Load Balancer (Node.js)

This project is a simple custom HTTP load balancer built using Node.js and Express.

It distributes incoming requests across multiple backend worker servers using a round-robin algorithm, performs periodic health checks, and exposes a metrics endpoint for monitoring.

The purpose of this project is to understand how load balancers work internally, including proxying, health checks, and request routing.

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Features

• Single entry point for client traffic (port 80)
• Multiple backend worker servers
• Round-robin request distribution
• Periodic health checks
• Automatic avoidance of unhealthy workers
• Metrics endpoint for observability

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Project Structure

load-balancer/
│
├── src/
│ ├── server.js # Load balancer entry point
│ ├── workers.js # Worker registry and round-robin logic
│ ├── health.js # Periodic health check logic
│ ├── proxy.js # Request proxying logic
│ ├── metrics.js # Metrics tracking
│ │
│ └── controllers/
│ └── metricsController.js # Metrics collection 
│
├── worker-service/
│ └── worker-server.js # Backend worker server
│
├── package.json
└── README.md

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How It Works

1. A client sends an HTTP request to the load balancer.
2. The load balancer selects the next healthy worker using round-robin scheduling.
3. The request is proxied to the selected worker.
4. The worker processes the request and sends a response.
5. The load balancer streams the response back to the client.

Separately:

• Health checks run independently at a fixed interval.
• Only health checks determine whether a worker is alive.
• Individual request failures do not affect worker health.

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Prerequisites

• Node.js 18 or later
• npm

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Installation

Clone the repository and install dependencies:

npm install

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Running the Project Locally

1. Start the worker servers

Open three separate terminals and run:

npm run worker1
npm run worker2
npm run worker3

Each worker runs the same code but listens on a different port.

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2. Start the load balancer

in another terminal:

npm run start

The load balancer listens on port 80.

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3. Test load balancing

Send requests to the load balancer:

curl http://localhost
curl http://localhost
curl http://localhost

You should see responses alternating between workers.

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4. View metrics

curl http://localhost/metrics

Example output:

requests_total 3
requests_failed 0
requests_active 0
worker_3001_alive 1
worker_3002_alive 1
worker_3003_alive 1
worker_3001_requests 1
worker_3002_requests 1
worker_3003_requests 1

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Health Checks

• Health checks run at a fixed interval
• They hit a dedicated /health endpoint on each worker
• HTTP response streams are properly consumed to avoid socket-related issues
• Health checks are the only authority for determining worker liveness

This approach prevents false positives and unstable worker state.

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Why This Project Exists

This project is intentionally built without external proxies in order to:

• Learn how HTTP proxying works internally
• Understand round-robin scheduling
• Explore health checks and failure handling
• Learn about streams, sockets, and connection reuse in Node.js
• See how real infrastructure problems appear even in small systems