We are having problems with understanding the different between the QUORUM and TWO ConsistencyLevel rules. We have a cluster defined with 3 nodes, a NetworkTopologyStrategy, and a SimpleSnitch. If the ConsistencyLevel is set to Quorum for writes, there is no problem. If we set the ConsistencyLevel to TWO for write, the write fails.
I believe in this case the both ConsistencyLevels require 2 nodes to write to but one fails and one succeeds. Any ideas?
What error message do you see?
What is your replication factor? This will affect the meaning of "quorum".
With RF=1, QUORUM only writes to one node, for example, which might explain why it works when TWO fails. See http://www.ecyrd.com/cassandracalculator/ for a handy calculator (though it doesn't support the CL=TWO level).
TWO and THREE are not intended for low RFs: see http://www.datastax.com/docs/0.7/consistency/index
For specialized cases where the replication factor is higher than three, Cassandra provides consistency levels of TWO and THREE