Event-Driven Design: Choosing Between SNS, SQS, and EventBridge

Event-driven design is a cornerstone of modern application architecture, especially in serverless systems where decoupled components must communicate efficiently.

In the world of AWS there are three messaging and event-routing services:

• Amazon Simple Notification Service (SNS)
• Amazon Simple Queue Service (SQS)
• Amazon EventBridge.

Each service excels in different scenarios, therefore understanding their nuances can help you design robust and scalable solutions.

Amazon SNS: Publish/Subscribe Messaging

Amazon SNS is a fully managed publish/subscribe (pub/sub) service. It allows a publisher to send messages to multiple subscribers simultaneously.

Key Features:

• Broadcast Messaging: SNS pushes messages to multiple subscribers in real-time.
• Multiple Protocol Support: Subscribers can receive messages over HTTP/S, email, SMS, or AWS services like Lambda and SQS.
• Fan-Out Pattern: Combine SNS with SQS to ensure message durability while maintaining pub/sub communication.

When to Use SNS:

• Real-Time Updates: Notify multiple systems or users about an event immediately (e.g., sending order confirmation emails and updating a dashboard).
• Fan-Out Architecture: Distribute messages to multiple processing pipelines (e.g., log processing, alerting systems).
• Cross-Region Notifications: Enable globally distributed applications to communicate efficiently.

Example:

A news publishing platform uses SNS to notify users of breaking news. Each subscriber chooses a preferred delivery channel—email, SMS, or mobile push notifications.

Example Architecture:

Amazon SQS: Decoupled Queuing

Amazon SQS is a fully managed message queuing service. It decouples components of distributed systems, allowing them to communicate asynchronously.

Key Features:

• Message Durability: Messages are stored until they are processed or expire.
• Processing Guarantee: Messages are delivered at least once (Standard Queue) or exactly once (FIFO Queue).
• Scalability: Supports massive throughput and unlimited queue size.

When to Use SQS:

• Task Queues: Process tasks asynchronously, such as image processing or order fulfillment.
• Message Retention: Ensure messages are not lost if a consumer is temporarily unavailable.
• Rate Limiting: Buffer requests to avoid overwhelming downstream systems.

Example:

An e-commerce platform uses SQS to manage order processing. Orders are added to a queue and processed asynchronously by a fleet of worker instances.

Example Architecture:

Amazon EventBridge: Event Bus for Rule-Based Routing

Amazon EventBridge is an event bus service that allows you to ingest and route events from various sources based on customizable rules.

Key Features:

• Event Routing: Route events to multiple targets based on rules.
• Integrated Sources: Ingest events from AWS services, SaaS applications, or custom applications.
• Schema Discovery: Automatically discover and use schemas for events.

When to Use EventBridge:

• Event-Driven Workflows: Trigger workflows or orchestrate tasks based on specific event patterns (e.g., serverless pipelines).
• Complex Routing: Implement fine-grained routing logic for event processing.
• Integrating SaaS Applications: Easily connect third-party SaaS events into your workflows.

Example:

A logistics company uses EventBridge to trigger a Lambda function when a shipment's status changes in its tracking system, enabling real-time notifications to customers and automated updates to internal dashboards.

Example Architecture:

Choosing the Right Service

The choice between SNS, SQS, and EventBridge depends on your use case:

Feature	SNS	SQS	EventBridge
Message Type	Pub/Sub	Queue	Event Bus
Delivery Model	Push	Pull	Push
Routing Capability	Limited	None	Advanced
Durability	Limited (requires SQS for durable fan-out)	High	High
Use Cases	Real-time notifications, fan-out	Task queues, async processing	Event-driven workflows

Hybrid Patterns

Combining these services can unlock even greater flexibility. For example:

• SNS + SQS: Use SNS for fan-out to multiple SQS queues, ensuring durability and decoupled processing.
• EventBridge + SQS: Route events to specific SQS queues based on event patterns for targeted processing.

Conclusion

SNS, SQS, and EventBridge each play a vital role in event-driven architectures. SNS excels in real-time pub/sub messaging, SQS in decoupled queuing, and EventBridge in advanced event routing. By evaluating your requirements, you can choose the right service or combination of services to build your robust and scalable solution.