Database / Apache Cassandra Intermediate and Advanced interview questions
What is the difference between consistency levels ONE, QUORUM, and ALL?
These are three of the most commonly used consistency levels, and each shifts the balance between speed, availability, and correctness differently.
| ONE | QUORUM | ALL |
| Only 1 replica must respond. | (N/2)+1 replicas must respond. | All N replicas must respond. |
| Fastest, most available. | Balances speed and correctness. | Strongest consistency, weakest availability. |
| Tolerates most node failures. | Tolerates a minority of node failures. | Fails if even one replica is down. |
With a replication factor of 3, QUORUM needs 2 of 3 replicas to agree, so the cluster can survive one node outage and still serve strongly-consistent reads/writes when paired correctly (QUORUM read + QUORUM write satisfies R+W>N).
ONE is popular for high-throughput, latency-sensitive workloads like clickstream logging where an occasional stale read is acceptable. ALL is rare in production because a single dead or slow replica blocks the entire request, hurting availability far more than QUORUM does for only a small consistency gain.
With replication factor 3, how many replicas must acknowledge at QUORUM?
Why is CL ALL rarely used in production Cassandra clusters?
More Related questions...
