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467 lines
14 KiB
Markdown
467 lines
14 KiB
Markdown
---
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name: architecture-patterns
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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.
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---
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# Architecture Patterns
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Master proven backend architecture patterns including Clean Architecture, Hexagonal Architecture, and Domain-Driven Design to build maintainable, testable, and scalable systems.
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## When to Use This Skill
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- Designing new backend systems from scratch
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- Refactoring monolithic applications for better maintainability
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- Establishing architecture standards for your team
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- Migrating from tightly coupled to loosely coupled architectures
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- Implementing domain-driven design principles
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- Creating testable and mockable codebases
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- Planning microservices decomposition
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## Core Concepts
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### 1. Clean Architecture (Uncle Bob)
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**Layers (dependency flows inward):**
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- **Entities**: Core business models
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- **Use Cases**: Application business rules
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- **Interface Adapters**: Controllers, presenters, gateways
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- **Frameworks & Drivers**: UI, database, external services
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**Key Principles:**
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- Dependencies point inward
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- Inner layers know nothing about outer layers
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- Business logic independent of frameworks
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- Testable without UI, database, or external services
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### 2. Hexagonal Architecture (Ports and Adapters)
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**Components:**
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- **Domain Core**: Business logic
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- **Ports**: Interfaces defining interactions
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- **Adapters**: Implementations of ports (database, REST, message queue)
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**Benefits:**
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- Swap implementations easily (mock for testing)
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- Technology-agnostic core
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- Clear separation of concerns
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### 3. Domain-Driven Design (DDD)
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**Strategic Patterns:**
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- **Bounded Contexts**: Separate models for different domains
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- **Context Mapping**: How contexts relate
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- **Ubiquitous Language**: Shared terminology
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**Tactical Patterns:**
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- **Entities**: Objects with identity
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- **Value Objects**: Immutable objects defined by attributes
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- **Aggregates**: Consistency boundaries
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- **Repositories**: Data access abstraction
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- **Domain Events**: Things that happened
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## Clean Architecture Pattern
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### Directory Structure
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```
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app/
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├── domain/ # Entities & business rules
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│ ├── entities/
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│ │ ├── user.py
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│ │ └── order.py
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│ ├── value_objects/
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│ │ ├── email.py
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│ │ └── money.py
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│ └── interfaces/ # Abstract interfaces
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│ ├── user_repository.py
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│ └── payment_gateway.py
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├── use_cases/ # Application business rules
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│ ├── create_user.py
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│ ├── process_order.py
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│ └── send_notification.py
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├── adapters/ # Interface implementations
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│ ├── repositories/
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│ │ ├── postgres_user_repository.py
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│ │ └── redis_cache_repository.py
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│ ├── controllers/
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│ │ └── user_controller.py
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│ └── gateways/
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│ ├── stripe_payment_gateway.py
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│ └── sendgrid_email_gateway.py
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└── infrastructure/ # Framework & external concerns
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├── database.py
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├── config.py
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└── logging.py
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```
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### Implementation Example
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```python
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# domain/entities/user.py
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from dataclasses import dataclass
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from datetime import datetime
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from typing import Optional
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@dataclass
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class User:
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"""Core user entity - no framework dependencies."""
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id: str
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email: str
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name: str
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created_at: datetime
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is_active: bool = True
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def deactivate(self):
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"""Business rule: deactivating user."""
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self.is_active = False
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def can_place_order(self) -> bool:
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"""Business rule: active users can order."""
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return self.is_active
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# domain/interfaces/user_repository.py
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from abc import ABC, abstractmethod
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from typing import Optional, List
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from domain.entities.user import User
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class IUserRepository(ABC):
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"""Port: defines contract, no implementation."""
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@abstractmethod
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async def find_by_id(self, user_id: str) -> Optional[User]:
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pass
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@abstractmethod
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async def find_by_email(self, email: str) -> Optional[User]:
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pass
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@abstractmethod
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async def save(self, user: User) -> User:
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pass
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@abstractmethod
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async def delete(self, user_id: str) -> bool:
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pass
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# use_cases/create_user.py
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from domain.entities.user import User
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from domain.interfaces.user_repository import IUserRepository
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from dataclasses import dataclass
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from datetime import datetime
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import uuid
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@dataclass
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class CreateUserRequest:
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email: str
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name: str
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@dataclass
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class CreateUserResponse:
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user: User
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success: bool
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error: Optional[str] = None
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class CreateUserUseCase:
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"""Use case: orchestrates business logic."""
