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Cronsul

Cronsul is a very simple distributed periodic job scheduler, built on cron and consul.

Problem Statement

Cronsul aims to solve a problem of:

  • I want to run a task periodically
  • My task may run on any one of a set of boxes
  • I want my task to run, even if some of those boxes are down.
  • I do not want my task to run more than once for a given period.

Cronsul does not currently attempt to solve any of the following problems:

  • My task should be retried if it fails.
  • My task is resource-intensive and needs to be scheduled on a box that has enough resources to spare.
  • I would like to run a task continuously.

Usage

cronsul is intended to be run by your cron daemon. You should schedule the same cronsul command on multiple boxes, at the same time. cronsul will attempt to grab a lock, and will exit silently if it fails to grab this lock; this ensures that only one box will run the command specified for a particular period.

A typical cronjob entry might look like this:

0 * * * *   root    cronsul cleanup_logs /usr/local/bin/cleanup_logs.sh

The cronsul script itself takes one or two arguments, in addition to a command:

  1. A task id. This is used to associate multiple cronsul runs from different boxes.
  2. Optionally, using --period, the length of the period (in seconds) during which you don't want your command to be run twice. This defaults to 60, which should generally be enough, but if your cron job might be started in different minutes on different boxes (e.g. due to different cron jobs or an overloaded box), you may increase this up to the frequency at which your job runs.

The current Unix time is rounded down to the next lowest multiple of period, and this value is used to calculate the key where cronsul's lock lives, so a lock created at 17:54 with a period of 3600 will expire at 18:00.

Dependencies

cronsul depends on:

  • bash 3+
  • curl
  • A running consul cluster.
  • some form of cron daemon.

cronsul-cleanup

Since cronsul leaves things lying around in consul's KV store even after exit, a helper script, cronsul-cleanup, is provided to help you clean up after cronsul.

It will delete any lock value that is more than two periods old.