socket_server.rb

require 'celluloid/io'
require 'json'
# require 'celluloid/autostart'

class TestServer
  include Celluloid::IO
  finalizer :shutdown

  def initialize(host, port)
    puts "*** Starting echo server on #{host}:#{port}"

    # Since we included Celluloid::IO, we're actually making a
    # Celluloid::IO::TCPServer here
    @server = TCPServer.new(host, port)
    async.run
  end

  def shutdown
    @server.close if @server
  end

  def run
    loop { async.handle_connection @server.accept }
  end

  def handle_connection(socket)
    _, port, host = socket.peeraddr
    puts "*** Received connection from #{host}:#{port}"
    # loop { socket.write socket.readpartial(4096) }

    # This is just a test, followed this format:
    #   https://github.com/soldair/pinoccio-server/blob/master/troop.js#L163
    cmd = {
      :type => "command",
      :to => 1,
      :timeout => 10000,
      :command => "led.on"
    }
    loop {
      puts socket.readpartial(4096)
      socket.write cmd.to_json # The light never turns on. Why?
      }
  rescue EOFError
    puts "*** #{host}:#{port} disconnected"
    socket.close
  end
end

supervisor = TestServer.supervise("0.0.0.0", 1234)
trap("INT") { supervisor.terminate; exit }
sleep

i think you might be missing a newline after your json command?
i have to study up a little on ruby tcp to help more. hey you should really make a repository just for your rb server!