Clean Architecture Patterns

Build maintainable, testable backends with SOLID principles and hexagonal architecture.

SOLID Principles (2026 Python)

S - Single Responsibility

python

# BAD: One class doing everything
class UserManager:
    def create_user(self, data): ...
    def send_welcome_email(self, user): ...
    def generate_report(self, users): ...

# GOOD: Separate responsibilities
class UserService:
    def create_user(self, data: UserCreate) -> User: ...

class EmailService:
    def send_welcome(self, user: User) -> None: ...

class ReportService:
    def generate_user_report(self, users: list[User]) -> Report: ...

O - Open/Closed (Protocol-based)

python

from typing import Protocol

class PaymentProcessor(Protocol):
    async def process(self, amount: Decimal) -> PaymentResult: ...

class StripeProcessor:
    async def process(self, amount: Decimal) -> PaymentResult:
        # Stripe implementation
        ...

class PayPalProcessor:
    async def process(self, amount: Decimal) -> PaymentResult:
        # PayPal implementation - extends without modifying
        ...

L - Liskov Substitution

python

# Any implementation of Repository can substitute another
class IUserRepository(Protocol):
    async def get_by_id(self, id: str) -> User | None: ...
    async def save(self, user: User) -> User: ...

class PostgresUserRepository:
    async def get_by_id(self, id: str) -> User | None: ...
    async def save(self, user: User) -> User: ...

class InMemoryUserRepository:  # For testing - fully substitutable
    async def get_by_id(self, id: str) -> User | None: ...
    async def save(self, user: User) -> User: ...

I - Interface Segregation

python

# BAD: Fat interface
class IRepository(Protocol):
    async def get(self, id: str): ...
    async def save(self, entity): ...
    async def delete(self, id: str): ...
    async def search(self, query: str): ...
    async def bulk_insert(self, entities): ...

# GOOD: Segregated interfaces
class IReader(Protocol):
    async def get(self, id: str) -> T | None: ...

class IWriter(Protocol):
    async def save(self, entity: T) -> T: ...

class ISearchable(Protocol):
    async def search(self, query: str) -> list[T]: ...

D - Dependency Inversion

python

from typing import Protocol
from fastapi import Depends

class IAnalysisRepository(Protocol):
    async def get_by_id(self, id: str) -> Analysis | None: ...

class AnalysisService:
    def __init__(self, repo: IAnalysisRepository):
        self._repo = repo  # Depends on abstraction, not concrete

# FastAPI DI
def get_analysis_service(
    db: AsyncSession = Depends(get_db)
) -> AnalysisService:
    repo = PostgresAnalysisRepository(db)
    return AnalysisService(repo)

Hexagonal Architecture (Ports & Adapters)

code

┌─────────────────────────────────────────────────────────────┐
│                      DRIVING ADAPTERS                        │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐  ┌──────────┐    │
│  │ FastAPI  │  │   CLI    │  │  Celery  │  │  Tests   │    │
│  │ Routes   │  │ Commands │  │  Tasks   │  │  Mocks   │    │
│  └────┬─────┘  └────┬─────┘  └────┬─────┘  └────┬─────┘    │
│       │             │             │             │           │
│       ▼             ▼             ▼             ▼           │
│  ╔═══════════════════════════════════════════════════════╗  │
│  ║                    INPUT PORTS                        ║  │
│  ║  ┌─────────────────┐  ┌─────────────────────────────┐ ║  │
│  ║  │ AnalysisService │  │ UserService                 │ ║  │
│  ║  │ (Use Cases)     │  │ (Use Cases)                 │ ║  │
│  ║  └────────┬────────┘  └──────────────┬──────────────┘ ║  │
│  ╠═══════════╪══════════════════════════╪════════════════╣  │
│  ║           ▼          DOMAIN          ▼                ║  │
│  ║  ┌─────────────────────────────────────────────────┐  ║  │
│  ║  │  Entities  │  Value Objects  │  Domain Events   │  ║  │
│  ║  │  Analysis  │  AnalysisType   │  AnalysisCreated │  ║  │
│  ║  └─────────────────────────────────────────────────┘  ║  │
│  ╠═══════════════════════════════════════════════════════╣  │
│  ║                   OUTPUT PORTS                        ║  │
│  ║  ┌──────────────────┐  ┌────────────────────────────┐ ║  │
│  ║  │ IAnalysisRepo    │  │ INotificationService       │ ║  │
│  ║  │ (Protocol)       │  │ (Protocol)                 │ ║  │
│  ║  └────────┬─────────┘  └──────────────┬─────────────┘ ║  │
│  ╚═══════════╪══════════════════════════╪════════════════╝  │
│              ▼                          ▼                   │
│  ┌───────────────────┐  ┌────────────────────────────────┐ │
│  │ PostgresRepo      │  │ EmailNotificationService       │ │
│  │ (SQLAlchemy)      │  │ (SMTP/SendGrid)                │ │
│  └───────────────────┘  └────────────────────────────────┘ │
│                      DRIVEN ADAPTERS                        │
└─────────────────────────────────────────────────────────────┘

