Clean Architecture

Clean Architecture, popularized by Robert C. Martin (Uncle Bob), separates software into layers to ensure independence from frameworks, databases, and UIs. The core principle is the Dependency Rule: source code dependencies can only point inwards.

When to Use

• Building enterprise applications with complex business logic.
• Long-lived projects where frameworks/databases might change over time.
• Large teams requiring clear separation of concerns to work in parallel.

Quick Start

// 1. Entity (Enterprise Logic) - Inner Layer
class User {
  constructor(
    public id: string,
    public name: string,
  ) {
    if (name.length < 2) throw new Error("Name too short");
  }
}

// 2. Use Case (Application Logic)
class CreateUserUseCase {
  constructor(private userRepository: UserRepository) {} // Depends on interface

  async execute(name: string): Promise<User> {
    const user = new User(crypto.randomUUID(), name);
    await this.userRepository.save(user);
    return user;
  }
}

// 3. Interface Adapter (Repository Interface)
interface UserRepository {
  save(user: User): Promise<void>;
}

// 4. Frameworks & Drivers (Implementation) - Outer Layer
class SqlUserRepository implements UserRepository {
  async save(user: User): Promise<void> {
    await db.query("INSERT INTO users ...", [user.id, user.name]);
  }
}

Core Concepts

The Dependency Rule

Inner layers (Entities) know nothing about outer layers (Controllers, Presenters). Outer layers depend on inner layers.

Entities

Enterprise-wide business rules. These are the least likely to change when something external changes (e.g., page navigation security).

Application Business Rules (Use Cases)

Orchestrate the flow of data to and from the entities. They contain the specific business rules of the application (e.g., "Create Order").

Common Patterns

Dependency Injection

The glue that makes Clean Architecture possible. Outer layers inject concrete implementations (e.g., SqlUserRepository) into inner layers (which expect UserRepository interface).

DTOs (Data Transfer Objects)

Use simple objects (DTOs) to cross boundaries. Do not pass Entities to the UI or Database rows to the Use Case.

Best Practices

Do:

• Define Interfaces in the layer that uses them (Interface Segregation).
• Test Use Cases in isolation using mocks for repositories.
• Keep Frameworks (React, NestJS, Spring) at the outermost layer.

Don't:

• Don't let database entities (ORM models) leak into the inner layers. Map them to domain Entities.
• Don't skip layers "for speed" (e.g., Controller calling DB directly) in complex apps.

Troubleshooting

Error	Cause	Solution
Circular Dependency	Violating the dependency rule.	Use Dependency Inversion (Interfaces) to break the cycle.
Boilerplate Overload	Creating strict layers for simple CRUD.	Consider "Vertical Slice Architecture" or Modular Monolith for simpler domains.

References

• The Clean Architecture Blog
• Clean Architecture vs. Hexagonal