Database / ScyllaDB Interview questions
Which is better and why: LOCAL_QUORUM or ONE for a globally distributed app?
Neither is universally better; the choice trades off consistency strength against latency and availability, and the right answer depends on what the read or write is for.
| ONE | LOCAL_QUORUM |
| Fastest possible latency; only one replica must respond. | Slightly higher latency; needs a majority in the local DC. |
| Can return stale data if the responding replica hasn't received the latest write. | Guarantees read-your-writes consistency within the local datacenter (when paired with equally strong writes). |
| Most tolerant of a single replica being down. | Tolerates a minority of local replicas being down. |
ONE is the right choice for workloads where absolute lowest latency matters more than freshness, such as non-critical telemetry ingestion or best-effort caching, and where the application can tolerate occasionally reading slightly stale data. LOCAL_QUORUM is the better default for most application data, like user profiles, orders, or inventory, where the application logic depends on reads reflecting recent writes within the region, since combining LOCAL_QUORUM reads and writes guarantees that overlap. The practical guidance is to default to LOCAL_QUORUM for correctness-sensitive paths and reserve ONE for specific, well-understood cases where staleness is genuinely acceptable.
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