Backend Architecture Enforcer

Enforce FastAPI Clean Architecture with BLOCKING validation.

When to Use

• Building FastAPI endpoints
• Creating services or repositories
• Reviewing backend architecture
• Refactoring legacy code

Architecture Overview

+-------------------------------------------------------------------+
|                        ROUTERS LAYER                               |
|  HTTP concerns only: request parsing, response formatting          |
|  Files: router_*.py, routes_*.py, api_*.py                        |
+-------------------------------------------------------------------+
|                        SERVICES LAYER                              |
|  Business logic: orchestration, validation, transformations        |
|  Files: *_service.py                                              |
+-------------------------------------------------------------------+
|                      REPOSITORIES LAYER                            |
|  Data access: database queries, external API calls                 |
|  Files: *_repository.py, *_repo.py                                |
+-------------------------------------------------------------------+
|                        MODELS LAYER                                |
|  Data structures: SQLAlchemy models, Pydantic schemas             |
|  Files: *_model.py (ORM), *_schema.py (Pydantic)                 |
+-------------------------------------------------------------------+

Validation Rules (BLOCKING)

File Naming Conventions

Quick Reference

Layer Separation Summary

Routers (HTTP Only)

• Request parsing and response formatting
• HTTP status codes and auth checks
• Delegate to services via Depends()

Services (Business Logic)

• Validation and orchestration
• Data transformations
• Raise domain exceptions (NOT HTTPException)

Repositories (Data Access)

• Database queries and persistence
• External API calls
• Return domain objects or None

Dependency Injection Quick Reference

# deps.py - Dependency providers
def get_user_repository(
    db: AsyncSession = Depends(get_db),
) -> UserRepository:
    return UserRepository(db)

def get_user_service(
    repo: UserRepository = Depends(get_user_repository),
) -> UserService:
    return UserService(repo)

# router_users.py - Usage
@router.get("/{user_id}")
async def get_user(
    user_id: int,
    service: UserService = Depends(get_user_service),
):
    return await service.get_user(user_id)

Blocked DI Patterns

# BLOCKED - Direct instantiation
service = UserService()

# BLOCKED - Global instance
user_service = UserService()

# BLOCKED - Missing Depends()
async def get_users(db: AsyncSession):  # Missing Depends()

Common Violations

Exception Pattern

# Domain exceptions (services/repositories)
class UserNotFoundError(DomainException):
    def __init__(self, user_id: int):
        super().__init__(f"User {user_id} not found")

# Router converts to HTTP
@router.get("/{user_id}")
async def get_user(user_id: int, service: UserService = Depends(get_user_service)):
    try:
        return await service.get_user(user_id)
    except UserNotFoundError:
        raise HTTPException(404, "User not found")

Async Rules

# GOOD - Async all the way
result = await db.execute(select(User))

# BLOCKED - Sync in async function
result = db.execute(select(User))  # Missing await

# For sync code, use executor
await loop.run_in_executor(None, sync_function)

References

For detailed patterns and examples, see:

Related Skills

• clean-architecture - DDD patterns
• fastapi-advanced - Advanced FastAPI patterns
• dependency-injection - DI patterns
• project-structure-enforcer - Folder structure

Capability Details

layer-separation

Keywords: router, service, repository, layer, clean architecture, separation Solves:

• Prevent database operations in routers
• Block business logic in route handlers
• Ensure proper layer boundaries

dependency-injection

Keywords: depends, dependency injection, DI, fastapi depends, inject Solves:

• Enforce use of FastAPI Depends() pattern
• Block direct instantiation in routers
• Ensure testable code structure

file-naming

Keywords: naming convention, file name, router_, _service, _repository Solves:

• Enforce consistent file naming patterns
• Validate router/service/repository naming
• Maintain codebase consistency

async-patterns

Keywords: async, await, sync, blocking call, asyncio Solves:

• Detect sync calls in async functions
• Prevent blocking operations in async code
• Ensure async consistency