Backend Architecture | VoidX Academy

7. Initializing Your First Server with Express/Flask

First Server

Initializing Your First Server with Express/Flask

A server is not a physical machine — it's a software program. More specifically, it is an application that runs in an infinite loop, permanently "listening" on a specific network port for incoming connections. When a connection arrives carrying an HTTP request, it wakes up, executes logic, sends a response, and goes back to listening.

In this module, we build a working server in both JavaScript (Node.js with Express) and Python (Flask), explaining every single line of code.

IP Addresses & Ports — Where Does the Traffic Go?

Every computer on a network is identified by an IP Address — a unique numeric label like 192.168.1.105 or the public address 104.21.8.3. But a computer can run hundreds of programs simultaneously. How does incoming traffic know which program to go to? That's the job of Ports.

A port is a numbered door (0–65535) on a computer. Each running program listens on a unique port. Think of the IP address as the building's street address, and the port as the specific apartment number inside that building.

🌐Port 80

Standard HTTP traffic. When you browse a non-HTTPS site, your browser connects to port 80 by default.

🔒Port 443

Standard HTTPS traffic. All encrypted web traffic goes through port 443.

🛠️Port 3000 / 8000

Common conventions for local development servers. Not special — just popular choices that developers agree on by habit.

🗄️Port 5432 / 3306

PostgreSQL listens on 5432. MySQL listens on 3306 by default.

💡localhost and 127.0.0.1: When developing on your own machine, localhost and 127.0.0.1 are aliases that mean "this computer, right here." A request to http://localhost:3000 stays entirely inside your machine — no internet connection needed.

Setting Up a Node.js + Express Server

Before writing any server code, you need Node.js installed. Verify your installation by running both commands in your terminal:

node --version
npm --version

Now create a fresh project folder and initialize it:

mkdir my-first-server
cd my-first-server
npm init -y
npm install express

Create a file called server.js and add the following code:

const express = require('express');
const app = express();
const PORT = process.env.PORT || 3000;

app.use(express.json());

app.get('/ping', (req, res) => {
    res.status(200).json({
        message: 'Pong!',
        timestamp: new Date().toISOString(),
        status: 'Server is alive and running'
    });
});

app.listen(PORT, () => {
    console.log(`✅ Server is running on http://localhost:${PORT}`);
    console.log(`   Try visiting http://localhost:${PORT}/ping in your browser`);
});

Run your server:

node server.js

Now open your browser to http://localhost:3000/ping. You'll see the JSON response! You have just built a working server.

What Just Happened? The Event Loop.

Notice that when you run node server.js, the terminal doesn't return to the command prompt like a normal script would. It stays open, printing that one line. This is because app.listen() enters Node.js's Event Loop.

Node.js is single-threaded. Instead of creating a new thread for every incoming request (which is how Java/Apache works), it uses a single thread with an event loop that efficiently handles thousands of simultaneous connections by pausing and resuming work as needed. When a request arrives, Node puts it in the event queue, processes it when ready, and goes back to waiting.

💡To stop your server: Press Ctrl + C in the terminal. This sends an interrupt signal that terminates the process.

Building a Server in Python (Flask)

If you prefer Python, Flask is a lightweight micro-framework that produces a nearly identical mental model to Express. First, install it:

pip install flask

Create a file called server.py:

from flask import Flask, jsonify
from datetime import datetime
import os

app = Flask(__name__)

@app.route('/ping', methods=['GET'])
def ping():
    return jsonify({
        'message': 'Pong!',
        'timestamp': datetime.utcnow().isoformat(),
        'status': 'Server is alive and running'
    }), 200

if __name__ == '__main__':
    port = int(os.environ.get('PORT', 8000))
    app.run(host='0.0.0.0', port=port, debug=True)

Run it:

python server.py

Handling Multiple Routes

A real server has dozens or hundreds of routes. Here is how you add more:

const express = require('express');
const app = express();

app.use(express.json());

const users = [
    { id: 1, name: 'Ama',   role: 'admin' },
    { id: 2, name: 'Kofi',  role: 'user' },
    { id: 3, name: 'Abena', role: 'user' }
];

app.get('/', (req, res) => {
    res.json({ message: 'Welcome to the VoidX API', version: '1.0.0' });
});

app.get('/users', (req, res) => {
    res.json({ count: users.length, users });
});

app.get('/users/:id', (req, res) => {
    const id = parseInt(req.params.id);
    const user = users.find(u => u.id === id);
    if (!user) {
        return res.status(404).json({ error: `User with id ${id} not found` });
    }
    res.status(200).json(user);
});

app.post('/users', (req, res) => {
    const { name, role } = req.body;
    if (!name) {
        return res.status(400).json({ error: 'Name is required' });
    }
    const newUser = {
        id: users.length + 1,
        name,
        role: role || 'user'
    };
    users.push(newUser);
    res.status(201).json({ message: 'User created', user: newUser });
});

app.listen(3000, () => console.log('Server running on port 3000'));

⚠️ Port Already in Use Error:

If you see Error: listen EADDRINUSE: address already in use :::3000, another process is already using that port. Either change your port number in the code, or find and kill the existing process. On Mac/Linux: lsof -i :3000 then kill -9 [PID].

