Interview prompt
Explain replication improves availability at a consistency cost to an engineer who understands the surrounding system but has not used this technique. Walk from its contract to a concrete operation, then discuss where it fails or becomes expensive.
A strong answer
Replication stores copies of data on multiple machines to improve availability, read capacity, or recovery. The system must define how writes reach replicas and what reads are allowed to observe. “Consistent” is not one universal behavior; the contract should name ordering and freshness guarantees.
With asynchronous replication, a write can be acknowledged by the leader before a follower receives it, so an immediate read from that follower may return an older value. Read-your-writes can be supported by routing a client to an up-to-date replica or carrying a version constraint.
A complete answer also calls out the assumptions that control correctness. Synchronous replication can reduce the window of acknowledged data loss but may add latency or reduce write availability when a replica is unreachable. A quorum is meaningful only with a defined membership and failure model. Replicas do not replace backups against accidental deletion or corruption.
Close by describing one representative test or measurement. A user updates an address and then reads the previous address from another region. State which consistency promise is missing and one routing or replication strategy that can provide it.
Follow-up questions
Answer the follow-ups in the frontmatter. Use the linked article for the concept and the trace to make the explanation concrete.