AWS SQS, SNS & Event Bridge — When to use what?

Amazon Web Services (AWS) offers multiple messaging and event services, including Amazon Simple Queue Service (SQS), Amazon Simple Notification Service (SNS), and Amazon EventBridge. Each of these services is designed for specific use cases. Here's a detailed breakdown of when to use each service:

Amazon Simple Queue Service (SQS)

SQS is a fully managed message queuing service that allows you to decouple and scale microservices, distributed systems, and serverless applications. It provides two types of message queues: Standard and FIFO (First-In-First-Out).

Use Cases:

1. Decoupling Microservices:

- When you need to decouple components of a system so they can run independently, improving resilience and scalability.

- Example: A user uploads a photo, which triggers an SQS message that a processing service consumes to resize the image.

2. Message Processing:

- When tasks need to be processed asynchronously.

- Example: Background processing of orders in an e-commerce application.

3. Load Leveling:

- To handle traffic spikes by queuing messages that can be processed by one or more consumers at a controlled rate.

- Example: Handling large volumes of user requests without overwhelming the backend.

4. FIFO Processing:

- When the order of message processing is critical and no duplicates are allowed.

- Example: Financial transactions or inventory updates.

Amazon Simple Notification Service (SNS)

SNS is a fully managed pub/sub-messaging service that enables you to decouple microservices, distributed systems, and serverless applications. It supports push-based, many-to-many messaging.

Use Cases:

1. Broadcast Messaging:

- When you need to send the same message to multiple recipients at once.

- Example: Sending alerts to multiple systems or users.

2. Fan-out Pattern:

- To send a message to multiple SQS queues or Lambda functions.

- Example: A new user registration event needs to update several systems (CRM, analytics, etc.) simultaneously.

3. Mobile Push Notifications:

- To send push notifications to mobile devices.

- Example: Sending notifications to users about promotional offers or updates.

4. Application Integration:

- When integrating various AWS services and third-party applications.

- Example: Publishing messages from an application to trigger AWS Lambda functions, SQS queues, or HTTP/S endpoints.

Amazon EventBridge

EventBridge (formerly known as CloudWatch Events) is a serverless event bus that makes it easy to connect application data from your own applications, SaaS applications, and AWS services. It offers more advanced event-routing capabilities than SNS.

Use Cases:

1. Event-Driven Architectures:

- When building applications that respond to events from various sources.

- Example: Automatically triggering a Lambda function when a file is uploaded to an S3 bucket.

2. Complex Event Routing:

- To filter and route events to specific targets based on event patterns.

- Example: Directing security events to a security monitoring system and operational events to an operations dashboard.

3. SaaS Integration:

- To integrate with third-party SaaS providers.

- Example: Triggering events in AWS based on activity in a SaaS application like Zendesk or Datadog.

4. Cross-Account Event Delivery:

- When you need to route events between different AWS accounts.

- Example: Centralized monitoring and alerting setup where events from multiple accounts are routed to a central account.

5. Schema Registry and Discovery:

- To manage event schemas and versioning.

- Example: Defining and discovering schemas for events in a central repository.

Summary

- SQS: Use for message queuing when you need to decouple components, handle asynchronous processing, or ensure message order and uniqueness (FIFO).

- SNS: Use for pub/sub messaging when you need to broadcast messages to multiple subscribers, implement a fan-out pattern, or send mobile push notifications.

- EventBridge: Used for building event-driven architectures with complex routing, integrating with SaaS applications, and handling cross-account event delivery.

Choosing the right service depends on your specific use case and architectural requirements. In many scenarios, these services can complement each other, providing a robust and scalable messaging and event-driven architecture.