This guide helps you rebuild your project in 2 stages:

1. Stage 1: Reverse Proxy only
2. Stage 2: Add Load Balancing

Use Stage 1 first, then move to Stage 2.

• docker-compose.yml
• app/
• nginx/nginx.conf

• Docker Desktop installed
• Docker Compose available

Before Stage 1 or Stage 2, create these files inside app/.

const express = require("express");
const os = require("os");

const app = express();
const PORT = 3000;

const INSTANCE = process.env.INSTANCE_NAME || os.hostname();

app.get("/", (req, res) => {
  res.send(`Hello World from ${INSTANCE}`);
});

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT} - ${INSTANCE}`);
});

{
  "name": "lb-demo",
  "version": "1.0.0",
  "main": "server.js",
  "dependencies": {
    "express": "^4.18.2"
  }
}

FROM node:18

WORKDIR /app

COPY package*.json ./
RUN npm install

COPY . .

EXPOSE 3000

CMD ["node", "server.js"]

Why this is required:

• Your compose services use build: ./app.
• So Docker needs app/Dockerfile to build the app image.
• Nginx does not need a Dockerfile in this setup because it uses nginx:latest directly.

In this stage, Nginx forwards requests to one backend app.

Replace docker-compose.yml with:

version: "3.9"

services:
  app1:
    build: ./app
    environment:
      - INSTANCE_NAME=App-1

  nginx:
    image: nginx:latest
    ports:
      - "8080:80"
    volumes:
      - ./nginx/nginx.conf:/etc/nginx/nginx.conf
    depends_on:
      - app1

Replace nginx/nginx.conf with:

events {}

http {
    server {
        listen 80;

        location / {
            proxy_pass http://app1:3000;
        }
    }
}

docker compose up --build

Open:

• http://localhost:8080

Expected:

• You always see response from App-1

This proves Reverse Proxy is working.

Now upgrade from one backend to three backends.

Replace docker-compose.yml with:

version: "3.9"

services:
  app1:
    build: ./app
    environment:
      - INSTANCE_NAME=App-1

  app2:
    build: ./app
    environment:
      - INSTANCE_NAME=App-2

  app3:
    build: ./app
    environment:
      - INSTANCE_NAME=App-3

  nginx:
    image: nginx:latest
    ports:
      - "8080:80"
    volumes:
      - ./nginx/nginx.conf:/etc/nginx/nginx.conf
    depends_on:
      - app1
      - app2
      - app3

Replace nginx/nginx.conf with:

events {}

http {
    upstream backend {
        server app1:3000;
        server app2:3000;
        server app3:3000;
    }

    server {
        listen 80;

        location / {
            proxy_pass http://backend;
        }
    }
}

docker compose up --build

Refresh this URL multiple times:

• http://localhost:8080

Expected:

• You should see different instance names (App-1, App-2, App-3) over multiple requests.

This proves Load Balancing is working.

• Reverse Proxy: Nginx accepts client request and forwards to backend service.
• Load Balancing: Nginx distributes requests across multiple backend services.

If you want a clean restart:

docker compose down -v
docker compose up --build