Run FastAPI background tasks

FastAPI has two ways to run work outside the request. BackgroundTasks runs a function in the web process right after it sends the response: fine for a few seconds of work, but the task is lost if the instance restarts, and it shares the web process’s CPU. For anything longer, Celery runs the tasks in a separate background worker process on Sevalla, with a Redis database holding the queue.

Before you begin

A FastAPI project deployed on Sevalla, as in Deploy FastAPI.

Run a short task after the response

Add a BackgroundTasks parameter to a route and queue the function on it. It needs no setup on Sevalla:

storefront/main.py

from fastapi import BackgroundTasks


def write_audit_log(message: str) -> None:
    print(message)


@app.post("/orders/{order_id}/cancel")
def cancel_order(order_id: int, background_tasks: BackgroundTasks):
    background_tasks.add_task(write_audit_log, f"Order {order_id} cancelled")
    return {"status": "cancelled"}

Create a Redis database

Create the database

In the sidebar, click Databases, then click Create database. Under Database, choose Redis, or Valkey: the code in this guide works with both. Enter storefront-redis as the Display name, choose the same Data center as the application and a Size, then click Create database.

Connect it to the application

In the sidebar, click Applications, then click the application. Click the Networking tab. Under Connected services, click Add internal connection. Choose storefront-redis as the Service, turn on Add environment variables to the application, and click Add internal connection. Sevalla adds REDIS_HOST, REDIS_PORT, REDIS_PASSWORD and REDIS_URL. See Connect an application.

Add Celery to the project

Install Celery with its Redis support, and update requirements.txt:

pip install "celery[redis]"
pip freeze > requirements.txt

Create the Celery application and a task:

storefront/worker.py

import os

from celery import Celery

# A Valkey database's URL starts with valkey://, which Celery doesn't accept.
broker_url = os.environ.get("REDIS_URL", "redis://localhost:6379/0").replace("valkey://", "redis://", 1)

celery = Celery("storefront", broker=broker_url)
celery.conf.broker_connection_retry_on_startup = True


@celery.task
def send_order_confirmation(order_id: int) -> None:
    print(f"Sending the confirmation for order {order_id}")

Queue the task from a route with delay:

storefront/main.py

from storefront.worker import send_order_confirmation


@app.post("/orders/{order_id}/confirm")
def confirm_order(order_id: int):
    send_order_confirmation.delay(order_id)
    return {"status": "queued"}

Commit and push the changes.

Run the worker on Sevalla

Create the background worker

On the application’s Processes tab, click Create process. Choose Background worker, enter Celery worker as the Name and this Custom start command:

celery -A storefront.worker worker --loglevel INFO --concurrency 2

Choose a Resource and click Create process.

Deploy

Click Deploy in the undeployed changes bar at the bottom of the page.

Set --concurrency to match the size. Without it, Celery starts one worker process per CPU of the machine it runs on, which can be more than the process’s memory holds. Two processes fit in H1 (0.3 GB of memory). To process more tasks at once, add instances in the process’s Scaling section, or choose a larger size and raise --concurrency. See Scaling.

The Create process sheet with Background worker selected, the Name and Custom start command fields, the resource and the scaling settings

Run tasks on a schedule

Run scheduled work as a cron job: create a Cron job process with a Schedule in cron syntax, such as 0 3 * * * for every day at 03:00 UTC, and a Custom start command that runs a script, such as python -m storefront.cleanup. See Cron jobs.

Verify

Queue a task, with your application’s domain:
curl -X POST https://storefront-x7k2m.sevalla.app/orders/42/confirm
The response is {"status":"queued"}.
Click the Logs tab and look at the worker’s runtime logs. They show Task storefront.worker.send_order_confirmation[...] received, the line Sending the confirmation for order 42, and Task storefront.worker.send_order_confirmation[...] succeeded.

On the Processes tab, the Celery worker process shows Running. If it shows Crashed, the logs show why, for example Error 111 connecting when REDIS_URL is missing or wrong.

Migrate

JavaScript

Python

Run FastAPI background tasks

Before you begin

Run a short task after the response

Create a Redis database

Add Celery to the project

Run the worker on Sevalla

Run tasks on a schedule

Verify

​Before you begin

​Run a short task after the response

​Create a Redis database

​Add Celery to the project

​Run the worker on Sevalla

​Run tasks on a schedule

​Verify

Before you begin

Run a short task after the response

Create a Redis database

Add Celery to the project

Run the worker on Sevalla

Run tasks on a schedule

Verify