The model
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.
A concrete walk-through
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.
Costs and failure cases
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.
Check your understanding
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.