Cron jobs show but are not executed in dockerized Django app - django

Question

I have "django_crontab" in my installed apps.
I have a cron job configured
CRONJOBS = [
('* * * * *', 'django.core.management.call_command',['dbbackup']),
]
my YAML looks like this:
web:
build: .
command:
- /bin/bash
- -c
- |
python manage.py migrate
python manage.py crontab add
python manage.py runserver 0.0.0.0:8000
build + up then I open the CLI:
$ python manage.py crontab show
Currently active jobs in crontab:
efa8dfc6d4b0cf6963932a5dc3726b23 -> ('* * * * *', 'django.core.management.call_command', ['dbbackup'])
Then I try that:
$ python manage.py crontab run efa8dfc6d4b0cf6963932a5dc3726b23
Backing Up Database: postgres
Writing file to default-858b61d9ccb6-2021-07-05-084119.psql
All good, but the cronjob never gets executed. I don't see new database dumps every minute as expected.

Answer 1

django-crontab doesn't run scheduled jobs itself; it's just a wrapper around the system cron daemon (you need to configure it with the location of crontab(1), for example). Since a Docker container only runs one process, you need to have a second container to run the cron daemon.
A setup I might recommend here is to write some other script that does all of the required startup-time setup, then runs some command that can be passed as additional arguments:
#!/bin/sh
# entrypoint.sh: runs as the main container process
# Gets passed the container's command as arguments
# Run database migrations. (Should be safe, if inefficient, to run
# multiple times concurrently.)
python manage.py migrate
# Set up scheduled jobs, if this is the cron container.
python manage.py crontab add
# Run whatever command we got passed.
exec "$#"
Then in your Dockerfile, make this script be the ENTRYPOINT. Make sure to supply a default CMD too, probably what would run the main server. With both provided, Docker will pass the CMD as arguments to the ENTRYPOINT.
# You probably already have a line like
# COPY . .
# which includes entrypoint.sh; it must be marked executable too
ENTRYPOINT ["./entrypoint.sh"] # must be JSON-array form
CMD python manage.py runserver 0.0.0.0:8000
Now in the docker-compose.yml file, you can provide both containers, from the same image, but only override the command: for the cron container. The entrypoint script will run for both but launch a different command at its last line.
version: '3.8'
services:
web:
build: .
ports:
- '8000:8000'
# use the Dockerfile CMD, don't need a command: override
cron:
build: .
command: crond -n # for Vixie cron; BusyBox uses "crond -f"
# no ports: