Amazon SNS: Simple Notification Service

Amazon Simple Notification Service (SNS) is a fully managed publish-subscribe (pub/sub) messaging service. While SQS provides point-to-point messaging (one producer, one consumer), SNS enables one-to-many communication — a single message published to a topic is delivered to multiple subscribers simultaneously.

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SQS vs SNS: The Fundamental Difference

Feature	SQS (Queue)	SNS (Topic)
Pattern	Point-to-point	Publish-subscribe
Consumers	One consumer per message	Multiple subscribers per message
Delivery	Consumer polls the queue	SNS pushes to subscribers
Persistence	Messages stored until deleted	No persistence — push and forget
Use case	Work queues, task processing	Notifications, fan-out, broadcasting

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How SNS Works

The Pub/Sub Model

1. Create a topic — a logical channel for messages
2. Add subscribers — endpoints that want to receive messages
3. Publish a message — the publisher sends a message to the topic
4. SNS delivers — the message is pushed to all subscribers simultaneously

Publisher  ——>  [SNS Topic]  ——>  Subscriber 1 (Email)
                             ——>  Subscriber 2 (SQS Queue)
                             ——>  Subscriber 3 (Lambda Function)
                             ——>  Subscriber 4 (HTTP Endpoint)

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SNS Topic Types

Standard Topics

• Nearly unlimited throughput — millions of messages per second
• Best-effort ordering — messages are generally delivered in order, but not guaranteed
• At-least-once delivery — messages may be delivered more than once to a subscriber
• Broad subscriber support — supports SQS, Lambda, HTTP/S, email, SMS, and mobile push

FIFO Topics

• Strict ordering — messages are delivered in the exact order they were published
• Exactly-once delivery — deduplication eliminates duplicate deliveries
• Limited subscribers — SQS FIFO queues and Lambda functions can subscribe to FIFO topics (Lambda support added 2022)
• Throughput: up to 300 publishes per second (3,000 with batching)

Feature	Standard Topic	FIFO Topic
Throughput	Nearly unlimited	300 msg/s (3,000 with batching)
Ordering	Best-effort	Strict
Delivery	At-least-once	Exactly-once
Subscribers	All types	SQS FIFO only
Topic name	Any valid name	Must end in .fifo

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Subscriber Types

SNS can push messages to a wide variety of endpoints:

Subscriber Type	Description	Use Case
Amazon SQS	Deliver to an SQS queue	Decoupled processing, fan-out
AWS Lambda	Invoke a Lambda function	Serverless event processing
HTTP/HTTPS	POST to a webhook URL	Third-party integrations
Email	Send an email (requires confirmation)	Alerts, notifications
SMS	Send a text message	Urgent notifications
Mobile push	Push notification (APNs, FCM)	Mobile app alerts
Amazon Kinesis Data Firehose	Stream to S3, Redshift, etc.	Data archiving

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Publishing Messages

Basic Publish

import boto3

sns = boto3.client('sns')

sns.publish(
    TopicArn='arn:aws:sns:eu-west-2:123456789012:order-events',
    Subject='New Order',
    Message='{"orderId": "ORD-12345", "status": "placed", "total": 49.99}'
)

Publish with Message Attributes

Message attributes are metadata attached to the message. They are critical for message filtering (covered below).