architecture-patterns

Architecture Patterns

Master proven backend architecture patterns including Clean Architecture, Hexagonal Architecture, and Domain-Driven Design to build maintainable, testable, and scalable systems.

Given: a service boundary or module to architect. Produces: layered structure with clear dependency rules, interface definitions, and test boundaries.

When to Use This Skill

• Designing new backend services or microservices from scratch
• Refactoring monolithic applications where business logic is entangled with ORM models or HTTP concerns
• Establishing bounded contexts before splitting a system into services
• Debugging dependency cycles where infrastructure code bleeds into the domain layer
• Creating testable codebases where use-case tests do not require a running database
• Implementing domain-driven design tactical patterns (aggregates, value objects, domain events)

Core Concepts

1. Clean Architecture (Uncle Bob)

Layers (dependency flows inward):

• Entities: Core business models, no framework imports
• Use Cases: Application business rules, orchestrate entities
• Interface Adapters: Controllers, presenters, gateways — translate between use cases and external formats
• Frameworks & Drivers: UI, database, external services — all at the outermost ring

Key Principles:

• Dependencies point inward only; inner layers know nothing about outer layers
• Business logic is independent of frameworks, databases, and delivery mechanisms
• Every layer boundary is crossed via an abstract interface
• Testable without UI, database, or external services

1. Hexagonal Architecture (Ports and Adapters)

Components:

• Domain Core: Business logic lives here, framework-free
• Ports: Abstract interfaces that define how the core interacts with the outside world (driving and driven)
• Adapters: Concrete implementations of ports (PostgreSQL adapter, Stripe adapter, REST adapter)

Benefits:

• Swap implementations without touching the core (e.g., replace PostgreSQL with DynamoDB)
• Use in-memory adapters in tests — no Docker required
• Technology decisions deferred to the edges

1. Domain-Driven Design (DDD)

Strategic Patterns:

• Bounded Contexts: Isolate a coherent model for one subdomain; avoid sharing a single model across the whole system
• Context Mapping: Define how contexts relate (Anti-Corruption Layer, Shared Kernel, Open Host Service)
• Ubiquitous Language: Every term in code matches the term used by domain experts

Tactical Patterns:

• Entities: Objects with stable identity that change over time
• Value Objects: Immutable objects identified by their attributes (Email, Money, Address)
• Aggregates: Consistency boundaries; only the root is accessible from outside
• Repositories: Persist and reconstitute aggregates; abstract over the storage mechanism
• Domain Events: Capture things that happened inside the domain; used for cross-aggregate coordination

Clean Architecture — Directory Structure

app/ ├── domain/ # Entities, value objects, interfaces │ ├── entities/ │ │ ├── user.py │ │ └── order.py │ ├── value_objects/ │ │ ├── email.py │ │ └── money.py │ └── interfaces/ # Abstract ports (no implementations) │ ├── user_repository.py │ └── payment_gateway.py ├── use_cases/ # Application business rules │ ├── create_user.py │ ├── process_order.py │ └── send_notification.py ├── adapters/ # Concrete 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 wiring, config, DI container ├── database.py ├── config.py └── logging.py

Dependency rule in one sentence: every import statement in domain/ and use_cases/ must point only toward domain/; nothing in those layers may import from adapters/ or infrastructure/.

Clean Architecture — Core Implementation

domain/entities/user.py

from dataclasses import dataclass from datetime import datetime

@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):
    self.is_active = False

def can_place_order(self) -> bool:
    return self.is_active

domain/interfaces/user_repository.py

from abc import ABC, abstractmethod from typing import Optional from domain.entities.user import User

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

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

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

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

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

use_cases/create_user.py

from dataclasses import dataclass from datetime import datetime from typing import Optional import uuid from domain.entities.user import User from domain.interfaces.user_repository import IUserRepository

@dataclass class CreateUserRequest: email: str name: str

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

class CreateUserUseCase: """Use case: orchestrates business logic, no HTTP or DB details."""

def __init__(self, user_repository: IUserRepository):
    self.user_repository = user_repository

async def execute(self, request: CreateUserRequest) -> CreateUserResponse:
    existing = await self.user_repository.find_by_email(request.email)
    if existing:
        return CreateUserResponse(user=None, success=False, error="Email already exists")

    user = User(
        id=str(uuid.uuid4()),
        email=request.email,
        name=request.name,
        created_at=datetime.now(),
    )
    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 of the user port."""

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:
    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 pydantic import BaseModel from use_cases.create_user import CreateUserUseCase, CreateUserRequest

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 only — no business logic lives here.""" response = await use_case.execute(CreateUserRequest(email=dto.email, name=dto.name)) if not response.success: raise HTTPException(status_code=400, detail=response.error) return {"user": response.user}

Hexagonal Architecture — Ports and Adapters

Core domain service — no infrastructure dependencies class OrderService: 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:
    if not order.is_valid():
        return OrderResult(success=False, error="Invalid order")

    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 (driving and driven interfaces)

class OrderRepositoryPort(ABC): @abstractmethod async def save(self, order: Order) -> Order: ...

