func CreateOrderHandler(w http.ResponseWriter, r *http.Request) {
    var req struct {
        UserID    int64   `json:"user_id"`
        ProductID int64   `json:"product_id"`
        Quantity  int     `json:"quantity"`
    }
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        http.Error(w, "bad request", 400)
        return
    }

    // Валидация - бизнес-логика прямо в handler
    if req.Quantity <= 0 || req.Quantity > 100 {
        http.Error(w, "invalid quantity", 400)
        return
    }

    // SQL - инфраструктура прямо в handler
    var price float64
    err := db.QueryRow("SELECT price FROM products WHERE id = $1", req.ProductID).Scan(&price)
    if err != nil {
        http.Error(w, "product not found", 404)
        return
    }

    total := price * float64(req.Quantity)

    // Ещё SQL
    _, err = db.Exec(
        "INSERT INTO orders (user_id, product_id, quantity, total) VALUES ($1,$2,$3,$4)",
        req.UserID, req.ProductID, req.Quantity, total,
    )
    if err != nil {
        http.Error(w, "internal error", 500)
        return
    }

    // Отправка email - ещё одна инфраструктурная деталь
    smtp.SendMail(smtpAddr, auth, "shop@example.com",
        []string{userEmail}, []byte("Your order is placed!"))

    w.WriteHeader(http.StatusCreated)
    json.NewEncoder(w).Encode(map[string]float64{"total": total})
}

<?php
// Аналогичный «толстый» controller в Symfony
final class OrderController
{
    public function __construct(
        private readonly \PDO $pdo,
        private readonly \Symfony\Component\Mailer\MailerInterface $mailer,
    ) {}

    public function createOrder(Request $request): JsonResponse
    {
        $req = json_decode($request->getContent(), true);
        $userId = (int) ($req['user_id'] ?? 0);
        $productId = (int) ($req['product_id'] ?? 0);
        $quantity = (int) ($req['quantity'] ?? 0);

        // Валидация - бизнес-логика прямо в controller
        if ($quantity <= 0 || $quantity > 100) {
            return new JsonResponse(['error' => 'invalid quantity'], 400);
        }

        // SQL - инфраструктура прямо в controller
        $stmt = $this->pdo->prepare('SELECT price FROM products WHERE id = :id');
        $stmt->execute(['id' => $productId]);
        $price = $stmt->fetchColumn();
        if ($price === false) {
            return new JsonResponse(['error' => 'product not found'], 404);
        }

        $total = (float) $price * $quantity;

        // Ещё SQL
        $insert = $this->pdo->prepare(
            'INSERT INTO orders (user_id, product_id, quantity, total) VALUES (:u, :p, :q, :t)',
        );
        $insert->execute(['u' => $userId, 'p' => $productId, 'q' => $quantity, 't' => $total]);

        // Отправка email - ещё одна инфраструктурная деталь
        $this->mailer->send((new \Symfony\Component\Mime\Email())
 ->from('shop@example.com')
 ->to('user@example.com')
 ->text('Your order is placed!'));

        return new JsonResponse(['total' => $total], 201);
    }
}

// domain/order.go - НЕЛЬЗЯ импортировать database/sql, net/http
package domain

type Order struct {
    ID        int64
    UserID    int64
    ProductID int64
    Quantity  int
    Total     float64
}

func NewOrder(userID, productID int64, quantity int, price float64) (*Order, error) {
    if quantity <= 0 || quantity > 100 {
        return nil, ErrInvalidQuantity
    }
    return &Order{
        UserID:    userID,
        ProductID: productID,
        Quantity:  quantity,
        Total:     price * float64(quantity),
    }, nil
}

<?php
// src/Domain/Order.php - НЕЛЬЗЯ импортировать PDO, Doctrine, Symfony\HttpFoundation
declare(strict_types=1);

namespace App\Domain;

final class Order
{
    private function __construct(
        private readonly int $userId,
        private readonly int $productId,
        private readonly int $quantity,
        private readonly float $total,
    ) {}

    public static function create(int $userId, int $productId, int $quantity, float $price): self
    {
        if ($quantity <= 0 || $quantity > 100) {
            throw new \DomainException('invalid quantity');
        }
        return new self(
            userId: $userId,
            productId: $productId,
            quantity: $quantity,
            total: $price * $quantity,
        );
    }

    public function total(): float { return $this->total; }
}