celluloid-worker.rb

require 'celluloid'

class URLResolver
  include Celluloid

  def resolve(num, url)
    sleep 1
    puts "[#{Time.now.to_f}] #{Thread.current.object_id} #{num} Resolving #{url}"
  end
end

Celluloid::Actor[:url_resolver] = URLResolver.pool(size: 5)

20.times.map {|i|
  Celluloid::Actor[:url_resolver].future(:resolve, i, 'http://example.com/')
}.map(&:value)