What is Cassandra's consistency level?
Answer
Cassandra consistency levels define how many replica nodes must respond for a read or write to succeed. Key levels: ONE: one replica responds. Fastest, lowest consistency. A write succeeds when one node confirms. A read returns data from the first node to respond. QUORUM: majority of replicas in the cluster must respond (RF/2 + 1 nodes). Balance of consistency and availability. LOCAL_QUORUM: quorum within the local datacenter only. Best for multi-DC deployments to avoid cross-DC latency on every operation. ALL: every replica must respond. Strongest consistency, least available. LOCAL_ONE: one replica in the local DC. EACH_QUORUM: quorum in each datacenter. Strong consistency condition: write_CL + read_CL > RF. With RF=3: QUORUM write (2 nodes) + QUORUM read (2 nodes) = 4 > 3 — guaranteed to read the latest write. Most production Cassandra uses LOCAL_QUORUM for consistency with low cross-DC latency.