Schedule Script Execution in a Node.js Docker Image with Cron

#docker #javascript #node #cron

Cron is a popular tool to schedule tasks, it comes pre-installed on almost any Linux image such as Debian or Ubuntu. Cron can execute any command at a predefined time like 4:30 AM or every 2 minutes. For this reason, it is the first choice of developers for scheduled script execution.

A docker image is nothing else than a stripped-down Linux operating system and comes with cron. Most answers on StackOverflow suggest installing extra packages or building a custom image, but neither approach is necessary. The vanilla Node.js Docker image has everything needed to schedule the execution of javascript files.

Example with Code

I want to run a javascript file every 30 minutes inside my docker image. I want to use docker-compose¹ to start and stop the docker container.

Here's my docker compose file:

version: "3.1"

services:
  price-daemon:
    image: node:14-alpine
    working_dir: /home/node/app
    volumes:
      - ./:/home/node/app
    command: "/usr/sbin/crond -f -l 0 -c /home/node/app/crontab -L /var/log/cron.log"

I use the lightweight node:14-alpine² image and mount my current working directory as volume.

The command starts the cron daemon and it will look for the file /home/node/app/crontab/root. Remove -l 0 if you don't like a super verbose log. In my working directory I created the file crontab/root with this contents:

# min hour day month weekday command
* * * * * /bin/date --rfc-2822 >> /var/log/cron.log
*/30 * * * * node /home/node/app/scheduled.js >> /var/log/cron.log 2>&1

The second line is just a timestamp printed into the log file for debugging purposes. The third line executes the scheduled.js file and logs its output into /var/log/cron.log

And that's it. Pretty simple right?