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From s n <svadhyaya.pr...@gmail.com>
Subject Re: Leader elections between 2 nodes.
Date Thu, 10 May 2018 22:24:25 GMT
*ReadOnlyMode Servers*

ReadOnlyMode allows clients sessions which requested ROM support to connect
to the server even when the server might be partitioned from the quorum. In
this mode ROM clients can still read values from the ZK service, but will
be unable to write values and see changes from other clients

Wouldn't this cause stale reads in case of network partition ?


On Thu, May 10, 2018 at 2:26 PM, Andor Molnar <andor@cloudera.com> wrote:

> You cannot read from ZooKeeper if there's no quorum, because it rejects
> client connections.
> But when you can, the request doesn't go to the leader, it served directly
> from the server that client is connected to.
>
> As a consequence, you cannot read stale data from ZooKeeper.
>
> Andor
>
>
>
> On Thu, May 10, 2018 at 1:36 PM, David Brower <david.brower@oracle.com>
> wrote:
>
> > Without quorum, the client has no idea how current the read data may be.
> > It might be from a replica that is hours/days/weeks out of data.
> > Sometimes this won't matter, sometimes it will be a critical failing.
> >
> > -dB
> >
> >
> >
> > On 5/9/2018 6:40 PM, Prasanth Mathialagan wrote:
> >
> >> For reads, is Quorum necessary? Reads are served from local the local
> >> replica, right?
> >>
> >> On Wed, May 9, 2018 at 8:29 AM, Jordan Zimmerman <
> >> jordan@jordanzimmerman.com
> >>
> >>> wrote:
> >>> A quorum means (n/2)+1 - In a 3 node cluster that is 2. So, if 2
> servers
> >>> are up you can achieve quorum.
> >>>
> >>> we need odd
> >>>> numbers in order to form a quorum
> >>>>
> >>> The "odd" in this formula refers to the number of configured servers
> not
> >>> how many are running at a given time. That's the reasoning for quorum
> in
> >>> the first place. Quorum I/O ensures that a majority of nodes is
> >>> referenced
> >>> for reads or writes (in ZooKeeper writes are what matter for quorum).
> >>>
> >>> -Jordan
> >>>
> >>> On May 9, 2018, at 10:26 AM, Kaushal Shriyan <kaushalshriyan@gmail.com
> >
> >>>>
> >>> wrote:
> >>>
> >>>> Hi,
> >>>>
> >>>> In a ensemble of 3 nodes with 1 leader and 2 followers, if the leader
> >>>>
> >>> goes
> >>>
> >>>> down then two servers can elect a leader among themselves. But we need
> >>>>
> >>> odd
> >>>
> >>>> numbers in order to form a quorum. I am confused. Please correct me
> if i
> >>>>
> >>> am
> >>>
> >>>> understanding it wrong.
> >>>>
> >>>> Any help will be highly appreciable
> >>>>
> >>>> Best Regards,
> >>>>
> >>>> Kaushal
> >>>>
> >>>
> >>>
> >
>

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