Using design patterns to build maintainable applications and reliable boundaries for AI-assisted development

Design patterns are a language, not a checklist

A pattern is useful when it names a real responsibility and makes an important part of a system easier to change. Adding Factory, Strategy or Repository simply to increase the class count has little value. Small PHP services can become less clear through needless layers; large applications can remain understandable through a few consistently protected boundaries.

That matters with AI-assisted code. A tool may generate valid syntax while placing a database query in a controller or an integration detail in business logic. Patterns and simple architectural rules give a team a language for reviewing that change.

A port is not its implementation

For order confirmation, the application can own a NotificationSender contract. SMTP, a provider API and a test implementation are adapters. Strategy makes sense where behaviour is selected at runtime from context; a Factory may select it. Where configuration fixes the implementation, ordinary dependency injection is often enough.

\\\text Use case ──► NotificationSender (port) ◄── SMTP / provider API / test adapter owned by application infrastructure boundary \\\

\\\`php interface NotificationSender { public function sendOrderConfirmation(OrderId $orderId): void; }

final readonly class SendOrderConfirmationService { public function __construct(private NotificationSender $sender) {}

public function send(OrderId $orderId): void { $this->sender->sendOrderConfirmation($orderId); } } \\\`

This is an illustrative excerpt: OrderId and the adapters are omitted. An interface implementation is not automatically a Strategy; that pattern needs interchangeable algorithms selected from runtime context.

Named operations and business rules

A Command can express intent and a Handler coordinate a use case. A Repository handles persistence, not an HTTP response; DTOs and Mappers earn their place where they clarify transformation or avoid unnecessary entity loading. A narrow read projection can be better than a mapper for every simple read.

A Specification is useful when a named business rule matters in more than one place. Ordinary form validation need not become a Specification. Ordered validators are not automatically Chain of Responsibility; that pattern needs its own passing-of-control contract.

Keep effects visible

A Decorator can add cache to a repository read without changing its contract. Recording a Domain Event does not ensure reliable asynchronous delivery or undo an external effect. Retry safety depends on idempotency, durable state and external side effects. Template Method fits a stable import lifecycle, but where only parsing or validation varies, Strategy or composition is often simpler; cleanup and failure handling still matter.

Reviewing AI-proposed changes

| Check | What it can establish | |---|---| | PHPStan | type problems and some contract mismatches | | Behavioural tests | whether an important use case still works | | Project rules | boundary violations, where separately implemented | | Human review | responsibility, side effects and trade-offs |

PHPStan and Pest do not enforce every architectural rule automatically. Project-specific rules, a dedicated tool or integration tests may be needed. Patterns do not make AI-generated code safe; they make a violated boundary easier to see.

Choose only what the operation needs

A document-generation operation does not need Command, Handler, Specification, Repository, Strategy, Factory, Adapter, Decorator and an event at once. Use components that solve the present problem. Good architecture is not a pattern catalogue; it is a set of decisions the next engineer or AI tool can understand, check and develop safely.