/architecture-patterns

name: architecture-patterns
description: Implement proven backend architecture patterns including Clean Architecture, Hexagonal Architecture, and Domain-Driven Design. Use this skill when designing clean architecture for a new microservice, when refactoring a monolith to use bounded contexts, when implementing hexagonal or onion architecture patterns, or when debugging dependency cycles between application layers.

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

Detailed patterns and worked examples

Detailed pattern documentation lives in `references/details.md`. Read that file when the navigation tier above is insufficient.

Testing — In-Memory Adapters

The hallmark of correctly applied Clean Architecture is that every use case can be exercised in a plain unit test with no real database, no Docker, and no network:

# 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="alice@example.com", name="Alice"))

    assert response.success
    assert response.user.email == "alice@example.com"
    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="alice@example.com", name="Alice"))
    response = await use_case.execute(CreateUserRequest(email="alice@example.com", 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