Chat Application Architecture | AWS | API Gateway | WebSockets | Microservices | System Design

In this blog, we’ll dive into one of the ways to build a scalable chat application using AWS Cloud. No fluff—let’s get straight to it!

Functional Requirement

User should be able to chat using we or mobile app.

️Tech Stack

Front End
Website: HTML5 / JavaScript (SPA) / Nextjs
Mobile App: Appropriate Tools

🤝 Interactions
All Client to Server interactions over WebSockets
Request/response payload: JSON

️ Backend
Backend will be Serverless as far as possible
Deployed on ️ Cloud
Scale on demand:

Logical Architecture

AWS Architecture

🧩 Components and Their Roles

1. Clients (Client 1, Client 2, … n Clients)️

   • These are WebSocket clients that connect to the API Gateway WebSocket API.
   • They send and receive messages through WebSockets.

2. API Gateway (WebSocket API)

   • API Gateway manages WebSocket connections.
   • It provides endpoints for WebSocket connection, disconnection, and message handling.
   • It routes events to the appropriate AWS Lambda functions.

3. Lambda Functions

   • Connect Lambda Function
     • Executes when a client establishes a WebSocket connection.
     • Stores the client’s connection ID in DynamoDB for message routing.
   • Disconnect Lambda Function
     • Triggers when a client disconnects.
     • Removes the connection ID from DynamoDB to prevent unnecessary message delivery.
   • onMessage Lambda Function
     • Handles incoming messages from WebSocket clients.
     • Retrieves connection IDs from DynamoDB to forward messages to the correct recipient.

4. DynamoDB

   • Stores active WebSocket connections (connection IDs).
   • Used for tracking which clients are online and enables message delivery to the correct recipients.

How This ️ Architecture Works

1. Client 1 and Client 2 connect to the WebSocket API via API Gateway.
2. API Gateway triggers the Connect Lambda function, which stores the connection ID in DynamoDB.
3. When a client sends a message, API Gateway invokes the onMessage Lambda function.
4. The onMessage Lambda function retrieves connection IDs from DynamoDB and forwards messages to the appropriate recipients using API Gateway.
5. When a client disconnects, API Gateway calls the Disconnect Lambda function, which removes the connection ID from DynamoDB.

How Far Can It Scale ?

• API Gateway supports up to millions of concurrent WebSocket connections.
• Lambda can handle thousands of requests per second (subject to AWS account limits).
• DynamoDB scales on-demand and can handle millions of transactions per second with proper indexing and partitioning.

Conclusion
This architecture is fully serverless, scalable, and cost-efficient for handling real-time WebSocket connections. It works well for medium to large-scale applications and can handle millions of users with minimal infrastructure management.

原文链接：Chat Application Architecture | AWS | API Gateway | WebSockets | Microservices | System Design