Nodemon — Auto-Restart on Save

Right now, every time you edit your server code, you have to manually Ctrl+C and restart. During development, this becomes exhausting within minutes. Nodemon is a tool that watches your files and automatically restarts the server whenever you save a change.

npm install --save-dev nodemon
npx nodemon server.js

Add a script to your package.json so you can just run npm run dev:

{
  "scripts": {
    "start": "node server.js",
    "dev":   "nodemon server.js"
  }
}

7. Initializing Your First Server with Express/Flask

First Server

Initializing Your First Server with Express/Flask

In this module, we build a working server in both JavaScript (Node.js with Express) and Python (Flask), explaining every single line of code.

IP Addresses & Ports — Where Does the Traffic Go?

🌐Port 80

Standard HTTP traffic. When you browse a non-HTTPS site, your browser connects to port 80 by default.

🔒Port 443

Standard HTTPS traffic. All encrypted web traffic goes through port 443.

🛠️Port 3000 / 8000

Common conventions for local development servers. Not special — just popular choices that developers agree on by habit.

🗄️Port 5432 / 3306

PostgreSQL listens on 5432. MySQL listens on 3306 by default.

Setting Up a Node.js + Express Server

Before writing any server code, you need Node.js installed. Verify your installation by running both commands in your terminal:

node --version
npm --version

Now create a fresh project folder and initialize it:

mkdir my-first-server
cd my-first-server
npm init -y
npm install express

Create a file called server.js and add the following code:

const express = require('express');
const app = express();
const PORT = process.env.PORT || 3000;

app.use(express.json());

app.get('/ping', (req, res) => {
    res.status(200).json({
        message: 'Pong!',
        timestamp: new Date().toISOString(),
        status: 'Server is alive and running'
    });
});

app.listen(PORT, () => {
    console.log(`✅ Server is running on http://localhost:${PORT}`);
    console.log(`   Try visiting http://localhost:${PORT}/ping in your browser`);
});

Run your server:

node server.js

Now open your browser to http://localhost:3000/ping. You'll see the JSON response! You have just built a working server.

What Just Happened? The Event Loop.

💡To stop your server: Press Ctrl + C in the terminal. This sends an interrupt signal that terminates the process.

Building a Server in Python (Flask)

If you prefer Python, Flask is a lightweight micro-framework that produces a nearly identical mental model to Express. First, install it:

pip install flask

Create a file called server.py:

from flask import Flask, jsonify
from datetime import datetime
import os

app = Flask(__name__)

@app.route('/ping', methods=['GET'])
def ping():
    return jsonify({
        'message': 'Pong!',
        'timestamp': datetime.utcnow().isoformat(),
        'status': 'Server is alive and running'
    }), 200

if __name__ == '__main__':
    port = int(os.environ.get('PORT', 8000))
    app.run(host='0.0.0.0', port=port, debug=True)

Run it:

python server.py

Handling Multiple Routes

A real server has dozens or hundreds of routes. Here is how you add more:

const express = require('express');
const app = express();

app.use(express.json());

const users = [
    { id: 1, name: 'Ama',   role: 'admin' },
    { id: 2, name: 'Kofi',  role: 'user' },
    { id: 3, name: 'Abena', role: 'user' }
];

app.get('/', (req, res) => {
    res.json({ message: 'Welcome to the VoidX API', version: '1.0.0' });
});

app.get('/users', (req, res) => {
    res.json({ count: users.length, users });
});

app.get('/users/:id', (req, res) => {
    const id = parseInt(req.params.id);
    const user = users.find(u => u.id === id);
    if (!user) {
        return res.status(404).json({ error: `User with id ${id} not found` });
    }
    res.status(200).json(user);
});

app.post('/users', (req, res) => {
    const { name, role } = req.body;
    if (!name) {
        return res.status(400).json({ error: 'Name is required' });
    }
    const newUser = {
        id: users.length + 1,
        name,
        role: role || 'user'
    };
    users.push(newUser);
    res.status(201).json({ message: 'User created', user: newUser });
});

app.listen(3000, () => console.log('Server running on port 3000'));

⚠️ Port Already in Use Error:

Nodemon — Auto-Restart on Save

npm install --save-dev nodemon
npx nodemon server.js

Add a script to your package.json so you can just run npm run dev:

{
  "scripts": {
    "start": "node server.js",
    "dev":   "nodemon server.js"
  }
}

7. Initializing Your First Server with Express/Flask

Initializing Your First Server with Express/Flask

IP Addresses & Ports — Where Does the Traffic Go?

Setting Up a Node.js + Express Server

What Just Happened? The Event Loop.

Building a Server in Python (Flask)

Handling Multiple Routes

Nodemon — Auto-Restart on Save

Knowledge Check

7. Initializing Your First Server with Express/Flask

Initializing Your First Server with Express/Flask

IP Addresses & Ports — Where Does the Traffic Go?

Setting Up a Node.js + Express Server

What Just Happened? The Event Loop.

Building a Server in Python (Flask)

Handling Multiple Routes

Nodemon — Auto-Restart on Save

Knowledge Check