A Simple Guide to Domain-Driven Design (DDD) in Laravel

Have you ever felt that as your Laravel project grows, things start getting a little out of hand? Controllers become bloated, models start doing too much, and suddenly, your codebase is like that drawer you’ve been meaning to organize for months. This is where Domain-Driven Design (DDD) can step in and make your life easier.

DDD is a way of designing your application so that its structure aligns closely with the problem you’re solving in the real world. It helps make your code cleaner, more scalable, and easier to manage as your project grows.

In this guide, we'll walk you through the basics of DDD in Laravel, explain how you can implement it, and show you some real-world examples along the way.

Table of Contents

1. What is Domain-Driven Design (DDD)?
2. Why Use DDD in Laravel?
3. The Components of DDD
   • Entities
   • Value Objects
   • Repositories
   • Services
4. Implementing DDD in Laravel
   • Example 1: Building an Order Management System
   • Example 2: Managing User Subscriptions
5. Pros and Cons of DDD in Laravel
6. Final Thoughts

What is Domain-Driven Design (DDD)?

Before we dive into Laravel specifics, let’s cover what Domain-Driven Design (DDD) is all about. DDD is a way to organize your application’s code by focusing on the business domain—the core problem your software is solving.

Instead of structuring your code around technical concepts like controllers or models, you structure it around real-world concepts. This could be things like orders, products, or customers, depending on what your application does.

In a nutshell, DDD helps you build an application that mirrors real-world processes, making the code easier to understand and maintain, especially as it grows.

Why Use DDD in Laravel?

If you're familiar with the MVC (Model-View-Controller) pattern that Laravel uses by default, you know it works great for most applications. But as your application scales, the MVC pattern can lead to a mess of interdependent code. Domain-Driven Design solves this problem by making your application easier to extend and maintain over time.

DDD also separates business logic from infrastructure code. This means your application logic won’t be tied to things like databases or APIs, making it easier to swap out technologies later.

The Components of DDD

To understand how DDD works, you need to know its key components. Let’s break them down:

Entities

Entities are objects in your domain that have a distinct identity. For example, an Order is an entity because each order is unique.

// app/Domain/Order/Order.php
class Order {
    private $id;
    private $status;

    public function __construct($id, $status) {
        $this->id = $id;
        $this->status = $status;
    }

    // Getter and other business logic
}

Value Objects

A Value Object is an object that doesn’t have an identity, but it represents a concept. A Money object, for example, represents a value but doesn’t need a unique ID.

// app/Domain/Order/Money.php
class Money {
    private $amount;
    private $currency;

    public function __construct($amount, $currency) {
        $this->amount = $amount;
        $this->currency = $currency;
    }

    public function getFormatted() {
        return "{$this->amount} {$this->currency}";
    }
}

Repositories

A Repository handles fetching and persisting domain objects like entities. Instead of your domain objects interacting with the database directly, repositories manage the data access.

// app/Domain/Order/OrderRepositoryInterface.php
interface OrderRepositoryInterface {
    public function findById($id): ?Order;
    public function save(Order $order): void;
}

// app/Infrastructure/Order/EloquentOrderRepository.php
class EloquentOrderRepository implements OrderRepositoryInterface {
    public function findById($id): ?Order {
        // Fetch order using Eloquent
    }

    public function save(Order $order): void {
        // Save order using Eloquent
    }
}

Services

A Service handles the business logic, like creating an order or processing a payment. Instead of putting this logic in your controllers, you encapsulate it in services.

// app/Domain/Order/CreateOrderService.php
class CreateOrderService {
    public function execute($data) {
        $order = new Order($data['id'], $data['status']);
        // Business logic for creating an order
    }
}

Implementing DDD in Laravel

Now that you understand the key components, let’s look at how we can implement DDD in Laravel with some real-world examples.

Example 1: Building an Order Management System

Let’s say you’re building an Order Management System for an e-commerce site. Here’s how DDD would help you organize your code:

1. Entities: You’d have an Order entity to represent each order, with attributes like id and status.
2. Value Objects: You might create a Money value object to represent prices.
3. Repositories: You’d create an OrderRepository to fetch and store orders in the database.
4. Services: A CreateOrderService would handle the logic for creating new orders.

Here’s a basic flow:

// app/Http/Controllers/OrderController.php
class OrderController {
    private $createOrderService;

    public function __construct(CreateOrderService $createOrderService) {
        $this->createOrderService = $createOrderService;
    }

    public function store(Request $request) {
        $this->createOrderService->execute($request->all());
        return response()->json(['message' => 'Order created!']);
    }
}

Example 2: Managing User Subscriptions

Imagine you’re managing user subscriptions for a SaaS platform. Using DDD, you’d create:

• A Subscription entity to represent each user’s subscription.
• A Duration value object to manage subscription periods.
• A SubscriptionRepository to handle data access.
• A SubscriptionService to handle business logic like renewing a subscription.

Here’s how you might handle a subscription renewal:

// app/Domain/Subscription/RenewSubscriptionService.php
class RenewSubscriptionService {
    private $subscriptionRepository;

    public function __construct(SubscriptionRepositoryInterface $subscriptionRepository) {
        $this->subscriptionRepository = $subscriptionRepository;
    }

    public function renew($subscriptionId) {
        $subscription = $this->subscriptionRepository->findById($subscriptionId);
        $subscription->renew();
        $this->subscriptionRepository->save($subscription);
    }
}

Pros and Cons of DDD in Laravel

Pros:

• Better organization: Code is neatly structured around the domain.
• Scalability: Easier to scale and manage large applications.
• Maintainability: Business logic is separated from infrastructure concerns.

Cons:

• Learning curve: DDD introduces new concepts, which can be overwhelming at first.
• Overhead for small projects: Implementing DDD for small, simple projects might feel like overkill.

Final Thoughts

DDD in Laravel may seem daunting, but once you start thinking in terms of business domains rather than technical layers, it starts to make sense. It’s all about keeping your code clean, maintainable, and scalable as your project grows.

Start small. Refactor one feature in your app using DDD principles and see how it feels. Over time, you’ll start to appreciate the organization and clarity it brings.

Good luck, and happy coding!