At-Least-Once vs Exactly-Once: Delivery Guarantees Compared
Overview Messaging and stream-processing systems must pick a delivery guarantee: does a message arrive at least one time (possibly more), or does its effect happen precisely once no matter how many retries occur? At-least-once favors simplicity and throughput by retrying until acknowledged, at the cost of possible duplicates; exactly-once layers on deduplication or transactional coordination so retries never produce a second effect. The choice matters because a duplicate side effect — a double charge, a double email, a double stock decrement — can be catastrophic or merely annoying depending on the domain. ...