Ruby on Rails: How to run things in the background?

Question

When a new resource is created and it needs to do some lengthy processing before the resource is ready, how do I send that processing away into the background where it won't hold up the current request or other traffic to my web-app?

in my model:

class User < ActiveRecord::Base
 after_save :background_check

 protected
 def background_check
  # check through a list of 10000000000001 mil different
  # databases that takes approx one hour :)
  if( check_for_record_in_www( self.username ) )
    # code that is run after the 1 hour process is finished.
    user.update_attribute( :has_record )
  end
 end
end

Answer 1

Start a separate process, which is probably most easily done with system, prepending a 'nohup' and appending an '&' to the end of the command you pass it. (Make sure the command is just one string argument, not a list of arguments.)

There are several reasons you want to do it this way, rather than, say, trying to use threads:

1. Ruby's threads can be a bit tricky when it comes to doing I/O; you have to take care that some things you do don't cause the entire process to block.

2. If you run a program with a different name, it's easily identifiable in 'ps', so you don't accidently think it's a FastCGI back-end gone wild or something, and kill it.

Really, the process you start should be "deamonized," see the Daemonize class for help.

Answer 2

you ideally want to use an existing background job server, rather than writing your own. these will typically let you submit a job and give it a unique key; you can then use the key to periodically query the jobserver for the status of your job without blocking your webapp. here is a nice roundup of the various options out there.

Answer 3

What about:

def background_check
   exec("script/runner check_for_record_in_www.rb #{self.username}") if fork == nil
end

The program "check_for_record_in_www.rb" will then run in another process and will have access to ActiveRecord, being able to access the database.

Answer 4

You should definitely check out the following Railscasts:

• http://railscasts.com/episodes/127-rake-in-background
• http://railscasts.com/episodes/128-starling-and-workling
• http://railscasts.com/episodes/129-custom-daemon

They explain how to run background processes in Rails in every possible way (with or without a queue ...)

Answer 5

I like to use backgroundrb, its nice it allows you to communicate to it during long processes. So you can have status updates in your rails app

Answer 6

I think spawn is a great way to fork your process, do some processing in background, and show user just some confirmation that this processing was started.