how does Local quorum consistency work ?? response from fastest node?

Question

i always have this doubt when a cassandra node got a read request for local quorum consistency does co-ordinator node asks all nodes with replicas in that DC for response or just the fastest responding nodes to it who's count satisfy the local quorum.

In this case RF is 3 Cassandra timeout during read query at consistency LOCAL_QUORUM (2 responses were required but only 1 replica responded).... does this mean coordinator asked only two replicas with fastest response for data and 1 out of 2 timed out or coordinator asked all nodes with replicas which means all 3 and 2 out of 3 timed out as i only got single response back.

Answer 1

The datastax docs describe LOCAL_QUORUM as follows:

Returns the record after a quorum of replicas in the current data center as the coordinator node has reported. Avoids latency of inter-data center communication.

The coordinator sends the read request to all of the replicas, and returns a result to the client when a quorum of nodes has replied to it. Your error indicates that 2 of the 3 replica nodes failed to reply within the ReadTimeout period. There could be various reason(s) for the error - like a node may be doing GC, or compacting, or under some other load which delays the reply. You would probably want to do some more troubleshooting to see what caused it.