system-design-patterns

# System Design Patterns Skill

Common patterns for designing scalable systems.

## Architectural Patterns

### Layered Architecture

```
┌─────────────────────────────┐
│     Presentation Layer      │  UI, API endpoints
├─────────────────────────────┤
│      Application Layer      │  Business logic, use cases
├─────────────────────────────┤
│        Domain Layer         │  Entities, business rules
├─────────────────────────────┤
│     Infrastructure Layer    │  Database, external services
└─────────────────────────────┘
```

```typescript
// Presentation Layer
@Controller('users')
class UserController {
  constructor(private userService: UserService) {}

  @Get(':id')
  getUser(@Param('id') id: string) {
    return this.userService.findById(id)
  }
}

// Application Layer
class UserService {
  constructor(private userRepository: UserRepository) {}

  async findById(id: string): Promise<User> {
    return this.userRepository.findById(id)
  }
}

// Domain Layer
class User {
  constructor(
    public id: string,
    public email: string,
    public name: string
  ) {}

  updateEmail(newEmail: string) {
    // Business validation
    if (!this.isValidEmail(newEmail)) {
      throw new ValidationError('Invalid email')
    }
    this.email = newEmail
  }
}

// Infrastructure Layer
class TypeOrmUserRepository implements UserRepository {
  async findById(id: string): Promise<User | null> {
    return this.repository.findOne({ where: { id } })
  }
}
```

### Microservices

```
                    ┌─────────────┐
                    │ API Gateway │
                    └──────┬──────┘
           ┌───────────────┼───────────────┐
           ▼               ▼               ▼
    ┌────────────┐  ┌────────────┐  ┌────────────┐
    │   User     │  │   Order    │  │  Payment   │
    │  Service   │  │  Service   │  │  Service   │
    └────────────┘  └────────────┘  └────────────┘
           │               │               │
           ▼               ▼               ▼
    ┌─────────