karlvr/scheduler repository overview

⁠Docker Scheduler

A cron-based scheduler service to run on a Docker Swarm to run tasks as Docker Swarm services.

Based on https://github.com/rayyanqcri/swarm-scheduler⁠

⁠Overview

The scheduler container runs cron. You provide crontab files to run commands in the scheduler container, or to start other services on your swarm to perform your tasks.

e.g.

30 8 * * * root <command>
41 9 * * * root run-service example

⁠Installation

You can declare the scheduler in a docker-compose.yml file. You must mount the docker.sock into the container, to enable the container to start services on the swarm (if you want to do that), and the container must run on a swarm manager in order to do that.

In this example we mount a host directory containing your crontab files to /etc/cron.d:

  scheduler:
    image: karlvr/scheduler
    volumes:
    - /var/run/docker.sock:/var/run/docker.sock
    - /srv/cron.d:/etc/cron.d
    deploy:
      placement:
        constraints:
        - node.role == manager

Or you can create a derived image and install your own files in /etc/cron.d during the build process.

⁠crontab

If you place your crontab files in /etc/cron.d then they must be in the format above with the username. You could also mount a file to /etc/crontab, which is in the same format.

⁠Swarm services

Large cron tasks are best run as swarm services, so they run on your swarm.

First create your services with the following deployment settings:

  example:
    ...
    deploy:
      replicas: 0
      restart_policy:
        condition: none

This is so they are declared, but not initially running, and so when they finish running they don't get restarted.

You could also set the restart_policy condition to on-failure so the job repeats if it fails.

Second, create a crontab file:

30 8 * * * root run-service example

The run-service script is part of the container, it simply uses the Docker cli to start the service on your swarm.