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From Max C <>
Subject Re: quick questions
Date Sun, 18 Dec 2016 01:02:52 GMT
As Matija mentioned, quorum is RF / 2 + 1:

RF=1, Quorum = 1
RF=2, Quorum = 2
RF=3, Quorum = 2
RF=4, Quorum = 3
RF=5, Quorum = 3
RF=6, Quorum = 4
RF=7, Quorum = 4

So no, you don’t have to have an odd RF to achieve a quorum, as you see above.  Most people
use RF=3 with a minimum of 3 nodes, though.  For RF=3, 2 of the 3 nodes need to be up in order
to satisfy a quorum read/write.

If you can’t achieve a quorum and you’re trying to read/write with quorum consistency
then the read/write operation will fail.  You could still do reads/writes with CL=ONE, though
(provided that at least 1 of the replicas was up).

- Max

> On Dec 17, 2016, at 1:21 pm, Kant Kodali <> wrote:
> I keep hearing that the minimum number of Cassandra nodes required to achieve Quorum
consensus is 4 I wonder why not 3? In fact, many container deployments by default seem to
deploy 4 nodes. Can anyone shine some light on this?
> What happens if I have 3 nodes and replication factor of 3 and consistency level: quorum?
I should be able to achieve quorum level consensus right.
> If Total node = 3, RF=2 and consistency level = Quorum. Then I understand the quorum
level consensus is not possible because the number of replica nodes here are 2.
> This also brings up another question does number of replica nodes always have to be an
odd number to achieve quorum level consensus? If so, what happens when a replica node goes
down ? it would still serve the requests but the quorum level consensus is not possible?
> Thanks
> kant

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