How an Outbox Pattern and NATS JetStream Fixed Duplicate Orders in a 10M-Message Warehouse System

A warehouse execution system handling 5–10 million daily messages for retail and manufacturing clients was silently producing duplicate orders and missed shipment confirmations. The root cause was a dual-write inconsistency: database commits and message publishes were separate operations with no shared transaction, so a crash between the two steps left both systems out of sync. At low volume the flaw was rare, but at scale it became a weekly operational problem. Engineers resolved it primarily by implementing the Outbox pattern, which writes business changes and pending event notifications in a single database transaction, eliminating the need to reconcile two separate systems. The team also migrated from ActiveMQ to NATS JetStream for simpler clustering and explicit message acknowledgment, though the architectural patterns were credited as the more impactful part of the fix.