I was working on a large community-driven website recently, which had always had the requirement to synchronize its user database with another (3rd party) site that we didn't control. The most we could get out of the other site was for them to send us regular XML dumps of the changes (additions, removals, deletions). Predictably, we wrote that as a rake task and added it to cron.

@hourly   cd /apps/product/current && export RAILS_ENV=production && rake product:synchronize_database

It worked great, until the client threw in ONE additional little snag; not only should it check for updates every hour, but an admin should be able to log in to the site and request an immediate synchronization. Unfortunately, this task can take anywhere from 10 minutes to 10 hours, depending on the size of the XML file and the number of employees involved. Clearly not a job for a simple backtick or %x{}.

There are a number of different options for running background processes in Rails, but since I had pretty simple requirements (and needed quick results) the best choice for me was clearly Spawn.

You can install the plugin from rubyforge:

script/plugin install http://spawn.rubyforge.org/svn/spawn/

Then implementing the client's request was as simple as adding a button with a remote_function and a controller action with:

spawn(:nice => 7) do
        exec("cd #{RAILS_ROOT} && export RAILS_ENV=#{RAILS_ENV} && rake product:synchronize_database")
end

The :nice option (vital, in my case!) will make sure that your process doesn't monopolize the CPU (just like its shell counterpart). There are only a couple of other options; you can choose to fork or thread your process (fork is the default), and you can wait for it to finish. Neither was necessary in my case. Problem solved and, in typical Rails fashion, it only took a few lines of code!