Start with `thin start` and then connect to the client with just ruby websocket-client.rb

config.ru

require 'eventmachine'
require 'rack'
require 'thin'
require 'faye/websocket'
Faye::WebSocket.load_adapter('thin')

App = lambda do |env|
  if Faye::WebSocket.websocket?(env)
    ws = Faye::WebSocket.new(env)

    ws.on :message do |event|
      ws.send(event.data)
    end

    ws.on :close do |event|
      p [:close, event.code, event.reason]
      ws = nil
    end

    # Return async Rack response
    ws.rack_response

  else
    # Normal HTTP request
    [200, {'Content-Type' => 'text/plain'}, ['Hello']]
  end
end

run App

websocket-client.rb

require 'faye/websocket'
require 'eventmachine'

EM.run {
  ws = Faye::WebSocket::Client.new("ws://#{ARGV[0]}:3000")

  ws.on :open do |event|
    p [:open]
    ws.send('Hello, world!')
  end

  ws.on :message do |event|
    p [:message, event.data]
  end

  ws.on :close do |event|
    p [:close, event.code, event.reason]
    ws = nil
  end
}