[FIXED] Use FastAPI to interact with async loop

Issue

I am running coroutines of 'workers' whose job it is to wait 5s, get values from an asyncio.Queue() and print them out continually.

q = asyncio.Queue()

def worker():
    while True:
        await asyncio.sleep(5)
        i = await q.get()
        print(i)
        q.task_done()

async def main(q):
    workers = [asyncio.create_task(worker()) for n in range(10)]
    await asyncio.gather(*workers)


if __name__ == "__main__":
    asyncio.run(main())

I would like to be able to interact with the queue through http requests using FastAPI. For example POST requests that would 'put' items in the queue for the workers to print.

I'm unsure how I can run the coroutines of the workers concurrently with FastAPI to achieve this effect. Uvicorn has its own event loop I believe and my attempts to use asyncio methods have been unsuccessful.

The router would look something like this I think.

@app.post("/")
async def put_queue(data:str):
    return q.put(data)

And I'm hoping there's something that would have an effect like this:

await asyncio.gather(main(),{FastApi() app run})

Solution

One option would be to add a task that wraps your main coroutine in a on startup event

import asyncio
@app.on_event("startup")
async def startup_event():
    asyncio.create_task(main())

This would schedule your main coroutine before the app has been fully started.

Important here is that you don't await the created task as it would basically block startup_event forever

Answered By - Simon Hawe