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

2. 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

3. 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(