Run Django background tasks

A background task is work that runs outside the request, such as sending email or resizing images. On Sevalla, Celery runs the tasks in a background worker process, built from the same image as the web process, and a Redis database holds the queue between them. Django 6.0 added a tasks framework (django.tasks) for defining and enqueuing tasks, but Django doesn’t run them: its built-in backends are for development and testing only. Celery provides both the queue and the worker.

Before you begin

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

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 next to settings.py:

storefront/celery.py

import os

from celery import Celery

os.environ.setdefault("DJANGO_SETTINGS_MODULE", "storefront.settings")

app = Celery("storefront")
app.config_from_object("django.conf:settings", namespace="CELERY")
app.autodiscover_tasks()

Load it when Django starts:

storefront/__init__.py

from .celery import app as celery_app

__all__ = ("celery_app",)

Point Celery at Redis in storefront/settings.py. Settings that start with CELERY_ configure Celery:

storefront/settings.py

# A Valkey database's URL starts with valkey://, which Celery doesn't accept.
CELERY_BROKER_URL = env("REDIS_URL", default="redis://localhost:6379/0").replace("valkey://", "redis://", 1)
CELERY_BROKER_CONNECTION_RETRY_ON_STARTUP = True

Add a task in one of your Django apps, for example an orders app listed in INSTALLED_APPS. Celery finds tasks.py in every installed app:

orders/tasks.py

from celery import shared_task


@shared_task
def send_order_confirmation(order_id):
    print(f"Sending the confirmation for order {order_id}")

Queue it from a view or anywhere else with send_order_confirmation.delay(order.id). 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 --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: Sevalla starts a new instance at each scheduled time, runs the command, and stops it. Put the work in a management command, then create the process:

Create the cron job

On the application’s Processes tab, click Create process. Choose Cron job, enter Clear sessions as the Name, python manage.py clearsessions as the Custom start command, and 0 3 * * * as the Schedule (every day at 03:00 UTC). Click Create process.

Deploy

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

Schedules run in UTC unless you turn on Set custom time zone. To keep the schedule in Celery instead, run Celery beat in a second background worker with the start command celery -A storefront beat --loglevel INFO, and keep that process at 1 instance: each beat instance sends every scheduled task.

Verify

Click the Web terminal tab, choose the Web process, and queue a task:

. /opt/venv/bin/activate
python manage.py shell -c "from orders.tasks import send_order_confirmation; send_order_confirmation.delay(42)"

Click the Logs tab and look at the worker’s runtime logs. They show Task orders.tasks.send_order_confirmation[...] received, the line Sending the confirmation for order 42, and Task orders.tasks.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 Django background tasks

Before you begin

Create a Redis database

Add Celery to the project

Run the worker on Sevalla

Run tasks on a schedule

Verify

​Before you begin

​Create a Redis database

​Add Celery to the project

​Run the worker on Sevalla

​Run tasks on a schedule

​Verify

Before you begin

Create a Redis database

Add Celery to the project

Run the worker on Sevalla

Run tasks on a schedule

Verify