Best practice for Rails App to run a long task in the background?

Question

I have a Rails application that unfortunately after a request to a controller, has to do some crunching that takes awhile. What are the best practices in Rails for providin

Answer 1

A simple solution that doesn't require any extra Gems or plugins would be to create a custom Rake task for handling the PDF generation. You could model the PDF generation process as a state machine with states such as submitted, processing and complete that are stored in the model's database table. The initial HTTP request to the Rails application would simply add a record to the table with a submitted state and return.

There would be a cron job that runs your custom Rake task as a separate Ruby process, so the main Rails application is unaffected. The Rake task can use ActiveRecord to find all the models that have the submitted state, change the state to processing and then generate the associated PDFs. Finally, it should set the state to complete. This enables your AJAX calls within the Rails app to monitor the state of the PDF generation process.

If you put your Rake task within your_rails_app/lib/tasks then it has access to the models within your Rails application. The skeleton of such a pdf_generator.rake would look like this:

namespace :pdfgenerator do
  desc 'Generates PDFs etc.'
  task :run => :environment do

    # Code goes here...
  end
end

As noted in the wiki, there are a few downsides to this approach. You'll be using cron to regularly create a fairly heavyweight Ruby process and the timing of your cron jobs would need careful tuning to ensure that each one has sufficient time to complete before the next one comes along. However, the approach is simple and should meet your needs.

Answer 2

I use Background Job (http://codeforpeople.rubyforge.org/svn/bj/trunk/README) to schedule tasks. I am building a small administration site that allows Site Admins to run all sorts of things you and I would run from the command line from a nice web interface.