Shipment Reminders Need Durable Idempotency, Not Just 5-Minute Queue Deduplication

A developer argues that a shipment reminder system should assume at-least-once queue delivery and rely on a durable idempotency key before sending, since queue deduplication only covers a 5-minute window. Acknowledging messages only after a send is safely persisted prevents lost notifications, but it can cause duplicate emails or SMS messages if a worker crashes after the provider accepts a send and before the completion is recorded. The piece recommends an atomic claim on a logical key such as user_id, reminder_id, and scheduled_at, with duplicates acknowledged without resending. It also suggests alerting on mismatches between delivery attempts and unique send keys, and sizing the key store for the full retry horizon.