DDD Tactical Patterns

Entity (Identity-based)

python

from dataclasses import dataclass, field
from uuid import UUID, uuid4

@dataclass
class Analysis:
    id: UUID = field(default_factory=uuid4)
    source_url: str
    status: AnalysisStatus
    created_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))

    def __eq__(self, other: object) -> bool:
        if not isinstance(other, Analysis):
            return False
        return self.id == other.id  # Identity equality

Value Object (Structural equality)

python

from dataclasses import dataclass

@dataclass(frozen=True)  # Immutable
class AnalysisType:
    category: str
    depth: int

    def __post_init__(self):
        if self.depth < 1 or self.depth > 3:
            raise ValueError("Depth must be 1-3")

Aggregate Root

python

class AnalysisAggregate:
    def __init__(self, analysis: Analysis, artifacts: list[Artifact]):
        self._analysis = analysis
        self._artifacts = artifacts
        self._events: list[DomainEvent] = []

    def complete(self, summary: str) -> None:
        self._analysis.status = AnalysisStatus.COMPLETED
        self._analysis.summary = summary
        self._events.append(AnalysisCompleted(self._analysis.id))

    def collect_events(self) -> list[DomainEvent]:
        events = self._events.copy()
        self._events.clear()
        return events

Directory Structure

code

backend/app/
├── api/v1/              # Driving adapters (FastAPI routes)
├── domains/
│   └── analysis/
│       ├── entities.py      # Domain entities
│       ├── value_objects.py # Value objects
│       ├── services.py      # Domain services (use cases)
│       ├── repositories.py  # Output port protocols
│       └── events.py        # Domain events
├── infrastructure/
│   ├── repositories/    # Driven adapters (PostgreSQL)
│   ├── services/        # External service adapters
│   └── messaging/       # Event publishers
└── core/
    ├── dependencies.py  # FastAPI DI configuration
    └── protocols.py     # Shared protocols

Anti-Patterns (FORBIDDEN)

python

# NEVER import infrastructure in domain
from app.infrastructure.database import engine  # In domain layer

# NEVER leak ORM models to API
@router.get("/users/{id}")
async def get_user(id: str, db: Session) -> UserModel:  # Returns ORM model
    return db.query(UserModel).get(id)

# NEVER have domain depend on framework
from fastapi import HTTPException
class UserService:
    def get(self, id: str):
        if not user:
            raise HTTPException(404)  # Framework in domain!

Key Decisions

Decision	Recommendation
Protocol vs ABC	Use Protocol (structural typing)
Dataclass vs Pydantic	Dataclass for domain, Pydantic for API
Repository granularity	One per aggregate root
Transaction boundary	Service layer, not repository
Event publishing	Collect in aggregate, publish after commit

Related Skills

•repository-patterns - Detailed repository implementations
•api-design-framework - REST API patterns
•database-schema-designer - Schema design

Capability Details

solid-principles

Keywords: SOLID, single responsibility, open closed, liskov, interface segregation, dependency inversion Solves:

•How do I apply SOLID principles in Python?
•My classes are doing too much

hexagonal-architecture

Keywords: hexagonal, ports and adapters, clean architecture, onion Solves:

•How do I structure my FastAPI app?
•How to separate infrastructure from domain?

ddd-tactical

Keywords: entity, value object, aggregate, domain event, DDD Solves:

•What's the difference between entity and value object?
•How to design aggregates?