Clean Architecture

Clean Architecture, Hexagonal Architecture & DDD for PHP/Symfony

Overview

This skill provides guidance for implementing Clean Architecture, Hexagonal Architecture (Ports & Adapters), and Domain-Driven Design patterns in PHP 8.3+ applications using Symfony 7.x. It ensures clear separation of concerns, framework-independent business logic, and highly testable code through layered architecture with inward-only dependencies.

When to Use

• Architecting new enterprise PHP applications with Symfony 7.x
• Refactoring legacy PHP code to modern, testable patterns
• Implementing Domain-Driven Design in PHP projects
• Creating maintainable applications with clear separation of concerns
• Building testable business logic independent of frameworks
• Designing modular PHP systems with swappable infrastructure

Instructions

1. Understand the Architecture Layers

Clean Architecture follows the dependency rule: dependencies only point inward.

+-------------------------------------+
|  Infrastructure (Frameworks)        |  Symfony, Doctrine, External APIs
+-------------------------------------+
|  Adapter (Interface Adapters)       |  Controllers, Repositories, Presenters
+-------------------------------------+
|  Application (Use Cases)            |  Commands, Handlers, DTOs
+-------------------------------------+
|  Domain (Entities & Business Rules) |  Entities, Value Objects, Domain Events
+-------------------------------------+

Hexagonal Architecture (Ports & Adapters):

• Domain Core: Business logic, framework-agnostic
• Ports: Interfaces (e.g., UserRepositoryInterface)
• Adapters: Concrete implementations (Doctrine, InMemory for tests)

DDD Tactical Patterns:

• Entities: Objects with identity (e.g., User, Order)
• Value Objects: Immutable, defined by attributes (e.g., Email, Money)
• Aggregates: Consistency boundaries with root entity
• Domain Events: Capture business occurrences
• Repositories: Persist/retrieve aggregates

2. Organize Directory Structure

Create the following directory structure to enforce layer separation:

src/
+-- Domain/                 # Innermost layer - no dependencies
|   +-- Entity/
|   |   +-- User.php
|   |   +-- Order.php
|   +-- ValueObject/
|   |   +-- Email.php
|   |   +-- Money.php
|   |   +-- OrderId.php
|   +-- Repository/
|   |   +-- UserRepositoryInterface.php
|   +-- Event/
|   |   +-- UserCreatedEvent.php
|   +-- Exception/
|       +-- DomainException.php
+-- Application/            # Use cases - depends on Domain
|   +-- Command/
|   |   +-- CreateUserCommand.php
|   |   +-- UpdateOrderCommand.php
|   +-- Handler/
|   |   +-- CreateUserHandler.php
|   |   +-- UpdateOrderHandler.php
|   +-- Query/
|   |   +-- GetUserQuery.php
|   +-- Dto/
|   |   +-- UserDto.php
|   +-- Service/
|       +-- NotificationServiceInterface.php
+-- Adapter/                # Interface adapters
|   +-- Http/
|   |   +-- Controller/
|   |   |   +-- UserController.php
|   |   +-- Request/
|   |       +-- CreateUserRequest.php
|   +-- Persistence/
|       +-- Doctrine/
|           +-- Repository/
|           |   +-- DoctrineUserRepository.php
|           +-- Mapping/
|               +-- User.orm.xml
+-- Infrastructure/         # Framework & external concerns
    +-- Config/
    |   +-- services.yaml
    +-- Event/
    |   +-- SymfonyEventDispatcher.php
    +-- Service/
        +-- SendgridEmailService.php

3. Implement Domain Layer

Start from the innermost layer (Domain) and work outward:

1. Create Value Objects with validation at construction time - they must be immutable using PHP 8.1+ readonly
2. Create Entities with domain logic and business rules - entities should encapsulate behavior, not just be data bags
3. Define Repository Interfaces (Ports) - keep them small and focused
4. Define Domain Events to decouple side effects from core business logic

4. Implement Application Layer

Build use cases that orchestrate domain objects:

1. Create Commands as readonly DTOs representing write operations
2. Create Queries for read operations (CQRS pattern)
3. Implement Handlers that receive commands/queries and coordinate domain objects
4. Define Service Interfaces for external dependencies (notifications, etc.)

5. Implement Adapter Layer

Create interface adapters that connect Application to Infrastructure:

1. Create Controllers that receive HTTP requests and invoke handlers
2. Create Request DTOs with Symfony validation attributes
3. Implement Repository Adapters that bridge domain interfaces to persistence layer

6. Configure Infrastructure

Set up framework-specific configuration:

1. Configure Symfony DI to bind interfaces to implementations
2. Create test doubles (In-Memory repositories) for unit testing without database
3. Configure Doctrine mappings for persistence

7. Test Without Framework

Ensure Domain and Application layers are testable without Symfony, Doctrine, or database. Use In-Memory repositories for fast unit tests.

