title: How to build mqtt broker from scratch?
body: <p>I need to build a MQTT broker with basic functions but I cannot find any documents about MQTT broker. </p>
  
  <p>Anyone have any idea how to do this? What do I need to read?</p>
  
  <p>Firstly, I just want broker can accept connection using <code>CONNECT</code> and <code>CONNACK</code>.</p>