async-dns-resolver.rb

require 'rubygems'
require 'eventmachine'
 
EM.run do
  resolver = proc do
    ['210.251.121.21', "8.8.8.8"].map do |addr|
      begin
        Resolv.getname addr
      rescue
        addr
      end
    end
  end
 
  handle_result = proc do |results|
    results.each {|result| p result }
    EM.stop
  end
 
  EM.defer(resolver, handle_result)
end