Examples

Example 1: Value Object with Validation

<?php
// src/Domain/ValueObject/Email.php

namespace App\Domain\ValueObject;

use InvalidArgumentException;

final readonly class Email
{
    public function __construct(
        private string $value
    ) {
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new InvalidArgumentException(
                sprintf('"%s" is not a valid email address', $value)
            );
        }
    }

    public function value(): string
    {
        return $this->value;
    }

    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }

    public function domain(): string
    {
        return substr($this->value, strrpos($this->value, '@') + 1);
    }
}

Example 2: Entity with Domain Logic

<?php
// src/Domain/Entity/User.php

namespace App\Domain\Entity;

use App\Domain\ValueObject\Email;
use App\Domain\ValueObject\UserId;
use App\Domain\Event\UserCreatedEvent;
use DateTimeImmutable;

class User
{
    private array $domainEvents = [];

    public function __construct(
        private UserId $id,
        private Email $email,
        private string $name,
        private DateTimeImmutable $createdAt,
        private bool $isActive = true
    ) {
        $this->recordEvent(new UserCreatedEvent($id->value()));
    }

    public static function create(
        UserId $id,
        Email $email,
        string $name
    ): self {
        return new self(
            $id,
            $email,
            $name,
            new DateTimeImmutable()
        );
    }

    public function deactivate(): void
    {
        $this->isActive = false;
    }

    public function canPlaceOrder(): bool
    {
        return $this->isActive;
    }

    public function id(): UserId
    {
        return $this->id;
    }

    public function email(): Email
    {
        return $this->email;
    }

    public function domainEvents(): array
    {
        return $this->domainEvents;
    }

    public function clearDomainEvents(): void
    {
        $this->domainEvents = [];
    }

    private function recordEvent(object $event): void
    {
        $this->domainEvents[] = $event;
    }
}

Example 3: Repository Port (Interface)

<?php
// src/Domain/Repository/UserRepositoryInterface.php

namespace App\Domain\Repository;

use App\Domain\Entity\User;
use App\Domain\ValueObject\Email;
use App\Domain\ValueObject\UserId;

interface UserRepositoryInterface
{
    public function findById(UserId $id): ?User;

    public function findByEmail(Email $email): ?User;

    public function save(User $user): void;

    public function delete(UserId $id): void;
}

Example 4: Command and Handler

<?php
// src/Application/Command/CreateUserCommand.php

namespace App\Application\Command;

final readonly class CreateUserCommand
{
    public function __construct(
        public string $id,
        public string $email,
        public string $name
    ) {
    }
}

<?php
// src/Application/Handler/CreateUserHandler.php

namespace App\Application\Handler;

use App\Application\Command\CreateUserCommand;
use App\Domain\Entity\User;
use App\Domain\Repository\UserRepositoryInterface;
use App\Domain\ValueObject\Email;
use App\Domain\ValueObject\UserId;
use InvalidArgumentException;

readonly class CreateUserHandler
{
    public function __construct(
        private UserRepositoryInterface $userRepository
    ) {
    }

    public function __invoke(CreateUserCommand $command): void
    {
        $email = new Email($command->email);

        if ($this->userRepository->findByEmail($email) !== null) {
            throw new InvalidArgumentException(
                'User with this email already exists'
            );
        }

        $user = User::create(
            new UserId($command->id),
            $email,
            $command->name
        );

        $this->userRepository->save($user);
    }
}

Example 5: Symfony Controller

<?php
// src/Adapter/Http/Controller/UserController.php

namespace App\Adapter\Http\Controller;

use App\Adapter\Http\Request\CreateUserRequest;
use App\Application\Command\CreateUserCommand;
use App\Application\Handler\CreateUserHandler;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpKernel\Attribute\AsController;
use Symfony\Component\Routing\Attribute\Route;
use Symfony\Component\Uid\Uuid;

#[AsController]
class UserController
{
    public function __construct(
        private CreateUserHandler $createUserHandler
    ) {
    }

    #[Route('/api/users', methods: ['POST'])]
    public function create(CreateUserRequest $request): JsonResponse
    {
        $command = new CreateUserCommand(
            id: Uuid::v4()->toRfc4122(),
            email: $request->email,
            name: $request->name
        );

        ($this->createUserHandler)($command);

        return new JsonResponse(['id' => $command->id], 201);
    }
}

Example 6: Request DTO with Validation

<?php
// src/Adapter/Http/Request/CreateUserRequest.php

namespace App\Adapter\Http\Request;

use Symfony\Component\Validator\Constraints as Assert;

class CreateUserRequest
{
    #[Assert\NotBlank]
    #[Assert\Email]
    public string $email;

    #[Assert\NotBlank]
    #[Assert\Length(min: 2, max: 100)]
    public string $name;
}

