concron

Concron - Cron for Container

• ✔ Dashboard included.
• ✔ Prometheus/OpenMetrics exporter included.

Quickstart

via Docker command

Place your crontab file to ./crontab, and run the below command.

$ docker run -v $(pwd)/crontab:/etc/crontab:ro -p 8000:80 ghcr.io/macrat/concron:latest

via Docker Compose

You can make docker-compose.yml like below,

version: "3"

services:
  concron:
    image: ghcr.io/macrat/concron:latest
    environment:
      CRON_TZ: Asia/Tokyo
      CONCRON_CRONTAB: |
        45 */3 * * *  docker run --rm busybox echo "you can do your task here!"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - ./cron-files:/etc/cron.d:ro
    ports:
      - "8000:80"
    restart: always

And then, start via docker-compose up -d command.

More examples are in examples directory.

without container

You can get Concron binary from releases page and execute it.

$ export CONCRON_PATH=/etc/crontab:/etc/cron.d CONCRON_LISTEN=":8000"  # These are default values.

$ crontab

Image variants

• latest, docker: Based on Docker image. To run Docker containers. This image supports less architectures than others.
• busybox: Minimal image based on Busybox (glibc). To execute simple shell scripts or your own binaries.
• slim: Based on Debian slim image. To execute complex scripts or programs that can not execute on busybox image.

All images are generated by the same Dockerfile.

Crontab

Concron searches crontab files from /etc/crontab or under the /etc/cron.d. These path is specified by CONCRON_PATH environment variable. You can also use CONCRON_CRONTAB environment variable instead of file.

In Windows, Concron searches C:\crontab, C:\cron.d, C:\%USERPROFILE%\crontab, and C:\%USERPROFILE%\cron.d.

Crontab file is like below.

SHELL=/bin/sh
CRON_TZ=Asia/Tokyo

#    schedule           command
# |------------| |--------------------|
   */10 * * * *   echo your-command
   @daily         echo another command

This file format is the same as common crontab.

But, there are two differences about the username column;

1. The username column is omitted in default, regardless of the file path. If you want to change execution user, please set yes to ENABLE_USER_COLUMN of environment variables.
2. You can omit the username using * instead of an actual username, even if you set ENABLE_USER_COLUMN.

Tasks that omitted username, will be executed as the same user as the execution user of Concron.

The crontab file that included username column is like below.

# This row is important!
ENABLE_USER_COLUMN = yes

#    schedule     user         command
# |------------| |----| |--------------------|
   */10 * * * *   root   echo your-command
   @daily         *      echo another command

The container has users that have the same name as UID, 1000 to 1010. So you can use specify UID in crontab like 1000 as user name.

Concron uses /bin/sh -c in Unix, %COMSPEC% /C in Windows, to execute command. You can change shell using SHELL and SHELL_OPTS. SHELL is /bin/sh or %COMSPEC% part, and SHELL_OPTS is -c or /C part.

In below example, all tasks will executed in the docker container.

# This is too long and ugly.
0 0 * * *  /usr/bin/docker run --rm busybox echo hello world


# Use docker command as a shell.
SHELL = /usr/bin/docker
SHELL_OPTS = run --rm
PARSE_COMMAND = yes

# This is short and cool!
0 0 * * *  busybox echo hello world

When the PARSE_COMMAND option in the above example is enabled, Concron executes comannd as "/usr/bin/docker" "run" "--rm" "busybox" "echo" "hello" "world" instead of "/usr/bin/docker" "run" "--rm" "busybox echo hello world". This option is useful if you want to use non-shell program as SHELL.

Dashboard

You can see dashboard on http://localhost:8000 in default. The listen address can change using CONCRON_LISTEN environment variable.

Metrics and Logging

The metrics for Prometheus in the OpenMetrics format is on http://localhost:8000/metrics. This listen address can change using CONCRON_LISTEN environment variable as the same as the address of dashboard.

In default, log level set to info. If you want get more information, please set debug to CONCRON_LOGLEVEL. Or, you can set warn or error to suppress log.

The Concron only writes log to the stdout. Please collect them using container engine's log collector or something.

Health check

Concron has 3 endpoints to check the status.

/healthz and /livez always return 200 OK while Concron running.

/readyz returns 200 OK while Concron is running and ready to execute tasks. Otherwise, it returns 503 Service Unavailable. For example, before done to load all tasks, or after receive interrupt signal.