 23:19:37 GMT
There are different cases for split brain and how to test the monitor code
depends on what signals you are using - but most usually, a split brain
case can be created by artificially split two quorum out of a single quorum
through manual configuration change (e.g. a 7 quorum servers can be split
into a 4 quorum with a 3 quorum - just exclude the servers from each
config).

Testing aside, internally we've been using the cardinality of QuorumPeer's
ServerState and this turns out to be a pretty reliable signal for dual
leader cases. For data consistency check in split brain case there is real
time data consistency check built in ZooKeeper in recent release (at least
for 3.6), and there are some built in metrics to signal the inconsistencies.

On Thu, Jun 18, 2020 at 5:01 AM Enrico Olivelli <eolivelli@gmail.com> wrote:

> I told about using iptables only in order to create a network partition.
>
> If you want a simple split brain just switch every server in the cluster to
> standalone mode (comment server.xx lines)
> I saw it happening in production, it was so nasty :-(
>
> Enrico
>
> Il giorno gio 18 giu 2020 alle ore 13:50 Jordan Zimmerman <
> jordan@jordanzimmerman.com> ha scritto:
>
> > Way back in 2013 I wrote about a scenario whereby ZooKeeper will have two
> > leader nodes. I don’t know if it still applies but maybe it will be
> helpful
> > to you:
> >
> http://zookeeper-user.578899.n2.nabble.com/Rolling-config-change-considered-harmful-td7578761.html
> >
> > ====================
> > Jordan Zimmerman
> >
> > > On Jun 18, 2020, at 4:07 AM, Tim Ward <tim.ward@featurespace.co.uk>
> > wrote:
> > >
> > > From: Szalay-Bekő Máté <szalay.beko.mate@gmail.com>
> > >> Honestly, I can not really think of any way to get split-brain. If
> > someone can, then let us know so that we can fix it ;)
> > >
> > > This issue arose because, apparently, we did get a split brain once
> upon
> > a time. The hints I've got however are that this happened in a pre-3.5
> > version of Zookeeper, involving a (botched?) rescaling from 3 to 7 nodes.
> > From which hints I'm guessing that something got fixed in 3.5 so that
> this
> > approach to deliberately provoking a split brain would not work with the
> > 3.5.7 we're now using.
> > >
> > > Tim Ward
> > >
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