cronc

A cron scheduler for containers.

cronc is an alternative to classic cron schedulers (cron, cronie, anacron, ...) suited for container environments.

The key motivation for its creation is that classical cron schedulers are not designed to pass their own environments on to their jobs. On a system with many scheduled jobs, this makes sense - each job should be responsible its own environment environment. However, in a container, one might only be scheduling a small handful of jobs relating to a single application. In such a case it would be convenient of the container's environment were available to the jobs.

A common way to achieve this is to have a custom entrypoint which saves environment variables to a file somewhere in the container. Jobs can then source those variables when they run. Another option is to save them to /etc/environment so they are automatically available to jobs.

Both of those options feel wrong to me, and so, here's cronc.

In summary, cronc:

• Passes its own environment through to jobs.
• Redirects the stdout/stderr of jobs its own stdout/stderr.
• Reads cron tabs from the following sources:
  • The file or directory given via the --cronPath option (default: /dev/crontab)
    • If the path is a directory, all files in that directory will be read
  • The environment variables given via the --cronVar option (default: CRONTAB)
• Does not support setting a per-job user - all jobs are run as the container user.
• Does not support setting environment variables directly in the crontab
• Runs in the foreground by default and responds appropriately to signals.