High CPU usage and poor performance in FastAPI with python-dependency-injector due to SQLAlchemy engine recreation

[Fred7b]

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Description

Problem

Our FastAPI application with python-dependency-injector is experiencing high CPU usage (100%+) and poor performance under load.

Example Implementation

class Container(containers.DeclarativeContainer):
    config = providers.Configuration()

    db = providers.Singleton(
        Database, 
        config.sqlalchemy_database_url,
        echo=config.postgres_echo,
        application_name=config.postgres_application_name,
        is_bouncer=config.postgres_pg_bouncer
    )
    
    my_repo = providers.Singleton(
        MyProfileRepository,
        async_session=db.provided.async_session,
    )
    
    user_service = providers.Singleton(
        UserService,
        users_repository=my_repo,
    )


@router.post('/hello', tags=['login'])
@inject
async def login(
    user_service: UserService = Depends(Provide[Container.user_service]),
):
    return await user_service.hello()


def app():
    container: Container = Container()
    container.wire(
        modules=[__name__],
        keep_cache=True
        )
    app: FastAPI = FastAPI()

Performance Issues

• Dependency injection overhead: 400ms+ per request
• CPU utilization: 100%+ under load

Profiling Results

Using pyinstruments profiling, we identified the bottlenecks:

• _patched in dependency_injector/wiring.py taking 400ms+

Environment

• FastAPI 0.103+
• python-dependency-injector 4.48.0
• SQLAlchemy 2.0+
• asyncpg
• Python 3.12