[FIXED] Get FastAPI to handle requests in parallel

Issue

Here is my trivial fastapi app:

from datetime import datetime
import asyncio

import uvicorn
from fastapi import FastAPI


app = FastAPI()

@app.get("/delayed")
async def get_delayed():
    started = datetime.now()
    print(f"Starting at: {started}")
    await asyncio.sleep(10)
    ended = datetime.now()
    print(f"Ending at: {ended}")
    return {"started": f"{started}", "ended": f"{ended}"}

if __name__ == "__main__":
    uvicorn.run("fastapitest.main:app", host="0.0.0.0", port=8000, reload=True, workers=2)

When I make 2 consecutive calls to it, the code in the function for the second one doesn't start executing until the first request finishes, producing an output like:

Starting at: 2021-09-17 14:52:40.317915
Ending at: 2021-09-17 14:52:50.321557
INFO:     127.0.0.1:58539 - "GET /delayed HTTP/1.1" 200 OK
Starting at: 2021-09-17 14:52:50.328359
Ending at: 2021-09-17 14:53:00.333032
INFO:     127.0.0.1:58539 - "GET /delayed HTTP/1.1" 200 OK

Given that the function is marked async and I am awaiting the sleep, I would expect a different output, like:

Starting at: ...
Starting at: ...
Ending at: ...
INFO:     127.0.0.1:58539 - "GET /delayed HTTP/1.1" 200 OK
Ending at: ...
INFO:     127.0.0.1:58539 - "GET /delayed HTTP/1.1" 200 OK

[for the calls I just opened up 2 browser tabs at localhost:8000/delayed ]

What am I missing?

Solution

It works in parallel as expected - it is just a browser thing: chrome on detecting the same endpoint being requested in different tabs, will wait for the first to be completly resolved to check if the result can be cached.

If instead you place 3 http requests from different processes in the shell, the results are as expected:

content-length: 77
content-type: application/json
date: Fri, 17 Sep 2021 19:51:39 GMT
server: uvicorn

{
    "ended": "2021-09-17 16:51:49.956629",
    "started": "2021-09-17 16:51:39.955487"
}


HTTP/1.1 200 OK
content-length: 77
content-type: application/json
date: Fri, 17 Sep 2021 19:51:39 GMT
server: uvicorn

{
    "ended": "2021-09-17 16:51:49.961173",
    "started": "2021-09-17 16:51:39.960850"
}


HTTP/1.1 200 OK
content-length: 77
content-type: application/json
date: Fri, 17 Sep 2021 19:51:39 GMT
server: uvicorn

{
    "ended": "2021-09-17 16:51:49.964156",
    "started": "2021-09-17 16:51:39.963510"
}

Adding a random, even if unused, query parameter on the URL for each browser tab will all cancel the trying-to-cache behavior.

related question: Chrome stalls when making multiple requests to same resource?

Answered By - jsbueno