Asynchronous Dependency Injection in FastAPI: Techniques for Managing Asynchronous Tasks' Dependencies

FastAPI's Dependency Injection (DI) is a core tool for managing resource sharing and reuse, especially in asynchronous scenarios, avoiding code duplication and coupling. At its core, dependencies are declared using `Depends()`, where functions only need to declare the required resources, and resource acquisition is handled externally. At a basic level, synchronous dependencies use regular functions (e.g., `get_sync_db`), while asynchronous dependencies use `async def` (e.g., `get_async_db`), with FastAPI automatically handling `await` calls. For example, the asynchronous route function `read_users` injects an asynchronous database connection via `db=Depends(get_async_db)`. Advanced techniques include nested dependencies (e.g., combining authentication dependencies with database dependencies) and passing dependencies in background tasks. Pitfalls to watch for include forgetting `await`, cyclic dependencies, and type mismatches. Mastering these concepts enables efficient construction of decoupled, scalable asynchronous applications, enhancing development efficiency through rational resource reuse.