Understanding Backend (part 2)

In Part 1, we understood the basic journey of a request:

URL → DNS → IP → Connection → HTTP Request → Backend → Database → Response → Browser

Now let's go one step deeper.

What actually happens inside the backend when a request reaches the server? 🤔

Let's understand it using a simple Spring Boot example.

1*) Request reaches the backend 🚀*

Suppose the frontend sends:

GET /api/products/10

The request reaches our backend server.

But the backend has many different URLs/endpoints.

So the first question is:

"Which piece of code should handle this request?"

This is where routing comes in.

For example:

@GetMapping("/api/products/{id}")
public ResponseEntity getProduct(@PathVariable Long id) {
return ResponseEntity.ok(productService.getProductById(id));
}

Spring Boot sees:

GET /api/products/10

and maps it to this controller method.

2*) Controller receives the request 🎯*

The Controller is usually the entry point of our application.

Its job is to:

Receive HTTP requests
Read request data
Call the appropriate service
Return an HTTP response

For example:

@RestController
@RequestMapping("/api/products")
public class ProductController {

@GetMapping("/{id}")
public ResponseEntity<Product> getProduct(@PathVariable Long id) {

    Product product = productService.getProductById(id);

    return ResponseEntity.ok(product);
}

}

Here:

GET /api/products/10
↓
ProductController

The controller doesn't ideally contain all the business logic.

So it passes the work to the Service layer.

3) Service layer handles business logic 🧠

The Service layer is where we generally put business rules and application logic.

For example:

public Product getProductById(Long id) {

return productRepository.findById(id)
        .orElseThrow(() -> new RuntimeException("Product not found"));

}

The service asks:

"I need product with ID 10. Where can I get it?"

So it calls the Repository.