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From Imesha Sudasingha <imesha...@cse.mrt.ac.lk>
Subject Re: Zookeeper - Network partitioning behaviour
Date Thu, 06 Oct 2016 07:04:22 GMT
Hi Rakesh,

Thanks for your great explanation. That is what I wanted to know.

Regards,
Imesha Sudasingha

On 6 October 2016 at 12:25, Rakesh Radhakrishnan <rakeshr@apache.org> wrote:

> Hi Imesha Sudasingha,
>
> For example, we have A,B,C,D,E five servers, that formed quorum and assume
> A is the Leader.  Again assume network partition happened between
> A,B(minority region)  and C,D,E. As we know, 3 is the majority factor to
> form quorum. Since Leader A is in the minority region, the entire quorum
> will get shutdown and all the servers will automatically moves to leader
> election phase. Now, A & B will do sending notifications each other and
> will never succeed to form quorum due to <3 factor. On the other side, C,D
> and E will participate each other and elect one of them as Leader.
>
> >>>>>But will the partition with a minority will also continue to serve
> read
> >>>>>requests until a write request comes to the leader in the minority
> >>>>>partition?
> After the partition, all the servers in the minority region will get
> shutdown and moves to leader election phase. All the client sessions
> connected to these servers will be disconnected and
> will receive "KeeperState.Disconnected" event to their watchers, if any
> registered.
>
> But ZooKeeper supports read-only server mode. In this mode, client can
> connect to the read-only server even when the server might be partitioned
> from the quorum.
>
> Reference:- Please read the section "Read Only Mode Server" in the apache
> document link, https://zookeeper.apache.org/doc/r3.4.9/zookeeperAdmin.html
>
>
> >>>>>In the above scenario, what will happen to the "watches" that have
> >>>>>already been registered to a node in the minority partition?
> Since the quorum is re-forming, all the client session watchers will
> receive "KeeperState.Disconnected" event. These client session will start
> sending connection request to all quorum servers(A,B,C,D,E) to re-establish
> the connection. In my above example, C,D,E re-forms quorum successfully and
> client sessions will reconnect to one of these servers in the quorum(am
> assuming zkclient has the C,D,E server host address configured). Clients
> automatically reset watches during successful session reconnect. There is a
> feature to disable this watch resetting, please read the configuration
> "zookeeper.disableAutoWatchReset" section in the following apache doc link
> to understand more on this,
> https://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html
>
> Regards,
> Rakesh
>
> On Wed, Oct 5, 2016 at 6:39 PM, Imesha Sudasingha <imesha.13@cse.mrt.ac.lk
> >
> wrote:
>
> > Hi all,
> >
> > When I was going through zookeeper, I noticed the following scenarios,
> >
> >    - Suppose we have a 5 member quorum operating as normal. In a network
> >    partitioning where the leader and a member node goes into one
> partition
> > and
> >    other 3 nodes get into the other network partition. I known then the
> > side
> >    with the majority will elect a new leader and carry on to serve
> > requests.
> >    But will the partition with a minority will also continue to serve
> read
> >    requests until a write request comes to the leader in the minority
> >    partition? How does zookeeper handle this occasion?
> >    - In the above scenario, what will happen to the "watches" that have
> >    already been registered to a node in the minority partition?
> >
> > Can you clarify the above scenarios?
> > Thanks in advance!
> >
> > --
> > *Imesha Sudasingha*
> > Undergraduate of Department of Computer Science and  Engineering,
> > University of Moratuwa.
> > +94717086160
> > View in Linkedin <https://lk.linkedin.com/in/imeshasudasingha>
> >
>



-- 
*Imesha Sudasingha*
Undergraduate of Department of Computer Science and  Engineering,
University of Moratuwa.
+94717086160
View in Linkedin <https://lk.linkedin.com/in/imeshasudasingha>

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