Clean Architecture

Clean Architecture Review Skill

Overview

This skill analyzes and reviews code based on Robert C. Martin's (Uncle Bob) Clean Architecture principles.

When to Use

• When user requests "clean architecture" review
• When asked about "architecture review", "dependency check", or "layer separation"
• When designing new features with clean architecture

Instructions

1. Project Structure Analysis

First, understand the project's directory structure:

• Check if layers are separated
• Examine module/package structure
• Identify dependency direction

2. The Dependency Rule

Core Principle: Source code dependencies must only point inward (toward higher-level policies).

┌─────────────────────────────────────────────────────────┐
│                  Frameworks & Drivers                    │
│  ┌─────────────────────────────────────────────────┐    │
│  │              Interface Adapters                  │    │
│  │  ┌─────────────────────────────────────────┐    │    │
│  │  │              Use Cases                   │    │    │
│  │  │  ┌─────────────────────────────────┐    │    │    │
│  │  │  │           Entities              │    │    │    │
│  │  │  └─────────────────────────────────┘    │    │    │
│  │  └─────────────────────────────────────────┘    │    │
│  └─────────────────────────────────────────────────┘    │
└─────────────────────────────────────────────────────────┘
                    ← Dependencies flow inward

Checklist:

• Do inner layers avoid referencing outer layers?
• Are Entities unaware of Use Cases, Adapters, and Infrastructure?
• Do Use Cases avoid directly referencing Adapters or Infrastructure?
• Is DIP applied when crossing boundaries?

2-1. Crossing Boundaries

The flow of control and source code dependency can point in opposite directions.

Controller → Use Case → Presenter
     ↓           ↓           ↑
[dependency] [dependency] [inverted!]

Example: When Use Case needs to call Presenter

• Directly referencing Presenter from Use Case violates the dependency rule
• Solution: Define Output Port (interface) inside Use Case, Presenter implements it

// Use Case Layer (inner)
interface OrderOutputPort {
  presentOrder(order: OrderData): void;
}

class CreateOrderUseCase {
  constructor(private outputPort: OrderOutputPort) {}
  execute() {
    // ... business logic
    this.outputPort.presentOrder(orderData);
  }
}

// Adapter Layer (outer) - depends on Use Case
class OrderPresenter implements OrderOutputPort {
  presentOrder(order: OrderData): void {
    // Transform for UI
  }
}

2-2. What Data Crosses Boundaries

Principle: Data crossing boundaries should be simple data structures (DTOs).

// Bad: Passing Entity directly
class GetUserUseCase {
  execute(): User { // Returning Entity directly
    return this.userRepository.findById(id);
  }
}

// Good: Transform to DTO
class GetUserUseCase {
  execute(): UserResponseDTO { // Simple data structure
    const user = this.userRepository.findById(id);
    return { id: user.id, name: user.name };
  }
}

Checklist:

• Are Entities or Database Rows not crossing boundaries directly?
• Are external framework data formats not penetrating inner layers?
• Are appropriate DTOs defined for each layer?

3. Layer Structure Review

Entities (Domain Layer)

• Contains core business rules
• No external dependencies
• Least likely to change

Checklist:

• Does Entity avoid depending on frameworks?
• Does it contain only pure business logic?
• Are there no external library imports?

Use Cases (Application Layer)

• Application-specific business rules
• Orchestrates Entities to implement features
• Defines interfaces for external systems

Checklist:

• Does each Use Case have a single responsibility?
• Are Input/Output ports clearly defined?
• Does it use Repository interfaces (not implementations)?

Interface Adapters

• Handles data transformation
• Contains Controllers, Presenters, Gateways
• Can apply patterns like MVC, MVVM

Checklist:

• Is transformation between DTOs and Entities happening here?
• Is the separation between external and internal formats clear?

Frameworks & Drivers (Infrastructure Layer)

• DB, Web Framework, external services
• Mostly glue code
• Most likely to change

Checklist:

• Are framework dependencies isolated to this layer?
• Are Repository implementations located here?

4. SOLID Principles Review

Object-oriented design principles used in Clean Architecture implementation. DIP is essential for Crossing Boundaries.

Output Format

Provide review results in this format:

# Clean Architecture Review Report

## Summary
- Overall Assessment: [Good/Needs Improvement/Critical]
- Key findings summary

## Dependency Rule Analysis
- List of violations
- Improvement suggestions

## Layer Structure Review
### Entities: [Good/Needs Improvement]
### Use Cases: [Good/Needs Improvement]
### Interface Adapters: [Good/Needs Improvement]
### Infrastructure: [Good/Needs Improvement]

## Recommendations
1. High priority improvements
2. Medium priority improvements
3. Long-term suggestions

Important Notes

Clean Architecture is Not a Silver Bullet

• Not all projects require exactly 4 layers
• Layer count is flexible as long as the dependency rule is followed
• Apply according to project scale and complexity

Common Pitfalls to Avoid

1. Over-obsession with layers: Layer separation is not the goal itself
2. Over-abstraction: Creating interfaces with only one implementation
3. Superficial adoption: Relying on IDE refactoring while ignoring actual dependency rules
4. Over-engineering for the future: Adding complexity for unlikely changes

Key Questions

Always consider these questions during review:

• "Does this separation actually provide benefits?"
• "Is the dependency rule truly being followed?"
• "Is the impact scope minimized when changes occur?"

Practical Verification

Ask yourself if you're really benefiting from Clean Architecture:

• Is it the layer separation that helps, or just IDE auto-refactoring?
• Is a Repository interface with only one implementation really necessary?
• Are you over-abstracting for a 0.001% chance of future change?
• Can you rename a class in outer modules without affecting inner modules?

Language-Specific Examples

TypeScript/JavaScript

// Good: Dependency Inversion
interface UserRepository {
  findById(id: string): Promise<User>;
}

class GetUserUseCase {
  constructor(private userRepository: UserRepository) {}
}

// Bad: Depending on concrete implementation
class GetUserUseCase {
  constructor(private userRepository: PostgresUserRepository) {}
}

Python

# Good: Interface definition using Protocol
from typing import Protocol

class UserRepository(Protocol):
    def find_by_id(self, id: str) -> User: ...

# Bad: Direct implementation import
from infrastructure.postgres import PostgresUserRepository

Java/Kotlin

// Good: Interface segregation
interface UserRepository {
    fun findById(id: String): User?
}

class GetUserUseCase(
    private val userRepository: UserRepository
)

// Bad: Using concrete implementation
class GetUserUseCase(
    private val userRepository: JpaUserRepository
)

References

• The Clean Architecture - Robert C. Martin
• Clean Architecture: A Craftsman's Guide to Software Structure and Design (Book)

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