title: How to implement MQTT for one-to-one message distribution
body: <p>I have been researching MQTT and I am curious how implementation works when trying to create a one-to-one communication system.</p>
  
  <p><strong>What I am trying to do:</strong> Set up a system that allows a IOT device to push notifications directly to its user &amp; the user to push notifications to their IOT device via a MQTT broker.</p>
  
  <p><strong>Thoughts on implementation:</strong> </p>
  
  <ol>
  <li>All devices have a separate topic that each user is subscribed to.</li>
  <li>Each device publishes to the same topic with a tag that identifies the user to pass the message to.</li>
  </ol>
  
  <p>Below is an image I created of how I see the implementation working. 
  <img src="https://i.stack.imgur.com/tCIzc.png" alt=""></p>
  
  <p><strong>More Info:</strong> I need an system that can scale to include 100k+ devices, but extremely low amount of messages(1 message a day)</p>
  
  <p><strong>Layout:</strong></p>
  
  <ul>
  <li>IOT device will send message via MQTT to the back end system.</li>
  <li>Back-end system will receive the message and store it in the back
  end.</li>
  <li>Web-service(Dispatcher) layer which will communicate between the
  back-end and mobile application.</li>
  <li>Mobile application will communicate directly to IOT device via web
  services.</li>
  </ul>
  
  <p><strong>Questions:</strong> </p>
  
  <ul>
  <li>How is the MQTT Broker implemented with regard to topics for one-to-one communication when large amounts of users are anticipated?</li>
  <li>Is there a limit on topics?</li>
  <li>What am I missing regarding the layout?</li>
  </ul>