Cron for containers
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README.md

Supercronic

Supercronic has an announcement blog post over here!

Supercronic is a crontab-compatible job runner, designed specifically to run in containers.

Why Supercronic?

Crontabs are the lingua franca of job scheduling, but typical server cron implementations are ill-suited for container environments:

  • They purge their environment before starting jobs. This is an important security feature in multi-user systems, but it breaks a fundamental configuration mechanism for containers.
  • They capture the output from the jobs they run, and often either want to email this output or simply discard it. In a containerized environment, logging task output and errors to stdout / stderr is often easier to work with.
  • They often don't respond gracefully to SIGINT / SIGTERM, and may leave running jobs orphaned when signaled. Again, this makes sense in a server environment where init will handle the orphan jobs and Cron isn't restarted often anyway, but it's inappropriate in a container environment as it'll result in jobs being forcefully terminated (i.e. SIGKILL'ed) when the container exits.
  • They often try to send their logs to syslog. This conveniently provides centralized logging when a syslog server is running, but with containers, simply logging to stdout or stderr is preferred.

Finally, they are often quiet, making these issues difficult to understand and debug!

Supercronic's goal is to behave exactly how you would expect cron running in a container to behave:

  • Your environment variables are available in jobs
  • Job output is logged to stdout / stderr
  • SIGTERM triggers a graceful shutdown (and so does SIGINT, which you can deliver via CTRL+C when used interactively)
  • Job return codes and schedules are logged to stdout / stderr

How does it work?

  • Install Supercronic (see below)
  • Point it at a crontab: supercronic CRONTAB
  • You're done!

Who is it for?

We (Aptible) originally created Supercronic to make it easy for customers of our Enclave container orchestration platform to incorporate periodic jobs in their apps, but it's more broadly applicable to anyone running cron jobs in containers.

Installation

Download

The easiest way to install Supercronic is to download a pre-built binary.

Navigate to the releases page, and grab the build that suits your system. The releases include example Dockerfile stanzas to install Supercronic that you can easily include in your own Dockerfile or adjust as needed.

Note: If you are unsure which binary is right for you, try supercronic-linux-amd64.

Build

You can also build Supercronic from source.

Supercronic uses dep for dependency management, so you'll need to install dep first.

Then, fetch Supercronic, install its dependencies, then install it:

go get github.com/aptible/supercronic
cd "${GOPATH}/src/github.com/aptible/supercronic"
dep ensure
go install

Crontab format

Broadly speaking, Supercronic tries to process crontabs just like Vixie cron does. In most cases, it should be compatible with your existing crontab.

There are, however, a few exceptions:

  • First, Supercronic supports second-resolution schedules: Under the hood, Supercronic uses the cronexpr package, so refer to its documentation to know exactly what you can do.
  • Second, Supercronic does not support changing users when running tasks. Setting USER in your crontab will have no effect. Changing users is usually best accomplished in container environments via other means, e.g., by adding a USER directive to your Dockerfile.

Here's an example crontab:

# Run every minute
*/1 * * * * echo "hello"

# Run every 2 seconds
*/2 * * * * * * ls 2>/dev/null

# Run once every hour
@hourly echo "$SOME_HOURLY_JOB"

Environment variables

Just like regular cron, Supercronic lets you specify environment variables in your crontab using a KEY=VALUE syntax.

However, this is only here for compatibility with existing crontabs, and using this feature is generally not recommended when using Supercronic.

Indeed, Supercronic does not wipe your environment before running jobs, so if you need environment variables to be available when your jobs run, just set them before starting Supercronic itself, and your jobs will inherit them.

For example, if you're using Docker, jobs started by Supercronic will inherit the environment variables defined using ENV directives in your Dockerfile, and variables passed when you run the container (e.g. via docker run -e SOME_VARIABLE=SOME_VALUE).

Unless you've used cron before, this is exactly how you expect environment variables to work!

Timezone

Supercronic uses your current timezone from /etc/localtime to schedule jobs. You can also override the timezone by setting the environment variable TZ when running Supercronic.

If you're unsure what timezone Supercronic is using, you can run it with the -debug flag to confirm.

Logging

Supercronic provides rich logging, and will let you know exactly what command triggered a given message. Here's an example:

$ cat ./my-crontab
*/5 * * * * * * echo "hello from Supercronic"

$ ./supercronic ./my-crontab
INFO[2017-07-10T19:40:44+02:00] read crontab: ./my-crontab
INFO[2017-07-10T19:40:50+02:00] starting                                      iteration=0 job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"
INFO[2017-07-10T19:40:50+02:00] hello from Supercronic                        channel=stdout iteration=0 job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"
INFO[2017-07-10T19:40:50+02:00] job succeeded                                 iteration=0 job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"
INFO[2017-07-10T19:40:55+02:00] starting                                      iteration=1 job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"
INFO[2017-07-10T19:40:55+02:00] hello from Supercronic                        channel=stdout iteration=1 job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"
INFO[2017-07-10T19:40:55+02:00] job succeeded                                 iteration=1 job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"

Debugging

If your jobs aren't running, or you'd simply like to double-check your crontab syntax, pass the -debug flag for more verbose logging:

$ ./supercronic -debug ./my-crontab
INFO[2017-07-10T19:43:51+02:00] read crontab: ./my-crontab
DEBU[2017-07-10T19:43:51+02:00] try parse(7): */5 * * * * * * echo "hello from Supercronic"[0:15] = */5 * * * * * *
DEBU[2017-07-10T19:43:51+02:00] job will run next at 2017-07-10 19:44:00 +0200 CEST  job.command="echo "hello from Supercronic"" job.position=0 job.schedule="*/5 * * * * * *"

Duplicate Jobs

Supercronic will wait for a given job to finish before that job is scheduled again (some cron implementations do this, others don't). If a job is falling behind schedule (i.e. it's taking too long to finish), Supercronic will warn you.

Here is an example:

$ cat ./my-crontab
# Sleep for 2 seconds every second. This will take too long.
* * * * * * * sleep 2

$ ./supercronic ./my-crontab
INFO[2017-07-11T12:24:25+02:00] read crontab: ./my-crontab
INFO[2017-07-11T12:24:27+02:00] starting                                      iteration=0 job.command="sleep 2" job.position=0 job.schedule="* * * * * * *"
INFO[2017-07-11T12:24:29+02:00] job succeeded                                 iteration=0 job.command="sleep 2" job.position=0 job.schedule="* * * * * * *"
WARN[2017-07-11T12:24:29+02:00] job took too long to run: it should have started 1.009438854s ago  job.command="sleep 2" job.position=0 job.schedule="* * * * * * *"
INFO[2017-07-11T12:24:30+02:00] starting                                      iteration=1 job.command="sleep 2" job.position=0 job.schedule="* * * * * * *"
INFO[2017-07-11T12:24:32+02:00] job succeeded                                 iteration=1 job.command="sleep 2" job.position=0 job.schedule="* * * * * * *"
WARN[2017-07-11T12:24:32+02:00] job took too long to run: it should have started 1.014474099s ago  job.command="sleep 2" job.position=0 job.schedule="* * * * * * *"

Questions and Support

Please feel free to open an issue in this repository if you have any question about Supercronic!

Note that if you're trying to use Supercronic on Aptible Enclave, we have a dedicated support article.

Contributing

PRs are always welcome! Before undertaking a major change, consider opening an issue for some discussion.

License

See LICENSE.md.

Copyright

Copyright (c) 2017 Aptible. All rights reserved.

@krallin