Docker, Rails and a cron tasks: how to set up a Docker container to run cron Rails rake tasks

Question

How can I set up properly a Docker instance to run cronjob tasks that execute Rails rake tasks ?

I have been trying several things and I have all these problems:

• The cron.d file is not loaded
• There is not log if a cron task fails
• The environment in the cron task session is empty

Answer 1

Because I have been dealing with all these problems for a while and I haven't find any solution in internet that solved all my problems I am pasting my solution here:

~/MyApp/crontab:

* * * * * root /bin/bash -l -c 'cd $RAILS_ROOT && bundle exec rake my_task' >> /var/log/app_cron.log 2>&1

~/MyApp/Dockerfile:

FROM ruby:2.5.1

# Install dependencies
RUN apt-get update && apt-get -y install cron

# Set an environment variable where the Rails app is installed to inside of Docker image:
ENV RAILS_ROOT /var/www/app
RUN mkdir -p $RAILS_ROOT

# Set our working directory inside the image
WORKDIR $RAILS_ROOT

# Setting env up
ENV RAILS_ENV="production"
ENV RACK_ENV="production"

# Adding gems
COPY Gemfile Gemfile
COPY Gemfile.lock Gemfile.lock
RUN bundle install --jobs 20 --retry 5 --without development test

# Adding project files
COPY . .

## Cron config

# Add crontab file to the cron.d directory
# Files in /etc/cron.d can not have names with "-" or ".". It can be problematic
COPY crontab /etc/cron.d/app

# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/app

# To load the env variables in cron sessions
# without this the user in the cron session won't be able to find commands and Gems
RUN printenv | grep -v "no_proxy" >> /etc/environment

# Run the command on container startup
CMD ["cron", "-f"]

Answer 2

For at simpler solution, you could use Arask . It runs timed tasks within Rails itself. No need to use crontab. It runs when rails is running. Just install gem and have your lines in config/initializers/arask.rb, for instance:

arask.create task: 'update:cache', cron: '*/5 * * * *' # Every 5 minutes