Many developers are switching to `async def` in FastAPI to handle multiple requests concurrently, but there's a crucial pitfall to be aware of. If you include a blocking call within an `async` route, it can block the event loop and halt your server's response capabilities. For instance:
In this example:
1. The first route uses an `async` definition, but the `time.sleep(0.5)` call is synchronous, effectively blocking the entire event loop for 500 milliseconds. Therefore, other requests cannot be processed during this time.
2. The second example correctly runs a blocking call in a synchronous function, allowing FastAPI to execute it in a thread pool, preventing the event loop from being blocked.
To avoid this issue, follow these guidelines:
- Default to using `def` for synchronous routes so that they don't block the event loop. FastAPI will handle these in a thread pool.
- Reserve `async def` for cases where the entire operation is non-blocking, such as using `httpx.AsyncClient` or `asyncpg`.
- If you find yourself mixing `async def` routes with synchronous code, use `await run_in_threadpool(...)` or `asyncio.to_thread(...)` to handle blocking sections effectively.
It's important to note that while synchronous routes utilize a thread pool (limited to a default of 40 threads in Starlette), switching to sync doesn't always guarantee zero blocking. Under high load, you can deplete the thread pool. For most applications, however, it's safer to use sync by default and be cautious with async implementations. What strategies do you personally use to keep your async routes from blocking the event loop?
5 Answers
You might want to check out `asyncio.sleep()`, which is designed to be non-blocking. If you need to perform a sleep in an async context, `await asyncio.sleep(1)` will work much better than a synchronous sleep. This way, the event loop can still operate while waiting. Also, consider using some custom async functions to handle longer tasks without blocking. There's also `run_in_executor`, which assigns a heavy workload to a separate thread, thereby keeping the loop running smoothly!
You should definitely be looking at the FastAPI documentation on async operations. It's a great resource that outlines best practices for defining async functions and avoiding blocking issues. Here’s the link: https://fastapi.tiangolo.com/async/#path-operation-functions
Blocking calls in async routes are a big no-no! The first route you mentioned should really raise some red flags for any seasoned dev. It’s basic knowledge at this point. If you’re going async, just remember—don’t include any blocking calls!
I hit the thread limit in Starlette (40 by default) once and it was a nightmare to debug. I think `asyncio.to_thread` is much cleaner and avoids a lot of headaches compared to `run_in_executor`. It’s a lifesaver!
A great trick is to set `PYTHONASYNCIODEBUG=1` in your testing environments. This gives you warnings if any coroutine takes more than 100ms to execute, which can highlight problematic blocking calls that might not show up in linter checks. If you know a task will take some time, definitely consider using `asyncio.to_thread()` to handle that without blocking the event loop.
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