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Provide a wrapper around cron jobs to manage failure more gracefully
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README.md

Cromer

Travis CI Status

Cromer is a utility that can be wrapped around any command, and subdues periodic/occasional failures at calling that exact same command. It maintains 'hidden' state between invocations of the command to enable it to do this. It is typically and particularly useful for wrapping around a cron job, so the user isn't continually notified about commands that sometimes fail.

Note that use is subject to the license conditions.

It is invoked like this:

cromer <cromer options> <the original command> <original command options>

e.g.:

cromer -X 1d ping some.remote.host

Typically, it might be used as a wrapper around a command in a cron job. Thus, your crontab might look like this:

*/5 * * * *   cromer -X 1d -t 4m do-some-command-that-fails-from-time-to-time

In the above example, the command do-some-command-that-fails-from-time-to-time will be run every 5 minutes inside cromer by cron. Because the -X parameter is set to one day, this command must successfully run and complete with no stdout/stderr output and a zero returncode at least once a day for cromer not to pass through stdout/stderr output to cron (and thus for cron to email the user or whatever other action it would normally take). There is also a 4-minute timeout on the command, so it will be killed (and considered by cromer to have failed) if it doesn't complete successfully within 4 minutes.

cromer provides a variety of command-line options and features, and more information can be found by invoking cromer --help. However, the main command-line options are:

  • -X - the original motivation for writing cromer. This can be set to any time period in weeks, days, hours, minutes, or seconds (e.g. -X5w7m). Assuming an identical command (with the same command-line parameters) has previously run successfully (probably by cron), cromer will ignore failures from invoking the exact same command again (again, probably from the same cron job) within that time period by swallowing the command's return code and stderr. If there hasn't been a success since at least the length of this period, and cromer is run by cron, failures will be reported as normal. State is kept by using a file of the form ~/.cromer/pingsomeremotehost.xyz123, where the first part of the filename is a 'compressed' readable version of the same (this is only added when the -r option is used), and the second part is an SHA1 hash of the complete command name (including options). This option effectively enables cron jobs to be set on a regular, fairly short timer (e.g. every hour), but occasional failures to be ignored - as long as the job passes at least once every period specified by this option.

  • -t - very similar to the Unix command timeout. However, the timeout is expressed using the same syntax as for -X above.

Pre-requisites

This is supported and tested on Python 3.3+. It is unlikely to work on an earlier version.

Installing

You can build a Debian package by executing make builddeb.

You need various Python modules installed as pre-requisites. If you aren't installing via the Debian package, you can install these the Python way with pip3 install -r requirements.txt.

cromer can optionally use the coloredlogs python module if it's present to make syserr-based logs more attractive. There is support in my sister project python-deb for building the coloredlogs module as a Debian package.

Hacking

Install pre-requisite packages with pip3 install -r requirements-hacking.txt.

Any pull requests/issues welcome.

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