FastAPI not behaving asynchronously

Question

I'm probably not understanding the asynchronous concept correctly in FastAPI.

I'm accessing the root endpoint of the following app from two clients at the same time. I'd expect FastAPI to print Started twice in a row at the start of the execution:

from fastapi import FastAPI
import asyncio

app = FastAPI()

@app.get("/")
async def read_root():
    print('Started')
    await asyncio.sleep(5)
    print('Finished')
    return {"Hello": "World"}

Instead I get the following, which looks very much non asynchronous:

Started
Finished
INFO: ('127.0.0.1', 49655) - "GET / HTTP/1.1" 200
Started
Finished
INFO: ('127.0.0.1', 49655) - "GET / HTTP/1.1" 200

What am I missing?

Answer 1

How do you ensure there are multiple simultaneous requests?

Your code is fine - try using following command for testing:

for n in {1..5}; do curl http://localhost:8000/ & ; done

Your browser might be caching subsequent requests to the same URL.

Answer 2

Well from what the demos in this github issue show, it's probably not due to FastAPI but the client that runs the requests.