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

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

Production adapter: Stripe

class StripePaymentAdapter(PaymentGatewayPort): def init(self, api_key: str): import stripe stripe.api_key = api_key self._stripe = stripe

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 self._stripe.error.CardError as e:
        return PaymentResult(success=False, error=str(e))

Test adapter: no external dependencies

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

DDD — Value Objects and Aggregates

Value Objects: immutable, validated at construction

from dataclasses import dataclass

@dataclass(frozen=True) class Email: value: str

def __post_init__(self):
    if "@" not in self.value or "." not in self.value.split("@")[-1]:
        raise ValueError(f"Invalid email: {self.value}")

@dataclass(frozen=True) class Money: amount: int # cents currency: str

def __post_init__(self):
    if self.amount < 0:
        raise ValueError("Money amount cannot be negative")
    if self.currency not in {"USD", "EUR", "GBP"}:
        raise ValueError(f"Unsupported currency: {self.currency}")

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

Aggregate root: enforces all invariants for its cluster of entities

class Order: def init(self, id: str, customer_id: str): self.id = id self.customer_id = customer_id self.items: list[OrderItem] = [] self.status = OrderStatus.PENDING self._events: list[DomainEvent] = []

def add_item(self, product: Product, quantity: int):
    if self.status != OrderStatus.PENDING:
        raise ValueError("Cannot modify a submitted order")
    item = OrderItem(product=product, quantity=quantity)
    self.items.append(item)
    self._events.append(ItemAddedEvent(order_id=self.id, item=item))

@property
def total(self) -> Money:
    totals = [item.subtotal() for item in self.items]
    return sum(totals[1:], totals[0]) if totals else Money(0, "USD")

def submit(self):
    if not self.items:
        raise ValueError("Cannot submit an empty order")
    if self.status != OrderStatus.PENDING:
        raise ValueError("Order already submitted")
    self.status = OrderStatus.SUBMITTED
    self._events.append(OrderSubmittedEvent(order_id=self.id))

def pop_events(self) -> list[DomainEvent]:
    events, self._events = self._events, []
    return events

Repository: persist and reconstitute aggregates

class OrderRepository(ABC): @abstractmethod async def find_by_id(self, order_id: str) -> Optional[Order]: ...

@abstractmethod
async def save(self, order: Order) -> None: ...
# Implementations persist events via pop_events() after writing state

Testing — In-Memory Adapters

tests/unit/test_create_user.py

import asyncio from typing import Dict, Optional from domain.entities.user import User from domain.interfaces.user_repository import IUserRepository from use_cases.create_user import CreateUserUseCase, CreateUserRequest

class InMemoryUserRepository(IUserRepository): def init(self): self._store: Dict[str, User] = {}

async def find_by_id(self, user_id: str) -> Optional[User]:
    return self._store.get(user_id)

async def find_by_email(self, email: str) -> Optional[User]:
    return next((u for u in self._store.values() if u.email == email), None)

async def save(self, user: User) -> User:
    self._store[user.id] = user
    return user

async def delete(self, user_id: str) -> bool:
    return self._store.pop(user_id, None) is not None

async def test_create_user_succeeds(): repo = InMemoryUserRepository() use_case = CreateUserUseCase(user_repository=repo)

response = await use_case.execute(CreateUserRequest(email="[email protected]", name="Alice"))

assert response.success
assert response.user.email == "[email protected]"
assert response.user.id is not None

async def test_duplicate_email_rejected(): repo = InMemoryUserRepository() use_case = CreateUserUseCase(user_repository=repo)

await use_case.execute(CreateUserRequest(email="[email protected]", name="Alice"))
response = await use_case.execute(CreateUserRequest(email="[email protected]", name="Alice2"))

assert not response.success
assert "already exists" in response.error

Troubleshooting

Use case tests require a running database

Business logic has leaked into the infrastructure layer. Move all database calls behind an IRepository interface and inject an in-memory implementation in tests (see Testing section above). The use case constructor must accept the abstract port, not the concrete class.

Circular imports between layers

A common symptom is ImportError: cannot import name X between use_cases and adapters. This happens when a use case imports a concrete adapter class instead of the abstract port. Enforce the rule: use_cases/ imports only from domain/ (entities and interfaces). It must never import from adapters/ or infrastructure/.

Framework decorators appearing in domain entities

If SQLAlchemy Column() or Pydantic Field() annotations appear on domain entities, the entity is no longer pure. Create a separate ORM model in adapters/repositories/ and map to/from the domain entity in the repository's _to_entity() method.

All logic ending up in controllers

When the controller grows beyond HTTP parsing and response formatting, extract the logic into a use case class. A controller method should do three things only: parse the request, call a use case, map the response.

Value objects raising errors too late

Validate invariants in __post_init__ (Python) or the constructor so an invalid Email or Money cannot be constructed at all. This surfaces bad data at the boundary, not deep inside business logic.

Context bleed across bounded contexts

If the Order context is importing User entities from the Identity context, introduce an Anti-Corruption Layer. The Order context should hold its own lightweight CustomerId value object and only call the Identity context through an explicit interface.

Advanced Patterns

For detailed DDD bounded context mapping, full multi-service project trees, Anti-Corruption Layer implementations, and Onion Architecture comparisons, see:

• references/advanced-patterns.md

Related Skills

• microservices-patterns — Apply these architecture patterns when decomposing a monolith into services
• cqrs-implementation — Use Clean Architecture as the structural foundation for CQRS command/query separation
• saga-orchestration — Sagas require well-defined aggregate boundaries, which DDD tactical patterns provide
• event-store-design — Domain events produced by aggregates feed directly into an event store