Building a Real-Time Chat System

Building a real-time chat system from scratch is an excellent way to learn about persistent connections, state management, and the challenges of scaling interactive applications.

The Architecture

At the core of any chat system is the requirement to push messages to clients instantly. Traditional HTTP polling isn't sufficient for this, so we rely on WebSockets.

WebSockets provide a full-duplex communication channel over a single, long-lived connection.

Core Components

1. Client Application: React frontend maintaining the WebSocket connection.
2. WebSocket Server: Node.js service managing active connections.
3. Pub/Sub System: Redis to broadcast messages across multiple server instances.
4. Database: PostgreSQL to persist the message history.

Handling Scalability

When running a single WebSocket server, things are simple. However, once you scale horizontally (adding more servers), you face a challenge: User A connects to Server 1, and User B connects to Server 2. How do they chat?

This is where Redis Pub/Sub shines.

// A simple example of publishing to Redis
import Redis from 'ioredis';

const redis = new Redis();

function broadcastMessage(channelId, message) {
  redis.publish(`chat:${channelId}`, JSON.stringify(message));
}

Every server subscribes to the relevant chat channels. When User A sends a message to Server 1, Server 1 publishes it to Redis. Server 2 receives the event from Redis and pushes it down the WebSocket connection to User B.

Key Takeaways

Building this taught me that real-time systems require a different mental model compared to standard REST APIs. You have to handle connection drops, reconnections, and ensure message delivery guarantees.