FastAPIasync def chạy trong event loop và phù hợp khi handler await async I/O như async database driver, HTTP client hoặc message broker. def sync handler được chạy trong threadpool để không block event loop.
Sai lầm thường gặp là viết async def nhưng gọi thư viện blocking như requests hoặc sync DB driver bên trong; điều đó vẫn block event loop. Nếu dependency/blocking library chưa async, dùng def route hoặc chuyển sang thư viện async tương ứng.
async def runs in the event loop and fits handlers that await async I/O such as async database drivers, HTTP clients or message brokers. A sync def handler runs in a threadpool so it does not block the event loop.
A common mistake is writing async def while calling blocking libraries such as requests or a sync DB driver inside it; that still blocks the event loop. If the library is not async, use a def route or switch to an async library.