Thanks! got it! On Sat, Dec 17, 2016 at 5:02 PM, Max C <mc_cassan...@core43.com> wrote:
> 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 > > > > > > > > > > >
