What happens when a replica node goes down, with consistency level of quorum?

Question

Scenario:

• Total Nodes: 3 [ A, B, C ]
• Replication factor: 2
• Write consistency: Quorum (2 replicas need to ack)
• Read consistency: Quorum

• Nodes partion ranges:

  • A [ primary 1-25, replica 26-50 ]
  • B [ primary 26-50, replica 51-75 ]
  • C [ primary 51-75, replica 1-25 ]

Question:

Suppose I need to insert data 30 and node A is down. What would be the behavior of Cassandra in this situation? Would Cassandra be able to write the data and report success back to the driver (even though the replica node is down and Cassandra needs 2 nodes to acknowledge a write)?

Answer 1

You only have 1 replica available for the write (B), so you'll get error on write (UnavailableException).

It's better to design your consistency levels / replication factor so that you can tolerate node's failure for a token range (consider bumping your RF to 3).

Also better not to try to solve the availability by following the eventual consistency path (R + W <= N), e.g. putting W=1 in this case. We've tried that and operationally it was not worth the effort.