What is a backend?

Two halves of every app

Most apps have two halves. The frontend is what you see and touch: buttons, forms, lists. The backend is hidden: it is a program running on a computer somewhere else, called a server.

A server is just a computer that stays on all the time and waits for requests. A request is a message from an app that says "please do this" or "please give me that".

What a backend does

• Stores data that must not be lost: users, orders, messages (in a database).
• Checks rules: is the password right? Is there enough money in the account? Is the seat still free?
• Does work the phone cannot or should not do: sending OTP SMS, charging a card, making a PDF ticket.
• Keeps secrets safe: payment keys and passwords never go to the phone.

One request, step by step

1. You tap "Book" in the app.
2. The app sends a request over the internet to the server.
3. The backend code reads the request and checks it.
4. It reads or writes the database.
5. It sends a response (an answer) back, for example "Booked, PNR 1234567890".
6. The app shows the answer on screen.

We can write this idea as a tiny JavaScript function. It takes a request object and returns a response object. Real backends are much bigger, but the shape is the same.

function handle(request) {
  if (request.path === '/hello') {
    return { status: 200, body: 'Hello from the backend!' };
  }
  return { status: 404, body: 'Not found' };
}

console.log(handle({ path: '/hello' }));
console.log(handle({ path: '/missing' }));

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