A modernization project separates order processing from customer notifications. Notification failure must not roll back accepted orders, and each service must scale independently. Which interaction is preferred?

1. Keep a synchronous notification call inside the order database transaction.
2. Publish durable events to independently consumed notification queues with retry and idempotency handling.
3. Retry notifications in a tight loop before acknowledging every order.
4. Share one process-memory list as the only event store.