عرض عملي: معالجة الرسالة نفسها مرتين بأمان

خطوة بخطوة

1. Publish one OrderPlaced event to inventory; stock drops by 2.
2. Replay the same record (reset the offset or resend); stock drops by 2 again. Show the bug.
3. Add an inbox table with the event id as primary key and insert it in the same transaction.
4. Planned mistake: keep processed ids in a HashSet. Restart the service, replay, and the duplicate is applied. Fix: durable inbox row.
5. Replay again: the unique violation is caught, stock is unchanged, and a log line says "duplicate skipped".