At-Most-Once Delivery

Overview

At-Most-Once Delivery is a delivery approach in which each message is attempted no more than once, preventing redelivery but allowing messages to be lost when transmission or processing fails. The production boundary for At-Most-Once Delivery should identify the authoritative system, responsible owner, accepted states, and recovery path.

Key risks include silent event loss, incomplete audit trails, missed status updates, stale caches, irrecoverable notifications, and business actions that never occur after a transient failure. It favors duplicate avoidance and low overhead, unlike at-least-once delivery, which retries and expects idempotent handling.

A production implementation should use it only where occasional loss is acceptable, make loss visible through counters or reconciliation, preserve authoritative state elsewhere, and document which events are noncritical. Monitoring for At-Most-Once Delivery should distinguish transport success, processing success, and the final external or financial result.

Useful measures include send failures, estimated loss rate, reconciliation gaps, consumer disconnects, unobserved state changes, and recovery actions triggered from the source of truth. At-Most-Once Delivery is closely connected to At-Least-Once Delivery, Message Queue, and Reconciliation API. Operational metrics for At-Most-Once Delivery should use stable denominators and separate technical activity from successful business completion.

For At-Most-Once Delivery, identifiers and timestamps should remain stable enough to trace the technical action to its final business outcome. Changes to At-Most-Once Delivery should be tested against normal, failed, delayed, duplicate, and recovery paths that apply to the operation.

Evidence for At-Most-Once Delivery should preserve the input, configuration version, actor or service, decision, downstream reference, and final outcome. The At-Most-Once Delivery runbook should define who can retry, cancel, replay, reconcile, communicate, and approve an exception.