What is an API, and why does every app need a backend?

Think of a restaurant

• The dining room and menu are the frontend. It is what the customer sees.
• The waiter is the API. He carries the order to the kitchen and brings the food back.
• The kitchen is the backend. It follows recipes (business rules) and prepares the order.
• The pantry is the database. It stores the ingredients (your data).

The customer never walks into the kitchen. In the same way, a browser or phone app never talks to your database directly. It always goes through the backend, using the API.

What a request looks like

When someone searches for shoes on a store, the app sends a request such as GET /api/products?q=shoes to the backend. The backend checks the request, asks the database for matching products and replies with a small package of data in a format called JSON, for example a list with each product’s name, price and whether it is in stock. The frontend then turns that JSON into product cards on the screen.

Why not put everything in the frontend?

• Security: anything in the frontend can be seen by users. Payment keys, passwords and private rules must stay on a server.
• Shared data: orders, users and stock must be the same for everyone, so they live in one place.
• Rules: discounts, permissions and validation must be enforced where users cannot change them.
• Many apps, one system: your website, Android app, iPhone app and admin panel can all use the same backend.

What a backend usually does

• Checks who the user is (login and permissions).
• Validates input so bad or harmful data does not get in.
• Reads and writes the database.
• Talks to other services such as payment gateways, SMS, WhatsApp and email.
• Sends back clean, predictable responses.

REST and GraphQL in one minute

These are two common styles for designing an API. REST gives you a separate address for each kind of data, such as products or orders. GraphQL lets the app ask for exactly the fields it needs in one request. Both work well. The right choice depends on how many different screens and apps will use the data.

Which language is it written in?

Backends can be written in PHP (Laravel), JavaScript (Node.js), Python, Go, Java and others. The language matters less than good structure, security and a database designed for how your business really works.