Concurrent HTTP requests with EventMachine

multi-em-http.rb

require 'eventmachine'
require 'em-http-request'

# Reference:
# https://github.com/igrigorik/em-http-request/wiki/Parallel-Requests
# http://rdoc.info/github/eventmachine/eventmachine/master/EventMachine/Iterator

urls = ['http://www.google.com', 'http://www.cloudamqp.com']

# always have 100 open request at any given time
concurrency = 100 

EM.run do
  EM::Iterator.new(urls, concurrency).each(
    proc { |url, iter|
      http = EventMachine::HttpRequest.new(url).get
      http.callback do |response| 
        p response.response
        iter.next
      end
      http.errback do
        p "Failed: #{url}"
        iter.next
      end
    }, 
    proc { 
      p 'All done!'
      EM.stop 
    })
end

Is there a way to have a certain amount of connections open at a time?