mobycron

mobycron

A simple cron deamon for docker written in go. It use the robfig cron library v3 engine and all cron jobs can be confgurated by two ways. The first mode will perform actions on others Docker containers based on cron schedule. The second mode is by a JSON file acting like a contab file.

The docker image include the official backup tool restic. This may be usefull for schedule prune job and cleanup backup snaphots directly on the restic server hosting REST repositories for optimal performance.

Versions

• latest available as pfillion/mobycron:latest at Docker Hub

Go packages

You can use the mobycron library directly by importing the package github.com/pfillion/mobycron/pkg/cron directly in your project.

Tools included in the docker image

• bash
• ca-certificates
• curl
• tzdata

Environnement variables

Cron job support any environnement variables specified by docker and replace it by the real value before executing the command.

MOBYCRON_DOCKER_MODE definde how mobycron will interact with Docker. The value possible is none (default), container or swarm. When mobycron is up and running in container or swarm mode, it watches Docker socket and try to find any containers with label mobycron.schedule and add them to the crontab based on the schedule. Go to docker mode section for more detail with this mode.

MOBYCRON_PARSE_SECOND is false by default. When activate, schedule accept an optional seconds field at the beginning of the cron spec. This is non-standard and has led to a lot of confusion. The new default parser conforms to the standard as described by the Cron wikipedia page.

MOBYCRON_CONFIG_FILE is file path to schedule all job like a crontab file. Go to configuration file section for more detail with this mode.

TZ configure local time zone of the container.

Arguments for the executing container

You can use argument instead of environnment variables. All variables as an equivalant command line option.

• --docker-mode, -d
• --parse-second, -s
• --config-file value, -f value

> docker run -v /var/run/docker.sock:/var/run/docker.sock pfillion/mobycron:latest --docker-mode=true --parse-second=false

Docker mode

Once mobycron is up and running in this mode, it watches Docker socket events for create, start and destroy events. If a container is found to have the label mobycron.schedule then it will be added to the crontab based on the schedule.

Cron scheduling rules and format is describe as follow: CRON Expression Format

CRON_TZ is now the recommended way to specify the timezone of a single schedule, which is sanctioned by the specification. The legacy TZ prefix will continue to be supported since it is unambiguous and easy to do so.

The container mode is the classic Docker mode. Labels can be applied are:

• mobycron.action is requied and indicate wich action must be performed on the container. Possible choices are start, restart, stop or exec.
• mobycron.command specifie the commande line to execute and is requied when the action is exec.
• mobycron.timeout override the default 10 second timeout to do the action.

The second mode is swarm mode. Docker need to be in a swarm node. Label can be applied is:

• mobycron.action is required and indicate which action must be performed on the container. Possible choices are only update due to the mechanic of services in Docker Swarm.

Examples

# Start mobycron in container mode
> docker run -d -e MOBYCRON_DOCKER_MODE=container -v /var/run/docker.sock:/var/run/docker.sock pfillion/mobycron:latest

# Start the job container and print date every minute
> docker run -d --label=mobycron.schedule="0/1 * * * *" --label=mobycron.action="start" busybox date

Configuration file

You can mount directly a file or use docker configuration to schedule all job like a crontab file. See the examples in the source code or below.

• /etc/mobycron/config.json

[
    {
        "schedule": "* * * * *",
        "command": "bash",
        "args": [
            "-c",
            "echo Hello $NAME"
        ]
    },
    {
        "schedule": "0/2 * * * *",
        "command": "curl",
        "args": [
            "-s",
            "-S",
            "-X",
            "GET",
            "https://example.com"
        ]
    }
]

This file will schedule three cron job.

• The first one will replace $NAME by the environnement variable configured in the container and print Hello + $NAME every minutes.
• The second will execute a curl command every 2 minutes. It may be usefull when you need to call any simple webcron or webhook URL like with EasyCron

Docker Secrets

As an alternative to passing sensitive information via environment variables, __FILE may be appended to any environment variables, causing the job to load the values for those variables from files present in the container. In particular, this can be used to load passwords from Docker secrets stored in /run/secrets/<secret_name> files.

Docker compose

See the docker swarm examples in the source code or below.

version: '3.7'
services:
  cron:
    image: pfillion/mobycron:latest
    environment:
      MOBYCRON_DOCKER_MODE: 'swarm'
      MOBYCRON_PARSE_SECOND: 'true'
      TZ: America/New_York
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
    deploy:
      restart_policy:
        condition: on-failure
        delay: 5s
        max_attempts: 3
        window: 30s
      replicas: 1

  busybox:
    image: busybox:latest
    command: echo 'Hello World!!'
    deploy:
      restart_policy:
        condition: none
      replicas: 6
      labels:
        mobycron.schedule: "*/30 * * * * *"
        mobycron.action: "update"

Authors

• pfillion

License

MIT