client.rb

require 'msgpack/rpc'
c = MessagePack::RPC::Client.new('localhost',9090)
c.call(:print, 'hello') #=> サーバー側に'hello'が表示される
c.notify(:print, 'hello') #=> 何も起きない

server.rb

require 'msgpack/rpc'

class MyHandler
  def print(msg)
    puts msg
  end
end

server = MessagePack::RPC::Server.new
server.listen('localhost', '9090', MyHandler.new)
server.run