FastAPI ndhukung loro-lorone async def lan def regular kanggo path operations. Pilihan kasebut penting kanggo performance: gunakake async def nalika sampeyan bisa non-blocking I/O, lan regular nalika kode sampeyan ngayak blocking (synchronous) operations.
awaitdef@app.get("/async")
async def async_endpoint():
data = await fetch_from_api() # await non-blocking I/O — efficient
return data
@app.get("/sync")
def sync_endpoint():
data = blocking_db_call() # ordinary blocking code
return data
async def → runs on the main event loop. Efficient ONLY if you await non-blocking calls.
⚠️ A BLOCKING call inside async def blocks the WHOLE event loop → kills concurrency!
def → FastAPI runs it in a THREAD POOL, so blocking code doesn't block the event loop.
Safe for synchronous/blocking libraries.
# ✅ async def — when you can await async libraries (httpx, async DB drivers)
async def get_user():
async with httpx.AsyncClient() as c:
return await c.get(url)
# ✅ plain def — when using SYNCHRONOUS/blocking libraries (requests, sync ORM)
def get_user():
return requests.get(url).json() # blocking → FastAPI runs it in a thread
# ❌ THE DANGEROUS MISTAKE — blocking call inside async def
async def bad():
return requests.get(url).json() # blocks the event loop! Use `def` or an async client
Aturan sing kritis: aja kaiyen ngokhake blocking calls ing async def — iku bakal blocking event loop lan numpes concurrency. Mbokmenawa gunakake async library karo await, utawa gunakake plain def (sing FastAPI ana njalanake kanthi aman ing thread pool).
Millih kanthi bener antara async def lan def langsung ngaruhi performance lan concurrency API sampeyan, lan yen salah iku minangka kesalahan sing umum lan serius.
Wongan sing paling penting yaiku async def mung ana manfaate nalika sampeyan await non-blocking operations — ngayak blocking call (kaya synchronous requests library utawa blocking database driver) ing dalam async def bakal blocking seluruh event loop, ndadeg semua concurrent requests lan numpes tujuane async.
Sebaliknya, FastAPI kanthi cerdas njalanake plain def endpoints ing thread pool, dadi synchronous/blocking code ana aman ing kono.
Ngerti aturan iki — gunakake async def karo truly async libraries, gunakake plain def kanggo blocking code, lan aja nggawe blocking calls ing async def — kabutuhan kanggo nulis performant FastAPI endpoints lan ngilangake kesalahan sing halus nanging menggeyeng saka accidentally serializing kabeh requests balik blocking event loop.