hickey/periodic repository overview

⁠Periodic Container

The Periodic container is based on the Alpine Linux distribution and is used to execute any number of scripts at a regular interval. Anything that can be specified in a cron time specification can be managed by the Periodic container.

Whether the container is executed as a standalone container or as an orchestrated set of containers, Periodic is able to manage an arbitrary number of cron jobs.

⁠Adding cron jobs and scripts

All of the scripts and crontab definitions are placed in directories under /data. Crontab definitions need to be placed in /data/crontabs and scripts can reside anywhwere within the /data hierarchy where /data/scripts is strongly suggested.

A quick note about providing scripts to be invoked under crond. Most container technologies do not provide a direct mechanism to mount the scripts into /data/scripts as executable files (i.e. they have read access only). So either the end user must have a mechanism to change the permissions on the scripts or the scripts must be called from crond with the interpreter for the script on the command line in the crontab file (i.e. /bin/sh /data/scripts/run_every_hour.sh)

As a result of using Alpine Linux, the crond process only supports the crontab convention for specifying files and not the cron.d convention. Any crontab files placed in /data/crontabs need to be named after a user on the system. All of the normal users are in the password file and are ready to be used. The most likely prospects would be:

• root
• bin
• daemon
• adm
• mail
• nobody

⁠Twelve Factor Controls

As discussed above all crontabs and scripts are mounted in the /data directory for most of the configuration of the container.

In addition the following environmental variables are used to configure aspects of the container:

• CRON_LOG_LEVEL: Set the verbosity of the logging output from the crond process. Default is set to 6 and the lower the number the more verbose the output.

⁠Additional Commands

Because semi⁠ is used as an entrypoint script, there are a few additional commands available through semi. Currently these are:

* help: Provide an overview of the additional commands supported
        through `semi`.
* README/readme: Print out this README file to the screen.