Example 7: Doctrine Repository Adapter

<?php
// src/Adapter/Persistence/Doctrine/Repository/DoctrineUserRepository.php

namespace App\Adapter\Persistence\Doctrine\Repository;

use App\Domain\Entity\User;
use App\Domain\Repository\UserRepositoryInterface;
use App\Domain\ValueObject\Email;
use App\Domain\ValueObject\UserId;
use Doctrine\ORM\EntityManagerInterface;

readonly class DoctrineUserRepository implements UserRepositoryInterface
{
    public function __construct(
        private EntityManagerInterface $entityManager
    ) {
    }

    public function findById(UserId $id): ?User
    {
        return $this->entityManager
            ->getRepository(User::class)
            ->find($id->value());
    }

    public function findByEmail(Email $email): ?User
    {
        return $this->entityManager
            ->getRepository(User::class)
            ->findOneBy(['email.value' => $email->value()]);
    }

    public function save(User $user): void
    {
        $this->entityManager->persist($user);
        $this->entityManager->flush();
    }

    public function delete(UserId $id): void
    {
        $user = $this->findById($id);
        if ($user !== null) {
            $this->entityManager->remove($user);
            $this->entityManager->flush();
        }
    }
}

Example 8: Symfony DI Configuration

# config/services.yaml
services:
    _defaults:
        autowire: true
        autoconfigure: true

    App\:
        resource: '../src/'
        exclude:
            - '../src/Domain/Entity/'
            - '../src/Kernel.php'

    # Repository binding - Port to Adapter
    App\Domain\Repository\UserRepositoryInterface:
        class: App\Adapter\Persistence\Doctrine\Repository\DoctrineUserRepository

    # In-memory repository for tests
    App\Domain\Repository\UserRepositoryInterface $inMemoryUserRepository:
        class: App\Tests\Infrastructure\Repository\InMemoryUserRepository

Example 9: In-Memory Repository for Testing

<?php
// tests/Infrastructure/Repository/InMemoryUserRepository.php

namespace App\Tests\Infrastructure\Repository;

use App\Domain\Entity\User;
use App\Domain\Repository\UserRepositoryInterface;
use App\Domain\ValueObject\Email;
use App\Domain\ValueObject\UserId;

class InMemoryUserRepository implements UserRepositoryInterface
{
    private array $users = [];

    public function findById(UserId $id): ?User
    {
        return $this->users[$id->value()] ?? null;
    }

    public function findByEmail(Email $email): ?User
    {
        foreach ($this->users as $user) {
            if ($user->email()->equals($email)) {
                return $user;
            }
        }
        return null;
    }

    public function save(User $user): void
    {
        $this->users[$user->id()->value()] = $user;
    }

    public function delete(UserId $id): void
    {
        unset($this->users[$id->value()]);
    }
}

Best Practices

1. Dependency Rule: Dependencies only point inward - domain knows nothing of application or infrastructure
2. Immutability: Value Objects MUST be immutable using readonly in PHP 8.1+ - never allow mutable state
3. Rich Domain Models: Put business logic in entities with factory methods like create() - avoid anemic models
4. Interface Segregation: Keep repository interfaces small and focused - do not create god interfaces
5. Framework Independence: Domain and application layers MUST be testable without Symfony or Doctrine
6. Validation at Construction: Validate in Value Objects at construction time - never allow invalid state
7. Symfony Attributes: Use PHP 8 attributes for routing (#[Route]), validation (#[Assert\]), and DI
8. Test Doubles: Always provide In-Memory implementations for repositories to enable fast unit tests
9. Domain Events: Dispatch domain events to decouple side effects - do not call external services from entities
10. XML/YAML Mappings: Use XML or YAML for Doctrine mappings instead of annotations in domain entities

Constraints and Warnings

Architecture Constraints

• Dependency Rule: Dependencies only point inward. Domain knows nothing of Application, Application knows nothing of Infrastructure. Violating this breaks the architecture.
• No Anemic Domain: Entities should encapsulate behavior, not just be data bags. Avoid getters/setters without business logic.
• Interface Segregation: Keep repository interfaces small and focused. Do not create god interfaces.

PHP Implementation Constraints

• Immutability: Value Objects MUST be immutable using readonly in PHP 8.1+. Never allow mutable state in Value Objects.
• Validation: Validate in Value Objects at construction time. Never allow invalid state to exist.
• Symfony Attributes: Use PHP 8 attributes for routing, validation, and DI (#[Route], #[Assert\Email], #[Autowire]).

Testing Constraints

• Framework Independence: Domain and Application layers MUST be testable without Symfony, Doctrine, or database.
• Test Doubles: Always provide In-Memory implementations for repository interfaces to enable fast unit tests.

Warnings

• Avoid Rich Domain Models in Controllers: Controllers should only coordinate, not contain business logic.
• Beware of Leaky Abstractions: Infrastructure concerns (like Doctrine annotations) should not leak into Domain entities. Use XML/YAML mappings instead.
• Command Bus Consideration: For complex applications, use Symfony Messenger for async processing. Do not inline complex orchestrations in handlers.
• Domain Events: Dispatch domain events to decouple side effects from core business logic. Do not call external services directly from entities.

References

• PHP Clean Architecture Patterns
• Symfony Implementation Guide

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