How to Run Background Tasks in a FastAPI Application?

How to Run a Thread in the Background for FastAPI Python

Challenge: Incorporating an Independent Background Function into FastAPI

When constructing a Python server using FastAPI, it can be necessary to incorporate a function that is unrelated to the API logic but needs to run in the background on a recurring basis. For instance, this could involve checking external APIs and printing information based on the responses.

Option 1: Using a Thread

One approach is to create a thread that executes the desired function. Here's how this can be achieved:

import threading

def start_worker():
    print('[main]: starting worker...')
    # Define and execute the worker function
    worker = my_worker.Worker()
    worker.working_loop()

if __name__ == '__main__':
    print('[main]: starting...')
    # Start the worker thread
    _worker_thread = Thread(target=start_worker, daemon=False)
    _worker_thread.start()

Option 2: Event Scheduling

An alternative method involves using an event scheduler for the background task:

import sched, time
from threading import Thread

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()

Option 3: Async Tasks with Event Loop

If the task is an async def function, it can be added to the current event loop using the asyncio.create_task() function:

from fastapi import FastAPI
import asyncio

async def print_task(s):
    while True:
        print('Hello')
        await asyncio.sleep(s)

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Add the async task to the event loop
    asyncio.create_task(print_task(5))
    yield
    print('Shutting down...')

app = FastAPI(lifespan=lifespan)

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