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 <k...@peernova.com> 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 > > > > >
