DDD Knowledge Base

Quick reference for DDD architecture patterns and PHP implementation guidelines.

Core Principles

Layer Dependencies (Clean Architecture)

Presentation → Application → Domain ← Infrastructure
                    ↓
              Domain (center)

Rule: Dependencies point INWARD. Domain has ZERO external dependencies.

Layer Responsibilities

| Layer | Contains | Depends On | |-------|----------|------------| | Domain | Entities, Value Objects, Aggregates, Domain Services, Repository Interfaces, Domain Events | Nothing | | Application | Use Cases, DTOs, Application Services | Domain | | Infrastructure | Repository Implementations, External APIs, DB, Cache, Queue | Domain, Application | | Presentation | Controllers, Actions, Request/Response, CLI | Application |

Quick Checklists

Domain Layer Checklist

[ ] No framework imports (Doctrine, Eloquent, Symfony)
[ ] Entities have behavior, not just data
[ ] Value Objects for domain concepts (Email, Money, Id)
[ ] Repository INTERFACES defined here
[ ] Enums for fixed value sets
[ ] Domain Events for side effects
[ ] No public function set*() methods

Application Layer Checklist

[ ] UseCases orchestrate, don't decide
[ ] DTOs for input/output
[ ] No business logic (if/switch on domain state)
[ ] Transaction boundaries here
[ ] No HTTP/CLI concerns

Infrastructure Layer Checklist

[ ] Implements Domain interfaces
[ ] No business logic in repositories
[ ] External service adapters
[ ] Caching, queuing implementations

Presentation Layer Checklist

[ ] Validates input
[ ] Maps to DTOs
[ ] Calls UseCase
[ ] Formats response
[ ] No business logic

Common Violations Quick Reference

| Violation | Where to Look | Severity | |-----------|---------------|----------| | use Doctrine\\ in Domain | Domain/.php | Critical | | use Illuminate\\ in Domain | Domain/.php | Critical | | use Infrastructure\\ in Domain | Domain/.php | Critical | | Only getters/setters in Entity | Domain/Entity/.php | Warning | | === 'pending' magic strings | Any PHP file | Warning | | public function set*() | Domain/Entity/.php | Warning | | Business logic in Controller | Presentation/.php | Warning | | Business logic in Repository | Infrastructure/*.php | Warning |

PHP 8.5 DDD Patterns

Value Object

final readonly class Email
{
    public function __construct(
        public string $value
    ) {
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new InvalidArgumentException('Invalid email');
        }
    }

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

Entity with Behavior

final class Order
{
    private OrderStatus $status;

    public function __construct(
        private readonly OrderId $id,
        private readonly CustomerId $customerId
    ) {
        $this->status = OrderStatus::Pending;
    }

    public function confirm(): void
    {
        if (!$this->status->canTransitionTo(OrderStatus::Confirmed)) {
            throw new DomainException('Cannot confirm order');
        }
        $this->status = OrderStatus::Confirmed;
    }
}

Repository Interface

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

References

For detailed information, load these reference files:

references/layer-architecture.md — Detailed layer rules and boundaries
references/domain-patterns.md — Entity, VO, Aggregate, Repository patterns
references/application-patterns.md — UseCase, DTO, Command/Query patterns
references/antipatterns.md — Common violations with detection patterns
references/php-specific.md — PHP 8.5 specific implementations

Assets

assets/report-template.md — Structured audit report template