Question

I’d like ExceptionNotifier to send out an email when an exception happens in a delayed job, just like for other exceptions. How can I achieve that?

Answer 1

I do this with Rails 3.2.6, delayed_job 3.0.3 and exception_notification 2.6.1 gem

# In config/environments/production.rb or config/initializers/delayed_job.rb

# Optional but recommended for less future surprises.
# Fail at startup if method does not exist instead of later in a background job 
[[ExceptionNotifier::Notifier, :background_exception_notification]].each do |object, method_name|
  raise NoMethodError, "undefined method `#{method_name}' for #{object.inspect}" unless object.respond_to?(method_name, true)
end

# Chain delayed job's handle_failed_job method to do exception notification
Delayed::Worker.class_eval do 
  def handle_failed_job_with_notification(job, error)
    handle_failed_job_without_notification(job, error)
    # only actually send mail in production
    if Rails.env.production?
      # rescue if ExceptionNotifier fails for some reason
      begin
        ExceptionNotifier::Notifier.background_exception_notification(error)
      rescue Exception => e
        Rails.logger.error "ExceptionNotifier failed: #{e.class.name}: #{e.message}"
        e.backtrace.each do |f|
          Rails.logger.error "  #{f}"
        end
        Rails.logger.flush
      end
    end
  end 
  alias_method_chain :handle_failed_job, :notification 
end

It’s probably a good idea to load this code in all environments to catch errors after bundle update etc before they reach production. I do this by having a config/initializers/delayed_job.rb file but you could duplicate the code for each config/environments/* environment.

Another tip is to tune the delayed job config a bit as default you may get a lot of duplicate exception mails when job fails.

# In config/initializers/delayed_job_config.rb
Delayed::Worker.max_attempts = 3

Update I had some problems with the delayed_job daemon silently exiting and it turned out to be when ExceptionNotifier fails to send mail and no one rescued the exception. Now the code rescues and log them.