Source: wshobson/agents Original Plugin: backend-development

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 (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 order.is_valid():
            return OrderResult(success=False, error="Invalid order")

        # Use ports (interfaces)
        payment = await self.payments.charge(
            amount=order.total,
            customer=order.customer_id
        )

        if not payment.success:
            return OrderResult(success=False, error="Payment failed")

        order.mark_as_paid()
        saved_order = await self.orders.save(order)

        await self.notifications.send(
            to=order.customer_email,
            subject="Order confirmed",
            body=f"Order {order.id} confirmed"
        )

        return OrderResult(success=True, order=saved_order)

# Ports (interfaces)
class OrderRepositoryPort(ABC):
    @abstractmethod
    async def save(self, order: Order) -> Order:
        pass

class PaymentGatewayPort(ABC):
    @abstractmethod
    async def charge(self, amount: Money, customer: str) -> PaymentResult:
        pass

class NotificationPort(ABC):
    @abstractmethod
    async def send(self, to: str, subject: str, body: str):
        pass

# Adapters (implementations)
class StripePaymentAdapter(PaymentGatewayPort):
    """Primary adapter: connects to Stripe API."""

    def __init__(self, api_key: str):
        self.stripe = stripe
        self.stripe.api_key = api_key

    async def charge(self, amount: Money, customer: str) -> PaymentResult:
        try:
            charge = self.stripe.Charge.create(
                amount=amount.cents,
                currency=amount.currency,
                customer=customer
            )
            return PaymentResult(success=True, transaction_id=charge.id)
        except stripe.error.CardError as e:
            return PaymentResult(success=False, error=str(e))

class MockPaymentAdapter(PaymentGatewayPort):
    """Test adapter: no external dependencies."""

    async def charge(self, amount: Money, customer: str) -> PaymentResult:
        return PaymentResult(success=True, transaction_id="mock-123")

Domain-Driven Design Pattern

PYTHON

# Value Objects (immutable)
from dataclasses import dataclass
from typing import Optional

@dataclass(frozen=True)
class Email:
    """Value object: validated email."""
    value: str

    def __post_init__(self):
        if "@" not in self.value:
            raise ValueError("Invalid email")

@dataclass(frozen=True)
class Money:
    """Value object: amount with currency."""
    amount: int  # cents
    currency: str

    def add(self, other: "Money") -> "Money":
        if self.currency != other.currency:
            raise ValueError("Currency mismatch")
        return Money(self.amount + other.amount, self.currency)

# Entities (with identity)
class Order:
    """Entity: has identity, mutable state."""

    def __init__(self, id: str, customer: Customer):
        self.id = id
        self.customer = customer
        self.items: List[OrderItem] = []
        self.status = OrderStatus.PENDING
        self._events: List[DomainEvent] = []

    def add_item(self, product: Product, quantity: int):
        """Business logic in entity."""
        item = OrderItem(product, quantity)
        self.items.append(item)
        self._events.append(ItemAddedEvent(self.id, item))

    def total(self) -> Money:
        """Calculated property."""
        return sum(item.subtotal() for item in self.items)

    def submit(self):
        """State transition with business rules."""
        if not self.items:
            raise ValueError("Cannot submit empty order")
        if self.status != OrderStatus.PENDING:
            raise ValueError("Order already submitted")

        self.status = OrderStatus.SUBMITTED
        self._events.append(OrderSubmittedEvent(self.id))

# Aggregates (consistency boundary)
class Customer:
    """Aggregate root: controls access to entities."""

    def __init__(self, id: str, email: Email):
        self.id = id
        self.email = email
        self._addresses: List[Address] = []
        self._orders: List[str] = []  # Order IDs, not full objects

    def add_address(self, address: Address):
        """Aggregate enforces invariants."""
        if len(self._addresses) >= 5:
            raise ValueError("Maximum 5 addresses allowed")
        self._addresses.append(address)

    @property
    def primary_address(self) -> Optional[Address]:
        return next((a for a in self._addresses if a.is_primary), None)

# Domain Events
@dataclass
class OrderSubmittedEvent:
    order_id: str
    occurred_at: datetime = field(default_factory=datetime.now)

# Repository (aggregate persistence)
class OrderRepository:
    """Repository: persist/retrieve aggregates."""

    async def find_by_id(self, order_id: str) -> Optional[Order]:
        """Reconstitute aggregate from storage."""
        pass

    async def save(self, order: Order):
        """Persist aggregate and publish events."""
        await self._persist(order)
        await self._publish_events(order._events)
        order._events.clear()

Resources

references/clean-architecture-guide.md: Detailed layer breakdown
references/hexagonal-architecture-guide.md: Ports and adapters patterns
references/ddd-tactical-patterns.md: Entities, value objects, aggregates
assets/clean-architecture-template/: Complete project structure
assets/ddd-examples/: Domain modeling examples

Best Practices

Dependency Rule: Dependencies always point inward
Interface Segregation: Small, focused interfaces
Business Logic in Domain: Keep frameworks out of core
Test Independence: Core testable without infrastructure
Bounded Contexts: Clear domain boundaries
Ubiquitous Language: Consistent terminology
Thin Controllers: Delegate to use cases
Rich Domain Models: Behavior with data

Common Pitfalls

Anemic Domain: Entities with only data, no behavior
Framework Coupling: Business logic depends on frameworks
Fat Controllers: Business logic in controllers
Repository Leakage: Exposing ORM objects
Missing Abstractions: Concrete dependencies in core
Over-Engineering: Clean architecture for simple CRUD