Example of running a docker cron service

This is a simple proof of concept and demo where a docker container can be used as a cron service to start other container tasks.

There are 2 docker images in this example:

• demo-service: a simple container that echos a message to the console and sleeps for 1 minute
• cron-service: a container which when started inserts a cron job via crontab to start the demo-service image every minute. The CONTENT environment variable is passed to the demo-service container and is echoed to the console, this is also logged to a log file in the cron-service container at /app/cron.log.

To see this in action, build the 2 images.

docker build -t demo-service -f DemoService.Dockerfile .
docker build -t cron-service -f CronService.Dockerfile .

Then run the cron-service container with the following command:

docker run -v /var/run/docker.sock:/var/run/docker.sock -e CONTENT="hello-3" -d cron-service

By binding the docker socket to the container, we do the docker in docker thing. This allows the cron-service container to start other containers on the host. Note also that we are passing an environment variable to the cron-service container to control the message that is echoed by the demo-service container.

After a while, when you run docker ps, you should see the container specified from the cron job running, look for the container with the image name demo-service.

If you do a docker log for the running demo-service container, you should see the message specified in the CONTENT environment variable, in this case hello-3.

Complete Commands

docker build -t demo-service -f DemoService.Dockerfile .
docker build -t cron-service -f CronService.Dockerfile .

docker run -v /var/run/docker.sock:/var/run/docker.sock -e CONTENT="hello-3" -d cron-service