---id: 0007-at-least-once-with-idempotency-keystype: decisionowner: justinstatus: acceptedreviewed: 2026-10-07---# 0007 At-least-once delivery, made safe with the order id## ContextBetween a payment and a shipment there are three hops that can each be repeated: Stripe redelivers webhooks, the queue redelivers messages, and fulfilment retries the warehouse call. Exactly-once delivery is not on offer from any of them.## DecisionAccept repeats everywhere and make every step idempotent by order id:- order state changes only move forward from a given status ([[payment-webhook]]), so a repeat changes nothing;- `order.paid` may be sent more than once ([[order-events]]);- fulfilment checks the order's status before acting and sends `Idempotency-Key: order-<id>` to the warehouse, which returns the existing shipment for a key it has seen ([[fulfilment]]).## Consequences- No dedupe table and no distributed locks.- Two copies of the same event handled at the same moment can both reach the warehouse. The idempotency key makes the second call return the first shipment, and `recordShipment` only applies once.- This depends on the warehouse honoring idempotency keys for at least as long as our retries last (about two hours). Theirs is 24 hours.