FastAPI BackgroundTasks: When to Use It and When to Switch to Celery

FastAPI's BackgroundTasks class, built on Starlette, lets developers offload lightweight work — such as sending emails or writing logs — so it runs after the HTTP response is sent without blocking the client. Tasks are added via the .add_task() method and execute within the same process and event loop as the request handler. Because they share the application's resources, developers must guard against issues like database session leaks, and blocking I/O must be wrapped using run_in_threadpool or asyncio.to_thread. Exceptions inside background tasks are logged but do not affect the HTTP response, meaning retry logic and failure alerts must be handled manually. For jobs that may run for minutes, require retries, or need to survive process restarts, dedicated task queues such as Celery, RQ, or Dramatiq are recommended instead.