Crontab in Docker with flask application - mysql

Question

I'm trying to create cronjob within the docker container, But the it doesn't work. Below is my code
Dockerfile
FROM python:3
LABEL image for a very management application
# We copy just the requirements.txt first to leverage Docker cache
COPY ./requirements.txt /app/requirements.txt
WORKDIR /app
RUN apt-get install -y default-libmysqlclient-dev
RUN pip3 install -r requirements.txt
RUN apt-get update && apt-get -y install cron
RUN touch /var/log/cron.log
COPY crontab /etc/cron.d/cjob
RUN chmod 0644 /etc/cron.d/cjob
ENV PYTHONUNBUFFERED 1
CMD cron -f
EXPOSE 5000
COPY . .
CMD ["cron", "-f"]
CMD [ "python3", "app.py" ]
crontab
*/5 * * * * root /test.py >> /logfile
test.py
from datetime import datetime
print("Our test works at", datetime.now())
I'm tring to run cronjob with the
docker-compose up
Is anything wrong i'm doing, Is their any other way for cronjob to work. Please can anyone help me. Thank you all

Answer 1

Since you can mainly run one CMD you will need to use the ENTRYPOINT directive. I can give you an example of how I made it work.
The theory behind the solution is that in the base image during build time you prepare all the necessary documents:
copy in the crontab file and apply it
Copy in the additional launch script
set ENTRYPOINT to the launch script
run CMD and start your app
Example:
COPY token-renewal-cron /etc/cron.d/token-renewal-cron
RUN crontab /etc/cron.d/token-renewal-cron
where crontab logic is inside token-renewal-cron
Then you need to copy the launch script into the /usr/local/bin/ directory
COPY launch.sh /usr/local/bin/launch.sh
RUN chmod 0777 /usr/local/bin/launch.sh #0777 as long as it works for POC :)
Where in our case launch.sh contains:
cron # this starts the cron process
TMP # some commands we need to run next to cron command (we prepare app env here)
# Append CMD from Dockerfile NOTE this is important so you can then use CMD after ENTRYPOINT inside DOCKERFILE
exec "$#"
Then at the end of the DOCKERFILE you just prepare ENTRYPOINT and CMD commands
#Launch our entrypoint script
ENTRYPOINT ["launch.sh"]
#Launch our application
CMD ["dotnet", "APPLICATION.dll"]