How to Run Background Tasks in FastAPI?

Background Task Execution in FastAPI

In FastAPI, developers may encounter a scenario where they need to run a function in the background, unrelated to their API. To achieve this, it's necessary to employ threading mechanisms.

Solution 1: Starting Thread before uvicorn.run

One approach involves starting the thread before invoking uvicorn.run, as uvicorn.run blocks the thread. This approach can be implemented as follows:

import time
import threading
from fastapi import FastAPI
import uvicorn

app = FastAPI()
class BackgroundTasks(threading.Thread):
    def run(self,*args,**kwargs):
        while True:
            print('Hello')
            time.sleep(5)
  
if __name__ == '__main__':
    t = BackgroundTasks()
    t.start()
    uvicorn.run(app, host="0.0.0.0", port=8000)

Alternatively, developers can leverage FastAPI's startup event to initiate the thread before the application's commencement:

@app.on_event("startup")
async def startup_event():
    t = BackgroundTasks()
    t.start()

Solution 2: Event Scheduler

Another option is to implement a recurring Event scheduler for the background task:

import sched, time
from threading import Thread
from fastapi import FastAPI
import uvicorn

app = FastAPI()
s = sched.scheduler(time.time, time.sleep)

def print_event(sc): 
    print("Hello")
    sc.enter(5, 1, print_event, (sc,))

def start_scheduler():
    s.enter(5, 1, print_event, (s,))
    s.run()

@app.on_event("startup")
async def startup_event():
    thread = Thread(target=start_scheduler)
    thread.start()

if __name__ == '__main__':
    uvicorn.run(app, host="0.0.0.0", port=8000)

Solution 3: Using the Event Loop

For asynchronous background tasks (async def functions), developers can utilize the current event loop with the asyncio.create_task() function:

from asyncio import create_task
from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def startup_task():
    await create_task(print_task(5))

In this approach, the event loop is created when initiating the uvicorn server.

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