DDD Layer Details

Domain Layer

The innermost layer — pure business logic with no external dependencies.

Entities

Entities have identity and lifecycle:

namespace WPECommerce\Core\Domain\Entities;

class Product
{
    private int $id;
    private string $title;
    private Money $price;
    private ?Money $salePrice;
    private int $stockQuantity;

    public function getTitle(): string { return $this->title; }
    public function getPrice(): Money { return $this->price; }
    public function isOnSale(): bool { return $this->salePrice !== null; }
    public function getEffectivePrice(): Money {
        return $this->isOnSale() ? $this->salePrice : $this->price;
    }
}

caution

Always use getter methods to access entity properties. Never use direct property access or array syntax.

// Correct
$product->getTitle();
$product->getFeaturedImageId();

// Wrong — will cause errors
$product['title'];
$product->title;  // Only 'title' field is public

Repository Interfaces

namespace WPECommerce\Core\Domain\Repositories;

interface ProductRepositoryInterface
{
    public function find(int $id): ?Product;
    public function findBySlug(string $slug): ?Product;
    public function save(Product $product): bool;  // Returns bool, not object
    public function delete(int $id): bool;
}

Application Layer

Orchestrates use cases by coordinating domain objects:

namespace WPECommerce\Core\Application\Services;

class CartService
{
    public function addItem(
        int $productId,
        ?int $variantId,  // variant BEFORE quantity!
        int $quantity
    ): Cart {
        // Business logic here
    }
}

danger

CartService::addItem() signature: addItem($product_id, $variant_id, $quantity) — variant before quantity! This is a common source of bugs.

Infrastructure Layer

Implements repository interfaces with actual database operations:

namespace WPECommerce\Core\Infrastructure\Persistence;

class ProductRepository implements ProductRepositoryInterface
{
    public function find(int $id): ?Product
    {
        // Uses ProductRepository::find($id), NOT findById()
        global $wpdb;
        $row = $wpdb->get_row($wpdb->prepare(
            "SELECT * FROM {$wpdb->prefix}ec_products WHERE id = %d",
            $id
        ));
        return $row ? $this->hydrate($row) : null;
    }

    public function save(Product $product): bool
    {
        // Returns bool — ID is set internally on the entity
    }
}

Presentation Layer

REST API controllers that handle HTTP and delegate to services:

namespace WPECommerce\Core\Presentation\Api;

class ProductApiController
{
    public function __construct(
        private ProductService $productService
    ) {}

    public function register_routes(): void
    {
        register_rest_route('ec/v1', '/products', [...]);
    }
}