Service Layer Pattern: Keeping Controllers Thin

As applications grow, controllers can become bloated. A service layer keeps code organized by separating responsibilities.

    Controller should focus on:
    - reading request
    - validation
    - returning response
    
    Service should focus on:
    - business rules
    - multi-step database updates
    - external API calls
    
    ## Example: OrderService
    
    ```php
    class OrderService {
      public function createOrder(User $user, array $items): Order {
        return DB::transaction(function () use ($user, $items) {
          $order = Order::create([
            'user_id' => $user->id,
            'status' => 'pending',
            'total' => 0,
          ]);
    
          $total = 0;
    
          foreach ($items as $it) {
            $order->items()->create([
              'product_id' => $it['product_id'],
              'qty' => $it['qty'],
              'price' => $it['price'],
            ]);
            $total += $it['qty'] * $it['price'];
          }
    
          $order->update(['total' => $total]);
    
          return $order;
        });
      }
    }
    ```
    
    Controller:
    
    ```php
    public function store(Request $request, OrderService $service) {
      $data = $request->validate([
        'items' => 'required|array|min:1',
        'items.*.product_id' => 'required|integer',
        'items.*.qty' => 'required|integer|min:1',
        'items.*.price' => 'required|numeric|min:0',
      ]);
    
      $order = $service->createOrder($request->user(), $data['items']);
      return response()->json($order, 201);
    }
    ```
    
    ## Graph: responsibility separation
    
    ```mermaid
    flowchart LR
      A[Controller] --> B[Service Layer]
      B --> C[Models/DB]
      B --> D[External APIs]
    ```
    
    In the next tutorial, we will build custom exceptions and consistent API error responses.