url.rb

class Url < ActiveRecord::Base
  def find_link(url)
    link = link_finder(url)
    EM::Run {
        link.callback {
          p "done"
        }
    }
  end
end

worker.rb

class MyWorker
  include Sidekiq::Worker

  def perfom(a_link)
    urls = Domain.urls.all
    
    urls.each { |url|
      url.find_link(a_link)
    }

    # slow down cowboy

    do_something_with(urls.first)
  end
end

worker_async.rb

class MyWorker
  include Sidekiq::Worker

  def perfom(a_link)
    urls = Domain.urls.all

    EM.run {
      EM::iterator.new(urls,10).map(proc { |url|
        url.find_link(a_link)
      }, proc {
        do_something_with(urls.first)
      })
    } 
  end
end