Broadcast Messaging

AWS SNS is a managed Pub/Sub service. One publisher sends a message to a **Topic**, and multiple subscribers (SQS, Lambda, Email, SMS) receive it instantly.

1. Fan-Out Pattern

This is the classic 'Architect' pattern. A user places an order, and the 'Order Service' publishes a message to an SNS Topic. Simultaneously, the 'Email Service', 'Inventory Service', and 'Shipping Service' (each with their own SQS queue) receive that message. This is how you build highly scalable, decoupled systems.

2. Message Filtering

SNS can filter messages so that a subscriber only receives what it needs. For example, the 'High-Value Shipping Service' only receives messages where the TotalAmount > 1000. This reduces unnecessary processing in your .NET workers.

3. Architect Insight

Q: "SNS vs SQS: Which should I use?"

Architect Answer: "Use **SNS** if you want to notify multiple listeners at once. Use **SQS** if you want to ensure that exactly one worker processes a specific task. In a professional architecture, you almost always use them together (**SNS-to-SQS** fan-out) to get the best of both worlds: parallel processing and reliable persistence."