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From Prabath Abeysekara <>
Subject Minimum Cluster size to accommodate a single node failure
Date Wed, 18 Jun 2014 06:08:57 GMT
Hi Everyone,

First of all, apologies if the $subject was discussed previously in this
list before. I've already gone through quite a few email trails on this but
still couldn't find a convincing answer which really made me raise this
question again here in this list.

If my understanding is correct, a *3 node Cassandra cluster* would survive
a single node failure while the Replication Factor is set to 3 with
consistency levels are assigned QUORUM for read/write operations. For
example, let's consider the following configuration.

* Number of nodes in the cluster : 3
* Replication Factor : 3
* Read/Write consistencies : QUORUM (this evaluates to 2 when RF is set to

Here's how I expect it to work.

Whenever a read operation takes place, the Cassandra cluster coordinator
node that receives the read request would try to read from at least two
replicas before responding to the client. With Read consistency being 2 (+
all raws being available in all three nodes), we should be able to survive
a single node failure in this particular instance for read operations.
Similarly, for write requests, even in the middle of a single node failure,
the writes should be allowed as the Write consistency is set to 2?

Can someone please confirm whether what's mentioned above is correct?
(Please note that I'm trying to figure out the *minimum* node numbers and I
indeed am aware of the fact that there are other factors also to be
considered in order to come up with the most optimal numbers for a given
cluster requirement).


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