4.1.2. Amazon SNS for Notifications

First Principle: Amazon SNS provides a scalable and reliable pub/sub messaging service for developers, enabling efficient fan-out communication and immediate notifications to various endpoints.

SNS pushes messages from one publisher to many subscribers simultaneously — the classic fan-out pattern. It's ideal for application-to-application (A2A) and application-to-person (A2P) communication.

• Publish/Subscribe (Pub/Sub) Model: Publishers send messages to a central topic, and all subscribers to that topic receive the message.
• Fan-out Capabilities: A single message published to an SNS topic can be delivered to multiple different types of subscribers simultaneously.
• Subscriber Types:
  • AWS Lambda functions: For event processing.
  • Amazon SQS queues: For reliable message delivery and decoupling.
  • HTTP/S endpoints: For webhooks.
  • Email, SMS, mobile push notifications.
• Message Filtering: Subscribers can filter messages received from a topic based on message attributes. Attach a filter policy to the individual subscription (e.g., {"eventType": ["order_placed"]}) so only that subscriber is filtered, with no extra topics and no filtering code in the consumer.
• Durability: Messages are stored durably across multiple Availability Zones.

Scenario: You're developing an image processing application. When a new image is uploaded and successfully processed, you need to notify several different downstream services (e.g., a database update, a search index rebuild) and also send an email notification to the user who uploaded the image.

⚠️ Exam Trap: SNS is push-based (subscribers receive immediately). SQS is pull-based (consumers poll for messages). If a question asks about notifying multiple services simultaneously, SNS is the answer. For buffering and decoupling, it's SQS.