How to Demonstrate Scalability & Elasticity using Docker

STEP 1 — Create a Simple Web App

Create a folder:

scaling-demo/
└── app.js
└── package.json
└── Dockerfile

package.json

{
  "name": "scale-demo",
  "version": "1.0.0",
  "dependencies": {
    "express": "^4.18.2"
  }
}

app.js

const express = require("express");
const app = express();
const port = 3000;

app.get("/", (req, res) => {
  const host = process.env.HOSTNAME;
  res.send(`Hello from container: ${host}`);
});

app.listen(port, () => console.log(`App running on port ${port}`));

Dockerfile

FROM node:18
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
CMD ["node", "app.js"]

STEP 2 — Build Docker Image

docker build -t scale-demo .

STEP 3 — Run SINGLE Container (No scaling yet)

docker run -p 3000:3000 scale-demo

Open browser → http://localhost:3000You will see:

Hello from container: <container_id>

STEP 4 — Add Load Balancer using Docker Compose

Create docker-compose.yml:

version: '3'
services:
  app:
    image: scale-demo
    deploy:
      replicas: 1
    ports:
      - "8080:3000"

Run it:

docker compose up --scale app=1

Access: http://localhost:8080

STEP 5 — SCALE UP (Show Scalability)

Increase containers from 1 → 3

docker compose up --scale app=3

Refresh your browser multiple times.You’ll notice the host name changing:

Hello from container: abc123
Hello from container: def556
Hello from container: ghi998

This proves scalability: more containers handle more load.