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From Andor Molnar <an...@cloudera.com>
Subject Re: Leader elections between 2 nodes.
Date Thu, 10 May 2018 21:26:29 GMT
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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