Delayed Job as a rake task

Delayed Job is a great and simple solution for background jobs in a Rails application. But if you don’t have the memory or resources to run a background thread 24hrs on a server what do you do?

We were in a similar situation for a client, who was strictly on a shoe string budget, running off Heroku. The background job was to process some applications, verify and sent emails if they had errors. This was to be done once a day. Running a background job just for this was a bad choice.

Another option was to run this as a rake task, at a set time, use Heroku Scheduler to run this at 04:00am every day. But we wanted to try a bit more generic solution.

We decided to stick to write Delayed Job classes, so that in future, it can serve 2 purposes

1. For more background processing, more jobs we just have to write new classes with perform methods
2. When the app is moved into it’s own private hosting VPS, run a background thread more often, to scale it.

So delayed job it was!

But in the current setup, we had to run the jobs as a rake task, triggered once a day at a specific time, while leaving room for a full fledged background process in the future.

Looking at the delayed_job source at Github we figured that this could be easily implemented. It internally creates a Delayed::Worker class and works off all the jobs in the delayed_jobs table.

The solution was really simple. Create a rake task in lib/tasks/delayed_job.rake

# File lib/tasks/delayed_job.rake

namespace :delayed_job do
  desc 'Run Delayed job worker'
  task work: :environment do
    worker = Delayed::Worker.new
    worker.work_off #by default runs 100 rows
  end
end

For more information on the sources:

• The rake task for starting delayed job - delayed/tasks.rb

• The Delayed::Worker start - delayed/worker.rb

• Delayed::Worker work_off definition - delayed/worker.rb#work_off

Credits: Header photo by Elisa Michelet on Unsplash.