For users of Heroku, the Scheduler add-on is a convenient and cheap way to run scheduled batch jobs. However, you get what you pay for: it’s a little bare bones. For example, it doesn’t support any sort of notifications when a job fails, a major issue unless you compulsively check your logs. For Rails apps with scheduled Rake tasks, the exception_notification-rake gem fixes this problem. This post walks you through configuring an app for email notifications about Rake tasks failing on the Heroku Scheduler.

Note: This guide was produced based on Rails 3.2. See the exception_notification-rake documentation for instructions with Rails 4 and 5.

A Failing Task

First things first. Let’s create a failing Rake task that we’re going to use for testing notification delivery. Since you’re already using Rake tasks with the scheduler, add another task to the .rake file where you keep them (most commonly lib/tasks/scheduler.rake but any .rake file in lib/tasks will do):

task :failing_task => :environment do
  puts "Failing task in environment #{Rails.env}..."
  FAIL!
end

Note that the new task depends on the :environment task. This is required for tasks you want notifications for. The Rails environment needs to be loaded because the configuration we’re going to add later happens during Rails initialization. Verify that this indeed does fail by running it:

$ rake failing_task

Which should produce something like this:

Failing task in environment development...
rake aborted!
undefined method `FAIL!' for main:Object
./lib/tasks/scheduler.rake:33:in `block in <top (required)>'
Tasks: TOP => failing_task

Install the Gem

Add the exception_notification-rake gem to your Gemfile:

gem 'exception_notification-rake', '0.0.4'

At the point of writing this, 0.0.4 is the latest version of the gem. You might want to check RubyGems to see if there is a newer version. Now tell Bundler to update your gems by running:

$ bundle update

Configure & Test Notifications in your Development Environment

Before you actually try this on a remote Heroku server where debugging is difficult, it’s a good idea to test everything end-to-end locally. For testing email delivery locally I have found mailcatcher to be very useful. It runs a simple SMTP server that you can query through a browser. Add mailcatcher to your Gemfile:

group :development do
  gem 'mailcatcher', '0.5.10'
end

We need this for development only. Again, you might want to check mailcatcher on RubyGems for the latest version.

Note: When I was adding mailcatcher to an app running on the Heroku Cedar stack, I encountered a weird gem dependency issue that only manifested itself in the production environment on Heroku. An explicit dependency on the “thin” server in my Gemfile (gem 'thin', '~> 1.5.0') fixed it. You might need the same, Heroku recommends thin anyway for production apps.

After you update your gems with bundle update you can run mailcatcher with:

$ mailcatcher -f

Point your browser to http://127.0.0.1:1080 and you should see an empty mailbox.

Configure your development environment to use the now running mailcatcher by adding this to your config/environments/development.rb file:

# 1. Point ActionMailer at mailcatcher
config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = { :host => "localhost", :port => 1025 }
config.action_mailer.raise_delivery_errors = true

# 2. Base configuration for ExceptionNotifier
config.middleware.use ExceptionNotifier,
  :exception_recipients => %w{exceptions@example.com},
  :ignore_if => lambda { true }

# 3. Enable Rake notifications
ExceptionNotifier::Rake.configure

The first part configures Rails’ ActionMailer (which is used under the hood to actually send mail) for mailcatcher. The second part sets up ExceptionNotifier, the underlying generic notification middleware (see its documentation for more background), and lastly we enable notifications about Rake failures by calling ExceptionNotifier::Rake.configure.

On the off chance that you’re already using ExceptionNotifier in your development environment, you can omit the second part. Note that in this example we are suppressing all notifications other than about Rake failures with the :ignore_if option.

We should be good to go. If you now run failing_task (in your development environment, which should be the default for a locally run task) a notification will pop up in mailcatcher. Run the task with:

$ rake failing_task

Then go to http://127.0.0.1:1080 to check out the email sent. The email includes the name of the Rake task that failed and a stacktrace of the exception thrown.

Email Configuration for your Heroku App

If all of this worked in your development environment, you’re ready to tackle your production environment on Heroku. But first you need to make sure email delivery in general is configured in your Heroku app. One of two cases will apply to you.

1. Your app is already sending email. Great, you don’t need to do anything since ExceptionNotifier will just use the configuration you already have in place. Skip to the next step.

2. Your app isn’t sending email yet (you haven’t explicitly configured anything or installed any email add-ons). If your app’s email needs are limited to failure notifications, then a simple add-on will work for you. I can recommend the SendGrid Starter add-on. It’s free and allows up to 200 emails per day, more than enough for a few failure notifications. To install it run:

$ heroku addons:add sendgrid:starter

If your app is on the Cedar stack at Heroku, you also need to manually configure SendGrid (it should be automatic on the Bamboo stack). The official documentation has all the details, but just adding this to your config/environments/production.rb file should do the job:

config.action_mailer.delivery_method = :smtp
config.action_mailer.smtp_settings = {
  :address => "smtp.sendgrid.net",
  :port => 587,
  :authentication => :plain,
  :user_name => ENV['SENDGRID_USERNAME'],
  :password => ENV['SENDGRID_PASSWORD'],
  :domain => 'heroku.com'
}

Configure & Test Notifications on Heroku

To enable Rake notifications in your production environment we add similar configuration to the config/environments/production.rb file like we did for the development environment:

# 1. Base configuration for ExceptionNotifier
config.middleware.use ExceptionNotifier,
  :sender_address => %{"Scheduler" <some.address@example.com>},
  :exception_recipients => %w{your.email@example.com},
  :ignore_if => lambda { true }

# 2. Enable Rake notifications
ExceptionNotifier::Rake.configure

Again, if you are already using ExceptionNotifier in your production environment, omit the first part.

This time, the concrete values actually matter. Replace the values of the :sender_address and :exception_recipients options with the actual addresses you want to use as sender and recipients (the sender isn’t actually very significant but could be helpful for example for filtering notification emails in your mail client).

It’s now time to commit your changes (the new failing_task as well as Gemfile and configuration changes) and push them to your Heroku app. If that succeeds, you can try out notification delivery by running failing_task on Heroku:

$ heroku run rake failing_task

Hopefully that failed successfully! All the addresses you listed under the :exception_recipients option should have received an email with details on the failure.

That’s all. Any Rake tasks you schedule from now on with the Heroku Scheduler will trigger notification emails if they fail.

Advanced Configuration

There are a few more configuration options that might be of interest, especially if you were already using ExceptionNotifier before. Check out the documentation for exception_notification-rake for details.