The scenario requires a one-to-many, asynchronous communication pattern to handle high-volume, event-driven notifications. A Publish/Subscribe (Pub/Sub) messaging bus is the ideal architectural solution. The website acts as the publisher, sending a single "new order" message to a message broker. The order management, warehouse, and billing systems act as independent subscribers, each receiving a copy of the message for processing. This pattern decouples the systems, allowing them to operate and scale independently while ensuring reliable, near-real-time data distribution, which aligns perfectly with the specified requirements.

B. Extract Transform Load (ETL): ETL is a batch-oriented process designed for bulk data movement, typically to a data warehouse, and is not suitable for near-real-time, event-driven notifications.

C. Enterprise Data Warehouse (EDW): An EDW is a system for data storage and analysis, not an integration pattern or a mechanism for real-time communication between operational systems.

D. Managed File Transfer (MFT): MFT is used for secure, scheduled, bulk file transfers. It is not designed for low-latency, message-based, one-to-many event notifications.

1. MuleSoft Official Documentation, Anypoint MQ: "Anypoint MQ is a multi-tenant, cloud messaging service that enables customers to perform advanced asynchronous messaging scenarios between their applications. Anypoint MQ is fully integrated with Anypoint Platform, offering role-based access control, client application management, and connectors." The documentation further explains publish-subscribe patterns using exchanges.

Source: MuleSoft Docs, "Anypoint MQ," Introduction section.

2. Hohpe, G., & Woolf, B. (2003). Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions. Addison-Wesley Professional. This foundational text, widely used in university curricula and professional training, defines the Publish-Subscribe Channel pattern.

Source: Chapter 4: Messaging Channels, "Publish-Subscribe Channel," pages 99-103. The pattern is described as a channel that "delivers a copy of a particular message to each of the attached receivers," which directly matches the scenario's need to notify three distinct systems.

3. MuleSoft Official Documentation, Event-Driven Architecture (EDA): "EDA is an architecture paradigm that enables an organization to detect “events” or important business moments (such as a transaction) and act on them in real time or near-real time... The publish/subscribe model allows for real-time communication between different applications without them being connected to each other."

Source: MuleSoft Whitepapers & Resources, "What is event-driven architecture (EDA)?" section.