Clean Architecture Skill

Comprehensive guide for implementing Clean Architecture, SOLID principles, and maintainable code structures.

When to Use This Skill

• Designing new service architecture
• Refactoring legacy code to clean architecture
• Implementing dependency injection
• Defining domain boundaries and layer separation
• Applying SOLID principles
• Reviewing architectural decisions

---

Architecture Layers

The Dependency Rule

Dependencies point inward. Inner layers must not know about outer layers.

┌─────────────────────────────────────────────────┐
│  External Layer (Web, CLI, GraphQL)             │
│  ┌───────────────────────────────────────────┐  │
│  │ Infrastructure (Repos, Adapters, ORM)     │  │
│  │ ┌───────────────────────────────────────┐ │  │
│  │ │ Application (Use Cases, Services)     │ │  │
│  │ │ ┌───────────────────────────────────┐ │ │  │
│  │ │ │ Domain (Entities, VOs, Services)  │ │ │  │
│  │ │ └───────────────────────────────────┘ │ │  │
│  │ └───────────────────────────────────────┘ │  │
│  └───────────────────────────────────────────┘  │
└─────────────────────────────────────────────────┘
        Dependencies point INWARD

1. Domain Layer

Business rules isolated from technical concerns:

• Entities: Objects with identity, business logic
• Value Objects: Immutable objects without identity
• Domain Services: Stateless operations on domain objects
• Repository Interfaces: Data access contracts

// Entity with behavior
export class User {
  constructor(
    public readonly id: UserId,
    private passwordHash: PasswordHash
  ) {}

  changePassword(newPassword: Password, hasher: PasswordHasher): void {
    this.passwordHash = hasher.hash(newPassword);
  }
}

// Value Object - immutable, validated
export class Email {
  private constructor(private readonly value: string) {}

  static create(email: string): Email {
    if (!this.isValid(email)) throw new InvalidEmailError(email);
    return new Email(email.toLowerCase());
  }

  private static isValid(email: string): boolean {
    return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
  }

  equals(other: Email): boolean {
    return this.value === other.value;
  }
}

// Repository interface - defines contract
export interface UserRepository {
  findById(id: UserId): Promise<User | null>;
  save(user: User): Promise<void>;
}

2. Application Layer

Orchestrates domain objects for use cases:

• Use Cases: Single responsibility operations
• DTOs: Data transfer at boundaries
• Ports: Interfaces for external dependencies

export class CreateUserUseCase {
  constructor(
    private readonly userRepository: UserRepository,
    private readonly passwordHasher: PasswordHasher
  ) {}

  async execute(input: CreateUserInput): Promise<CreateUserOutput> {
    const existing = await this.userRepository.findByEmail(
      Email.create(input.email)
    );
    if (existing) throw new EmailAlreadyExistsError();

    const user = new User(
      UserId.generate(),
      Email.create(input.email),
      this.passwordHasher.hash(input.password),
      new Date()
    );

    await this.userRepository.save(user);
    return user.toDTO();
  }
}

3. Infrastructure Layer

Implements interfaces from inner layers:

• Repository Implementations: Database access
• External Adapters: Third-party integrations
• ORM/Query Builders: Data persistence

export class PostgreSQLUserRepository implements UserRepository {
  constructor(private readonly db: Database) {}

  async findById(id: UserId): Promise<User | null> {
    const row = await this.db.query('SELECT * FROM users WHERE id = $1', [id.toString()]);
    return row ? this.toDomain(row) : null;
  }

  async save(user: User): Promise<void> {
    await this.db.query(
      `INSERT INTO users (id, email, password_hash) VALUES ($1, $2, $3) ON CONFLICT (id) DO UPDATE SET email = $2`,
      [user.id.toString(), user.email.toString(), user.passwordHash]
    );
  }

  private toDomain(row: UserRow): User {
    return new User(UserId.fromString(row.id), PasswordHash.fromString(row.password_hash));
  }
}

4. Presentation Layer

Entry points to the application:

• Controllers: HTTP handlers
• Resolvers: GraphQL endpoints
• CLI Commands: Command-line interfaces

export class UserController {
  constructor(private readonly createUserUseCase: CreateUserUseCase) {}

  async create(req: Request, res: Response): Promise<void> {
    try {
      const result = await this.createUserUseCase.execute(req.body);
      res.status(201).json(result);
    } catch (error) {
      if (error instanceof EmailAlreadyExistsError) {
        res.status(409).json({ error: error.message });
      }
    }
  }
}

---

Project Structure

src/
├── domain/
│   ├── entities/          (User, Order)
│   ├── value-objects/     (Email, Money, UserId)
│   ├── services/          (PricingService)
│   ├── repositories/      (Interfaces only)
│   └── errors/
├── application/
│   ├── use-cases/         (CreateUser, UpdateOrder)
│   ├── services/          (NotificationService)
│   ├── ports/             (EmailPort, PaymentPort)
│   └── dto/
├── infrastructure/
│   ├── repositories/      (PostgreSQL, MongoDB implementations)
│   ├── adapters/          (SendGrid, Stripe)
│   ├── orm/
│   └── config/
├── presentation/
│   ├── http/              (Controllers, Routes, Middleware)
│   ├── graphql/           (Resolvers)
│   └── cli/               (Commands)
├── shared/                (Utilities, Kernel helpers)
└── container/             (Dependency Injection setup)

