layer-arch-knowledge

Layered Architecture Knowledge Base

Quick reference for Layered (N-Tier) Architecture patterns and PHP implementation guidelines.

Core Principles

Layered Architecture Overview

┌─────────────────────────────────────────────────────────────────┐
│                     PRESENTATION LAYER                          │
│         (Controllers, Views, API Endpoints, CLI)                │
│                                                                 │
│  Responsibilities:                                              │
│  - Handle user input                                            │
│  - Display output                                               │
│  - Validate input format                                        │
│  - Route requests                                               │
└───────────────────────────┬─────────────────────────────────────┘
                            │ calls
                            ▼
┌─────────────────────────────────────────────────────────────────┐
│                     APPLICATION LAYER                           │
│           (Services, Use Cases, DTOs, Facades)                  │
│                                                                 │
│  Responsibilities:                                              │
│  - Orchestrate business operations                              │
│  - Transaction management                                       │
│  - Coordinate domain objects                                    │
│  - Map between layers                                           │
└───────────────────────────┬─────────────────────────────────────┘
                            │ calls
                            ▼
┌─────────────────────────────────────────────────────────────────┐
│                      DOMAIN LAYER                               │
│      (Entities, Value Objects, Domain Services, Rules)          │
│                                                                 │
│  Responsibilities:                                              │
│  - Business logic                                               │
│  - Business rules and invariants                                │
│  - Domain events                                                │
│  - Core algorithms                                              │
└───────────────────────────┬─────────────────────────────────────┘
                            │ calls
                            ▼
┌─────────────────────────────────────────────────────────────────┐
│                   INFRASTRUCTURE LAYER                          │
│       (Repositories, External APIs, Database, Cache)            │
│                                                                 │
│  Responsibilities:                                              │
│  - Data persistence                                             │
│  - External service integration                                 │
│  - Technical infrastructure                                     │
│  - Framework-specific code                                      │
└─────────────────────────────────────────────────────────────────┘

Core Rule: Each layer only communicates with the layer directly below it.

Layer Communication Rules

From Layer	Can Call	Cannot Call
Presentation	Application	Domain, Infrastructure
Application	Domain, Infrastructure	Presentation
Domain	Infrastructure (via interfaces)	Presentation, Application
Infrastructure	—	All upper layers

Quick Checklists

Presentation Layer Checklist

• Controllers are thin
• No business logic
• Input validation only
• Calls application services
• Transforms output for display
• Framework code contained here

Application Layer Checklist

• Orchestrates operations
• Transaction boundaries
• No direct DB access
• Uses domain objects
• DTOs for input/output
• No framework dependencies

Domain Layer Checklist

• Pure business logic
• No infrastructure code
• Rich entity behavior
• Value objects for concepts
• Business rules encapsulated
• Repository interfaces only

Infrastructure Layer Checklist

• Implements domain interfaces
• Database operations
• External API calls
• Caching logic
• No business logic

Common Violations Quick Reference

Violation	Where to Look	Severity
Skipping layers	Controller calling DB	Critical
Upward dependency	Domain using Application	Critical
Business in Controller	if/switch in controllers	Warning
Anemic services	Service = simple delegation	Warning
Fat repository	Logic in repository	Warning

PHP 8.4 Layered Architecture Patterns

Presentation Layer (Controller)

<?php

declare(strict_types=1);

namespace Presentation\Api\Order;

use Application\Order\Service\OrderService;
use Application\Order\DTO\CreateOrderDTO;
use Symfony\Component\HttpFoundation\JsonResponse;
use Symfony\Component\HttpFoundation\Request;

final readonly class OrderController
{
    public function __construct(
        private OrderService $orderService
    ) {}

    public function create(Request $request): JsonResponse
    {
        $dto = CreateOrderDTO::fromRequest($request);

        $result = $this->orderService->createOrder($dto);

        return new JsonResponse($result->toArray(), 201);
    }

    public function show(string $id): JsonResponse
    {
        $order = $this->orderService->getOrder($id);

        return new JsonResponse($order->toArray());
    }
}

Application Layer (Service)

<?php

declare(strict_types=1);

namespace Application\Order\Service;

use Application\Order\DTO\CreateOrderDTO;
use Application\Order\DTO\OrderDTO;
use Domain\Order\Entity\Order;
use Domain\Order\Repository\OrderRepositoryInterface;
use Domain\Order\ValueObject\OrderId;

final readonly class OrderService
{
    public function __construct(
        private OrderRepositoryInterface $orderRepository,
        private TransactionManagerInterface $transactionManager
    ) {}

    public function createOrder(CreateOrderDTO $dto): OrderDTO
    {
        return $this->transactionManager->transactional(function () use ($dto) {
            $order = Order::create(
                id: $this->orderRepository->nextIdentity(),
                customerId: $dto->customerId,
                lines: $dto->lines
            );

            $this->orderRepository->save($order);

            return OrderDTO::fromEntity($order);
        });
    }

    public function getOrder(string $id): OrderDTO
    {
        $order = $this->orderRepository->findById(new OrderId($id));

        if ($order === null) {
            throw new OrderNotFoundException($id);
        }

        return OrderDTO::fromEntity($order);
    }
}

Domain Layer (Entity)

<?php

declare(strict_types=1);

namespace Domain\Order\Entity;

use Domain\Order\ValueObject\OrderId;
use Domain\Order\ValueObject\CustomerId;
use Domain\Order\ValueObject\Money;
use Domain\Order\Enum\OrderStatus;

final class Order
{
    private OrderStatus $status;
    private array $lines = [];

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

    public static function create(OrderId $id, CustomerId $customerId, array $lines): self
    {
        $order = new self($id, $customerId);

        foreach ($lines as $line) {
            $order->addLine($line);
        }

        return $order;
    }

    public function confirm(): void
    {
        if (!$this->canBeConfirmed()) {
            throw new CannotConfirmOrderException();
        }

        $this->status = OrderStatus::Confirmed;
    }

    public function total(): Money
    {
        return array_reduce(
            $this->lines,
            fn (Money $carry, OrderLine $line) => $carry->add($line->total()),
            Money::zero('USD')
        );
    }

    private function canBeConfirmed(): bool
    {
        return $this->status === OrderStatus::Draft && !empty($this->lines);
    }
}

Infrastructure Layer (Repository)

<?php

declare(strict_types=1);

namespace Infrastructure\Persistence\Order;

use Domain\Order\Entity\Order;
use Domain\Order\Repository\OrderRepositoryInterface;
use Domain\Order\ValueObject\OrderId;
use Doctrine\ORM\EntityManagerInterface;

final readonly class DoctrineOrderRepository implements OrderRepositoryInterface
{
    public function __construct(
        private EntityManagerInterface $em
    ) {}

    public function findById(OrderId $id): ?Order
    {
        return $this->em->find(Order::class, $id->value);
    }

    public function save(Order $order): void
    {
        $this->em->persist($order);
        $this->em->flush();
    }

    public function nextIdentity(): OrderId
    {
        return OrderId::generate();
    }
}

References

For detailed information, load these reference files:

• references/layer-responsibilities.md — Detailed layer responsibilities
• references/layer-communication.md — Inter-layer communication patterns
• references/dto-patterns.md — Data Transfer Object patterns
• references/antipatterns.md — Common violations with detection patterns