An eventmachine-based test server forking child processes to do the work, connecting to a UNIX domain socket on which the parent process is listening.

gistfile1.rb

require 'eventmachine'
CRLF = "\r\n"

module Child
  attr_writer :child_number
  include EM::P::LineText2

  def post_init
    set_line_mode
  end

  def receive_line(line)
    send_data "#{@child_number}: #{line}#{CRLF}"
  end
end

CHILD_CONNS = []
OUTSIDE_CONNS = []

module ConnectionToChild
  include EM::P::LineText2

  def post_init
    CHILD_CONNS << self
  end

  def unbind
    CHILD_CONNS.delete self
  end

  def receive_line(line)
    OUTSIDE_CONNS.each do |conn|
      conn.send_data(line)
      conn.send_data(CRLF)
    end
  end
end

module OutsideServer
  include EM::P::LineText2

  def post_init
    @child_num = 0
    OUTSIDE_CONNS << self
  end

  def unbind
    OUTSIDE_CONNS.delete self
  end

  def receive_line(line)
    child = CHILD_CONNS[@child_num]
    @child_num = (@child_num + 1) % CHILD_CONNS.size
    if child.nil?
      STDERR.puts("no children connected!")
    else
      child.send_data(line)
      child.send_data(CRLF)
    end
  end
end

UDS_PATH = "robin_sock.sock"

EM.run do

  EM.start_server("0.0.0.0", 10000, OutsideServer)
  EM.start_server(UDS_PATH, ConnectionToChild)
  4.times do |i|
    EM.fork_reactor do
      EM.connect(UDS_PATH, Child) do |conn|
        conn.child_number = i
      end
    end
  end

end

Amazing :) Can be a nice common pattern, actually :) +1