CODEWITHSUNDEEP
cassandracassandra-2.0eventual-consistency
user1870400
user1870400

Reputation: 6364

How does R+W > N makes sense for large clusters?

Say I have a 50 node cluster and my replication factor = 3 and Write Consistency Level = Quorum which is (50/2) + 1 = 26 and Read Consistency Level = ONE. Now we satisfy the equation R + W > N because 26 + 1 > 3 but how does this guarantee strong consistency? what if my write goes to first 26 nodes and I happen to read from node 45. How can I get the most updated value?

Upvotes: 1

Views: 188

Answers (1)

MarcintheCloud
MarcintheCloud

Reputation: 1653

Consistency Level = The number of acknowledgements that REPLICA nodes need to give the coordinator in order to satisfy a request.

So in your example, CL=QUORUM = 2 because 2 is the majority of RF = 3.

Plugging that back into the equation... 2 + 1 = 3 3 !> 3 there for you don't have 100% consistency.

See: http://docs.datastax.com/en/cassandra/2.0/cassandra/dml/dml_config_consistency_c.html

Upvotes: 4

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