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def __init__(self, user_repository: IUserRepository):
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self.user_repository = user_repository
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async def execute(self, request: CreateUserRequest) -> CreateUserResponse:
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# Business validation
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existing = await self.user_repository.find_by_email(request.email)
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if existing:
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return CreateUserResponse(
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user=None,
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success=False,
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error="Email already exists"
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)
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# Create entity
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user = User(
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id=str(uuid.uuid4()),
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email=request.email,
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name=request.name,
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created_at=datetime.now(),
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is_active=True
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)
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# Persist
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saved_user = await self.user_repository.save(user)
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return CreateUserResponse(
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user=saved_user,
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success=True
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)
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# adapters/repositories/postgres_user_repository.py
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from domain.interfaces.user_repository import IUserRepository
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from domain.entities.user import User
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from typing import Optional
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import asyncpg
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class PostgresUserRepository(IUserRepository):
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"""Adapter: PostgreSQL implementation."""
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def __init__(self, pool: asyncpg.Pool):
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self.pool = pool
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async def find_by_id(self, user_id: str) -> Optional[User]:
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async with self.pool.acquire() as conn:
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row = await conn.fetchrow(
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"SELECT * FROM users WHERE id = $1", user_id
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)
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return self._to_entity(row) if row else None
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async def find_by_email(self, email: str) -> Optional[User]:
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async with self.pool.acquire() as conn:
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row = await conn.fetchrow(
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"SELECT * FROM users WHERE email = $1", email
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)
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return self._to_entity(row) if row else None
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async def save(self, user: User) -> User:
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async with self.pool.acquire() as conn:
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await conn.execute(
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"""
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INSERT INTO users (id, email, name, created_at, is_active)
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VALUES ($1, $2, $3, $4, $5)
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ON CONFLICT (id) DO UPDATE
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SET email = $2, name = $3, is_active = $5
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""",
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user.id, user.email, user.name, user.created_at, user.is_active
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)
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return user
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async def delete(self, user_id: str) -> bool:
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async with self.pool.acquire() as conn:
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result = await conn.execute(
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"DELETE FROM users WHERE id = $1", user_id
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)
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return result == "DELETE 1"
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def _to_entity(self, row) -> User:
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"""Map database row to entity."""
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return User(
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id=row["id"],
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email=row["email"],
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name=row["name"],
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created_at=row["created_at"],
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is_active=row["is_active"]
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)
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# adapters/controllers/user_controller.py
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from fastapi import APIRouter, Depends, HTTPException
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from use_cases.create_user import CreateUserUseCase, CreateUserRequest
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from pydantic import BaseModel
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router = APIRouter()
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class CreateUserDTO(BaseModel):
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email: str
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name: str
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@router.post("/users")
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async def create_user(
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dto: CreateUserDTO,
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use_case: CreateUserUseCase = Depends(get_create_user_use_case)
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):
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"""Controller: handles HTTP concerns only."""
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request = CreateUserRequest(email=dto.email, name=dto.name)
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response = await use_case.execute(request)
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if not response.success:
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raise HTTPException(status_code=400, detail=response.error)
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return {"user": response.user}
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```
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## Hexagonal Architecture Pattern
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```python
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# Core domain (hexagon center)
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class OrderService:
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"""Domain service - no infrastructure dependencies."""
