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Simple alerting system that lets you define checks and alerts in any language that are then magically run on Heroku.


~> 2.6


~> 1.2
~> 4.1
~> 2.33
 Project Readme

Skywatch (alpha)

A simple alerting system that lets you define checks and alerts in any language that are then magically run on Heroku. Nagios can go cry in a corner.

NoOps! Polyglot! Free monitoring of anything!


$ gem install skywatch

Usage / Quickstart

It's a fairly powerful tool. Run skywatch --help to see a list of subcommands. Here is the quickest way to something interesting:

$ mkdir demo && cd demo             # make a directory for your scripts
$ skywatch init                     # this will fail and require Heroku auth
$ skywatch init                     # run again, after logged in
$ skywatch enable check example     # enable the example check script
$ skywatch deploy                   # everything is shipped to Heroku
$ skywatch monitor                  # watch it run in the cloud


  • Lets you monitor or assert anything at any frequency
  • Scriptable alerts (email, sms, tweet, etc)
  • Runs in the cloud on Heroku for free
  • Completely automated deployment
  • Easily monitor activity logs in real-time
  • Enable / disable checks or alerts
  • Unphased by flapping. You get alerted once.
  • Entire system is in self-contained CLI tool
  • Simple enough for personal use, powerful enough for commercial use
  • Can be used for building adaptive systems?

What the hell is this amazing thing??

tl;dr, skywatch is a tool to run repeating check scripts on Heroku. It's the simplest idea wrapped into a convenient utility.

Skywatch is a command-line utility that manages checking and alerting scripts used by a small (50 lines) watcher service. Skywatch deploys these scripts and the service on Heroku where they can run and monitor anything from the cloud for free.

The watcher service runs check scripts that can assert anything at any frequency. If a check script returns a non-zero exit status, it will fire any enabled alert scripts, passing it the output of the check script. Alert scripts can then act on this assertion failure, such as send email, SMS, or webhook.

The check script will continue to run and potentially fail, but the alert script only runs once if it ran without error. Only until a reset signal is sent will it be ready to fire the alert again for any failed check script. In this way, alerts work like clip indicators in the audio recording world. They turn on once any clipping happens and remain on until you manually reset them.

You manage your scripts locally with the skywatch command, or by hand since they're just files in directories. When you want to deploy script changes, toggle enabled scripts, or reset the alert state, you can run a skywatch command and it will handle pushing changes to Heroku for you.

Using skywatch

The skywatch command manages a directory containing check scripts and alert scripts. You can make a new directory and let skywatch set this up for you:

$ mkdir skywatch-demo
$ cd skywatch-demo
$ skywatch init

It will have you authenticate with your Heroku credentials if you haven't already. Grab a free account if you don't have one. When you run skywatch init authenticated it will create some example alerts and checks, then deploy an empty watcher to Heroku. None of the checks or alerts are enabled by default. See the scripts it set up by just running skywatch from the directory:

$ skywatch
  Checks for fathomless-crag-3169
    example                  every 30s        disabled
    skywatch_watchers        every 3600s      disabled
  Alerts for fathomless-crag-3169
    email            disabled

Take a look at all the files in the directory. Checks and alerts are nothing more than scripts. Checks have a naming convention of <interval>.<name>, and enabling and disabling is just setting the execute bit on the scripts. There's nothing the skywatch command does that you can't easily do by hand. It just happens to be terribly convenient.

$ skywatch edit alert email

This will open your editor and you can see the example email alert script is using SendGrid. In fact, when you ran skywatch init, you were set up with a free SendGrid starter addon for 600 emails a day. So let's try it by putting your email address in the TO variable of the script. Now enable the alert:

$ skywatch enable alert email

Let's create a new check script in bash that fails so we can get the alert.

$ skywatch create check failure_test 30

The last argument is the interval. Intervals are always in seconds. All this did was create a new file under the checks directory with a little bit of boiler plate. Let's replace its contents with this:

#!/usr/bin/env bash
echo "Oh no, a failed check."
exit 255

Enable the check and then deploy:

$ skywatch enable check failure_test
$ skywatch deploy

It's going to move some files around and then deploy to Heroku. It keeps a staging directory called .skywatch, which is a Git repo used to push to Heroku. It automatically adds this to a .gitignore file, so you can version your scripts with Git and not worry about this implementation detail.

Once it's finished, you might want to run monitor to see how it went and what's going on. This is just tailing the Heroku logs of the watcher service:

$ skywatch monitor

You can run this whenever to see what it's doing. You'll probably see that it triggered the alert. Go check your email! That will be the only email you get, regardless of whether the check starts to work again and then fail again. No flapping. You have to manually reset:

$ skywatch reset

This should cause another alert email within 30 seconds. And of course, you can tear everything down with destroy:

$ skywatch destroy

This destroys the Heroku app and the .skywatch directory. It doesn't touch your scripts at all. In fact, you can run skywatch init again if you'd like.

The source code to all this is terribly simple. The watcher service is only about 50 lines of Ruby. Everything else is just file operations. In fact, the little state it maintains is kept in file metadata. For how automated it is, skywatch has to be one of the simplest monitoring services ever.

Writing Check Scripts

Check scripts are any executable script using the shebang to define the interpreter. Heroku has most common languages built-in to its Cedar stack, so feel free to use Python, Perl, Ruby, whatever. I like bash.

The only conventions of check scripts are the interval-in-the-filename and that a non-zero exit status will fire the alerts. Any output of the check script will be piped into STDIN of the alert script, so try be verbose but not too verbose.

If you're using bash, it's a good idea to use set -e so any failed subcommand will bubble up. Here's an example check script:

#!/usr/bin/env bash
set -e
curl --trace-ascii --silent --fail

Writing Alert Scripts

Like check scripts, alert scripts can be written in any language. Also, like check scripts, the exit status is important. If an alert script exit status is non-zero, it will run again with the next failure of the check script.

The alert script is given the output of the check script via STDIN. It's also given 2 arguments. The first is the name of the check script. The second is the exit status of the failed check script. Here's an example alert script:

#!/usr/bin/env bash
SUBJECT="[skywatch] $1"
BODY=`echo -e "Failure with status $2:\n\n$(cat)"`
set -e
curl \
  -X 'POST' \
  -F "api_user=$SENDGRID_USERNAME" \
  -F "api_key=$SENDGRID_PASSWORD" \
  -F "to=$TO" \
  -F "subject=$SUBJECT" \
  -F "text=$BODY" \
  -F "from=$TO" \
  --silent --fail ""

The output of an alert script is ignored. It might be a good idea to log the output of failed alert scripts. You'd then be able to see it via skywatch monitor. Sounds like a contribution idea.


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request