---

Dependency Injection

// src/container/container.ts
import { Container } from 'inversify';

const container = new Container();

// Bind implementations to interfaces
container.bind<UserRepository>(TYPES.UserRepository)
  .to(PostgreSQLUserRepository)
  .inSingletonScope();

container.bind<CreateUserUseCase>(TYPES.CreateUserUseCase)
  .to(CreateUserUseCase)
  .inTransientScope();

container.bind<UserController>(TYPES.UserController)
  .to(UserController)
  .inTransientScope();

export { container };

---

SOLID Principles

Single Responsibility

Each layer has one reason to change:

• Domain: Business rules
• Application: Use case coordination
• Infrastructure: Technical implementations
• Presentation: User interface

Open/Closed

Add features by creating new use cases, not modifying existing:

export class UpdateUserUseCase { /* ... */ }

Liskov Substitution

Repository implementations are fully interchangeable:

const repo: UserRepository = new PostgreSQLUserRepository(db);
const repo: UserRepository = new MongoUserRepository(client);
// Both satisfy the contract

Interface Segregation

Use focused interfaces, not fat ones:

// Good: Segregated
interface UserCreator { create(data): User; }
interface UserDeleter { delete(id): void; }

// Bad: Fat interface
interface UserService {
  create(): User;
  update(): User;
  delete(): void;
  sendEmail(): void;
  generateReport(): Report;
}

Dependency Inversion

Depend on abstractions, not implementations:

// Application defines the port
export interface EmailPort {
  send(to: string, subject: string, body: string): Promise<void>;
}

// Infrastructure implements
export class SendGridAdapter implements EmailPort {
  async send(to: string, subject: string, body: string): Promise<void> {
    await this.sendgrid.send({ to, subject, text: body });
  }
}

// Use cases depend on port
export class CreateUserUseCase {
  constructor(private readonly emailPort: EmailPort) {}
}

---

Testing

// Unit: Domain logic without infrastructure
describe('User', () => {
  it('should change password', () => {
    const hasher = new BCryptHasher();
    const user = new User(UserId.generate(), hasher.hash('oldpass'));
    user.changePassword(Password.create('newpass'), hasher);
    expect(user.validatePassword(Password.create('newpass'), hasher)).toBe(true);
  });
});

// Integration: Infrastructure with real DB
describe('PostgreSQLUserRepository', () => {
  it('should save and retrieve user', async () => {
    const repo = new PostgreSQLUserRepository(testDb);
    const user = createTestUser();
    await repo.save(user);
    const retrieved = await repo.findById(user.id);
    expect(retrieved).not.toBeNull();
  });
});

// E2E: Full stack via HTTP
describe('User API', () => {
  it('should create user via POST', async () => {
    const response = await request(app).post('/api/users').send({
      email: 'test@example.com',
      password: 'secure123'
    });
    expect(response.status).toBe(201);
  });
});

---

Anti-Patterns

Domain Logic in Controllers

// Bad: Business logic in controller
async create(req, res) {
  if (await this.db.query('SELECT * FROM users WHERE email = $1', [req.body.email])) {
    return res.status(409).json({ error: 'Email exists' });
  }
}

// Good: Delegate to use case
async create(req, res) {
  const result = await this.createUserUseCase.execute(req.body);
  res.status(201).json(result);
}

Infrastructure in Domain

// Bad: Infrastructure leak in entity
class User {
  async save() {
    await prisma.user.create({ data: this });
  }
}

// Good: Repository handles persistence
class User { /* pure domain */ }
class UserRepository {
  async save(user: User) { await prisma.user.create(...); }
}

Anemic Domain Model

// Bad: Entity is just data
class User {
  id: string;
  password: string;
}
class UserService {
  changePassword(user: User, pwd: string) {
    user.password = hash(pwd);  // Logic outside entity
  }
}

// Good: Rich domain model
class User {
  changePassword(newPassword: Password, hasher: PasswordHasher): void {
    if (!newPassword.isStrong()) throw new WeakPasswordError();
    this.passwordHash = hasher.hash(newPassword);
  }
}

---

Migration Path (Legacy → Clean Architecture)

1. Identify Boundaries: Find domain concepts
2. Extract Entities: Create domain objects with behavior
3. Define Interfaces: Create repository/port interfaces
4. Implement Adapters: Wrap existing data access
5. Create Use Cases: Extract business logic
6. Refactor Controllers: Delegate to use cases
7. Add DI Container: Wire dependencies
8. Write Tests: Cover each layer

---

Quick Checklist

• Domain is infrastructure-free
• All dependencies point inward
• Use cases are thin orchestrators
• Repository interfaces in domain
• Implementations in infrastructure
• Controllers delegate to use cases
• DTOs at layer boundaries
• Comprehensive test coverage (unit, integration, e2e)
• DI container wires all dependencies
• No anemic domain models