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def __init__(
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self,
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order_repository: OrderRepositoryPort,
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payment_gateway: PaymentGatewayPort,
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notification_service: NotificationPort
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):
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self.orders = order_repository
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self.payments = payment_gateway
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self.notifications = notification_service
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async def place_order(self, order: Order) -> OrderResult:
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# Business logic
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if not order.is_valid():
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return OrderResult(success=False, error="Invalid order")
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# Use ports (interfaces)
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payment = await self.payments.charge(
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amount=order.total,
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customer=order.customer_id
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)
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if not payment.success:
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return OrderResult(success=False, error="Payment failed")
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order.mark_as_paid()
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saved_order = await self.orders.save(order)
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await self.notifications.send(
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to=order.customer_email,
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subject="Order confirmed",
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body=f"Order {order.id} confirmed"
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)
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return OrderResult(success=True, order=saved_order)
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# Ports (interfaces)
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class OrderRepositoryPort(ABC):
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@abstractmethod
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async def save(self, order: Order) -> Order:
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pass
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class PaymentGatewayPort(ABC):
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@abstractmethod
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async def charge(self, amount: Money, customer: str) -> PaymentResult:
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pass
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class NotificationPort(ABC):
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@abstractmethod
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async def send(self, to: str, subject: str, body: str):
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pass
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# Adapters (implementations)
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class StripePaymentAdapter(PaymentGatewayPort):
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"""Primary adapter: connects to Stripe API."""
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def __init__(self, api_key: str):
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self.stripe = stripe
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self.stripe.api_key = api_key
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async def charge(self, amount: Money, customer: str) -> PaymentResult:
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try:
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charge = self.stripe.Charge.create(
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amount=amount.cents,
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currency=amount.currency,
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customer=customer
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)
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return PaymentResult(success=True, transaction_id=charge.id)
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except stripe.error.CardError as e:
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return PaymentResult(success=False, error=str(e))
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class MockPaymentAdapter(PaymentGatewayPort):
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"""Test adapter: no external dependencies."""
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async def charge(self, amount: Money, customer: str) -> PaymentResult:
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return PaymentResult(success=True, transaction_id="mock-123")
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```
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## Domain-Driven Design Pattern
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```python
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# Value Objects (immutable)
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from dataclasses import dataclass
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from typing import Optional
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@dataclass(frozen=True)
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class Email:
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"""Value object: validated email."""
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value: str
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def __post_init__(self):
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if "@" not in self.value:
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raise ValueError("Invalid email")
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@dataclass(frozen=True)
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class Money:
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"""Value object: amount with currency."""
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amount: int # cents
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currency: str
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def add(self, other: "Money") -> "Money":
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if self.currency != other.currency:
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raise ValueError("Currency mismatch")
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return Money(self.amount + other.amount, self.currency)
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# Entities (with identity)
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class Order:
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"""Entity: has identity, mutable state."""
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def __init__(self, id: str, customer: Customer):
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self.id = id
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self.customer = customer
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self.items: List[OrderItem] = []
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self.status = OrderStatus.PENDING
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self._events: List[DomainEvent] = []
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def add_item(self, product: Product, quantity: int):
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"""Business logic in entity."""
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item = OrderItem(product, quantity)
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self.items.append(item)
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self._events.append(ItemAddedEvent(self.id, item))
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def total(self) -> Money:
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"""Calculated property."""
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return sum(item.subtotal() for item in self.items)
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def submit(self):
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"""State transition with business rules."""
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if not self.items:
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raise ValueError("Cannot submit empty order")
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if self.status != OrderStatus.PENDING:
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raise ValueError("Order already submitted")
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self.status = OrderStatus.SUBMITTED
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self._events.append(OrderSubmittedEvent(self.id))
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# Aggregates (consistency boundary)
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class Customer:
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"""Aggregate root: controls access to entities."""
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def __init__(self, id: str, email: Email):
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self.id = id
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self.email = email
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self._addresses: List[Address] = []
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self._orders: List[str] = [] # Order IDs, not full objects
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def add_address(self, address: Address):
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"""Aggregate enforces invariants."""
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if len(self._addresses) >= 5:
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raise ValueError("Maximum 5 addresses allowed")
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self._addresses.append(address)
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@property
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def primary_address(self) -> Optional[Address]:
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return next((a for a in self._addresses if a.is_primary), None)
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# Domain Events
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@dataclass
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class OrderSubmittedEvent:
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order_id: str
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occurred_at: datetime = field(default_factory=datetime.now)
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# Repository (aggregate persistence)
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class OrderRepository:
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"""Repository: persist/retrieve aggregates."""
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async def find_by_id(self, order_id: str) -> Optional[Order]:
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"""Reconstitute aggregate from storage."""
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pass
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async def save(self, order: Order):
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"""Persist aggregate and publish events."""
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await self._persist(order)
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await self._publish_events(order._events)
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order._events.clear()
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```
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