architecture-patterns

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.

# 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/
│   │   ├── user.py
│   │   └── order.py
│   ├── value_objects/
│   │   ├── email.py
│   │   └── money.py
│   └── interfaces/   # Abstract interfaces
│       ├── user_repository.py
│       └── payment_gateway.py
├── use_cases/        # Application business rules
│   ├── create_user.py
│   ├── process_order.py
│   └── send_notification.py
├── adapters/         # Interface implementations
│   ├── repositories/
│   │   ├── postgres_user_repository.py
│   │   └── redis_cache_repository.py
│   ├── controllers/
│   │   └── user_controller.py
│   └── gateways/
│       ├── stripe_payment_gateway.py
│       └── sendgrid_email_gateway.py
└── infrastructure/   # Framework & external concerns
    ├── database.py
    ├── config.py
    └── logging.py
```

### Implementation Example

```python
# domain/entities/user.py
from dataclasses import dataclass
from datetime import datetime
from typing import Optional

@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, List
from domain.entities.user import User

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

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

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

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

    @abstractmethod
    async def delete(self, user_id: str) -> bool:
        pass

# use_cases/create_user.py
from domain.entities.user import User
from domain.interfaces.user_repository import IUserRepository
from dataclasses import dataclass
from datetime import datetime
import uuid

@dataclass
class CreateUserRequest:
    email: str
    name: str

@dataclass
class CreateUserResponse:
    user: User
    success: bool
    error: Optional[str] = None

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
from domain.interfaces.user_repository import IUserRepository
from domain.entities.user import User
from typing import Optional
import asyncpg

class PostgresUserRepository(IUserRepository):
    """Adapter: PostgreSQL implementation."""

    def __init__(self, pool: asyncpg.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 find_by_email(self, email: str) -> Optional[User]:
        async with self.pool.acquire() as conn:
            row = await conn.fetchrow(
                "SELECT * FROM users WHERE email = $1", email
            )
            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

    async def delete(self, user_id: str) -> bool:
        async with self.pool.acquire() as conn:
            result = await conn.execute(
                "DELETE FROM users WHERE id = $1", user_id
            )
            return result == "DELETE 1"

    def _to_entity(self, row) -> User:
        """Map database row to entity."""
        return User(
            id=row["id"],
            email=row["email"],
            name=row["name"],
            created_at=row["created_at"],
            is_active=row["is_active"]
        )

# adapters/controllers/user_controller.py
from fastapi import APIRouter, Depends, HTTPException
from use_cases.create_user import CreateUserUseCase, CreateUserRequest
from pydantic import BaseModel

router = APIRouter()

class CreateUserDTO(BaseModel):
    email: str
    name: str

@router.post("/users")
async def create_user(
    dto: CreateUserDTO,
    use_case: CreateUserUseCase = Depends(get_create_user_use_case)
):
    """Controller: handles HTTP concerns only."""
    request = CreateUserRequest(email=dto.email, name=dto.name)
    response = await use_case.execute(request)

    if not response.success:
        raise HTTPException(status_code=400, detail=response.error)

    return {"user": response.user}
```

## Hexagonal Architecture Pattern

```python
# Core domain (hexago