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+---
+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.
+---
+
+# 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
+
+1. **Dependency Rule**: Dependencies always point inward
+2. **Interface Segregation**: Small, focused interfaces
+3. **Business Logic in Domain**: Keep frameworks out of core
+4. **Test Independence**: Core testable without infrastructure
+5. **Bounded Contexts**: Clear domain boundaries
+6. **Ubiquitous Language**: Consistent terminology
+7. **Thin Controllers**: Delegate to use cases
+8. **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