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FastAPI

Blocking the event loop

Running synchronous, long-running code inside an async def route, stalling every concurrent request.

Why interviewers ask about it

The single most common FastAPI performance bug. Use plain def (FastAPI runs it in a threadpool) or an async driver - async def around blocking code is the worst option.

Related terms

asyncio event loop

The single-threaded scheduler running coroutines, resuming each when the thing it awaited is ready.

ASGI

The async successor to WSGI, supporting long-lived connections and async callables.

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