Is it right to run crontab service and wsgi within a single docker container

Question

Currently, I have a Django wsgi to handle web traffic. (I have a nginx as front end to forward traffic to wsgi)

Now, I would like to start a crobtab service, to run periodic background job.

This is what I had done

---

django\Dockerfile

FROM python:3.6.4-alpine3.4
...

RUN chmod +x entrypoint.sh

ENTRYPOINT ["sh", "entrypoint.sh"]

CMD /usr/local/bin/gunicorn web.wsgi:application -b django:5000 --log-level=info --error-logfile=/var/log/gunicorn3.err.log

django\entrypoint.sh

#!/bin/sh

...

python manage.py crontab add

# https://stackoverflow.com/questions/37015624/how-to-run-a-cron-job-inside-a-docker-container
#
# "crond -help" yields:
#
# -f      Foreground
# -b      Background (default)
# -S      Log to syslog (default)
# -l N    Set log level. Most verbose:0, default:8
# -d N    Set log level, log to stderr
# -L FILE Log to FILE
# -c DIR  Cron dir. Default:/var/spool/cron/crontabs

# start cron
/usr/sbin/crond -f -l 8

exec "$@"

docker-compose.yml

  django:
    build:
      context: ./django
      dockerfile: Dockerfile
    restart: always  
    depends_on:
      - pgbouncer
    expose:
      - "5000"
    volumes:
      - static_data:/app/static

---

If I use the above setup, I notice that.

1. My schedule cron worker is running periodically.
2. But, wsgi unable to serve web traffic.

First, I try to change in django\entrypoint.sh

# start cron
/usr/sbin/crond -f -l 8

to

# start cron
/usr/sbin/crond -b -l 8

After making above change,

1. My schedule cron worker is no longer running. Not sure why.
2. wsgi can serve web traffic.

---

May I know why is it so? How can I make my django container to serve web traffic, and run cron job at the same time?

Or, is it not the right way to do such thing in Docker? I should use 2 containers?

Answer 1

I'd run a full on second container naming the service in the compose file cron or something of the likes (maybe more specific to the actual job in the event you have multiple crons). 1 process per container is general practice. In the "cron" container I wouldn't even run it via crond I'd have whatever I'm using on the host machine to handle the scheduling of the container. I'd change your job from being a django-cron to a custom django-admin command since you won't be managing the running of it with the django app anymore. You could still build the second container off the django one image just change the CMD with the docker-compose.yml file command: ["django-admin", "mycommand"]. Likely would be unnecessary to expose the ports on the second container. Invoke as a normal service docker-compose run mycronservice