title: mqtt-Where mosquitto store all topics?
body: <p>I’m tracing mosquitto for a few days
  I want to check some questions:</p>
  
  <ol>
  <li><p>Would MQTT broker store all of the pub/sub topics when they pub/sub? if yes,where?</p></li>
  <li><p>Which struct(or file?) when publish send a message, publisher will search the struct whether topic has been used,and then reused it?</p></li>
  </ol>
  
  <p>Thank you all for answer for mqtt  beginner.</p>