/architecture-patterns

name: architecture-patterns
description: Implement proven backend architecture patterns including Clean Architecture, Hexagonal Architecture, and Domain-Driven Design. Use when architecting complex backend systems or refactoring existing applications for better maintainability.
license: MIT

Architecture Patterns

Master proven backend architecture patterns including Clean Architecture, Hexagonal Architecture, and Domain-Driven Design to build maintainable, testable, and scalable systems.

When to Use This Skill

• Designing new backend systems from scratch
• Refactoring monolithic applications for better maintainability
• Establishing architecture standards for your team
• Migrating from tightly coupled to loosely coupled architectures
• Implementing domain-driven design principles
• Creating testable and mockable codebases
• Planning microservices decomposition

Core Concepts

1. Clean Architecture (Uncle Bob)

**Layers (dependency flows inward):**

• **Entities**: Core business models
• **Use Cases**: Application business rules
• **Interface Adapters**: Controllers, presenters, gateways
• **Frameworks & Drivers**: UI, database, external services

**Key Principles:**

• Dependencies point inward
• Inner layers know nothing about outer layers
• Business logic independent of frameworks
• Testable without UI, database, or external services

2. Hexagonal Architecture (Ports and Adapters)

**Components:**

• **Domain Core**: Business logic
• **Ports**: Interfaces defining interactions
• **Adapters**: Implementations of ports (database, REST, message queue)

**Benefits:**

• Swap implementations easily (mock for testing)
• Technology-agnostic core
• Clear separation of concerns

3. Domain-Driven Design (DDD)

**Strategic Patterns:**

• **Bounded Contexts**: Separate models for different domains
• **Context Mapping**: How contexts relate
• **Ubiquitous Language**: Shared terminology

**Tactical Patterns:**

• **Entities**: Objects with identity
• **Value Objects**: Immutable objects defined by attributes
• **Aggregates**: Consistency boundaries
• **Repositories**: Data access abstraction
• **Domain Events**: Things that happened

Clean Architecture Pattern

Directory Structure

app/
├── domain/           # Entities & business rules
│   ├── entities/
│   ├── value_objects/
│   └── interfaces/   # Abstract interfaces
├── use_cases/        # Application business rules
├── adapters/         # Interface implementations
│   ├── repositories/
│   ├── controllers/
│   └── gateways/
└── infrastructure/   # Framework & external concerns

Implementation Example

# domain/entities/user.py
from dataclasses import dataclass
from datetime import datetime

@dataclass
class User:
    """Core user entity - no framework dependencies."""
    id: str
    email: str
    name: str
    created_at: datetime
    is_active: bool = True

    def deactivate(self):
        """Business rule: deactivating user."""
        self.is_active = False

    def can_place_order(self) -> bool:
        """Business rule: active users can order."""
        return self.is_active

# domain/interfaces/user_repository.py
from abc import ABC, abstractmethod
from typing import Optional

class IUserRepository(ABC):
    """Port: defines contract, no implementation."""

    @abstractmethod
    async def find_by_id(self, user_id: str) -> Optional[User]:
        pass

    @abstractmethod
    async def save(self, user: User) -> User:
        pass

# use_cases/create_user.py
from dataclasses import dataclass
import uuid
from datetime import datetime

@dataclass
class CreateUserRequest:
    email: str
    name: str

class CreateUserUseCase:
    """Use case: orchestrates business logic."""

    def __init__(self, user_repository: IUserRepository):
        self.user_repository = user_repository

    async def execute(self, request: CreateUserRequest) -> CreateUserResponse:
        # Business validation
        existing = await self.user_repository.find_by_email(request.email)
        if existing:
            return CreateUserResponse(user=None, success=False, error="Email already exists")

        # Create entity
        user = User(
            id=str(uuid.uuid4()),
            email=request.email,
            name=request.name,
            created_at=datetime.now(),
            is_active=True
        )

        # Persist
        saved_user = await self.user_repository.save(user)
        return CreateUserResponse(user=saved_user, success=True)

# adapters/repositories/postgres_user_repository.py
class PostgresUserRepository(IUserRepository):
    """Adapter: PostgreSQL implementation."""

    def __init__(self, pool):
        self.pool = pool

    async def find_by_id(self, user_id: str) -> Optional[User]:
        async with self.pool.acquire() as conn:
            row = await conn.fetchrow("SELECT * FROM users WHERE id = $1", user_id)
            return self._to_entity(row) if row else None

    async def save(self, user: User) -> User:
        async with self.pool.acquire() as conn:
            await conn.execute(
                """INSERT INTO users (id, email, name, created_at, is_active)
                VALUES ($1, $2, $3, $4, $5)
                ON CONFLICT (id) DO UPDATE SET email = $2, name = $3, is_active = $5""",
                user.id, user.email, user.name, user.created_at, user.is_active
            )
            return user

Hexagonal Architecture Pattern

# Core domain (hexagon center)
class OrderService:
    """Domain service - no infrastructure dependencies."""

    def __init__(
        self,
        order_repository: OrderRepositoryPort,
        payment_gateway: PaymentGatewayPort,
        notification_service: NotificationPort
    ):
        self.orders = order_repository
        self.payments = payment_gateway
        self.notifications = notification_service

    async def place_order(self, order: Order) -> OrderResult:
        # Business logic
        if not orde