title: Must a MQTT client publish to topic(s) if there are no subscribers?
body: <p>In the context of MQTT and pub/sub, let's assume my embedded micro-controller IOT device has a significant number of variables (i.e., various sensors, values, settings, outputs, etc) which can be mapped to "topics". </p>
  
  <p>Is it really necessary to publish (to a broker) on topics if there are no subscribers for the topics?<br>
  I'd like to think I have a fairly decent grasp of MQTT, but as I thought through the implementation for this device, this question jumped out at me.  It would not be trivial for this device to <em>always</em> publish on <em>all</em> topics.</p>
  
  <p>Typically in this case, a subscriber will be interested in a small subset of topics, not all.  It seems very unnecessary to publish on ALL topics if only a small subset are being subscribed to.  Why not only publish on topics being subscribed to?</p>
  
  <p>MQTT subscriber clients typically connect to a broker, then send their subscribe message to the broker.  I don't see a mechanism by which subscribers can indicate to publishers what topics they are interested in, thereby allowing publishers to only publish on the necessary topics. </p>
  
  <p>Am I misunderstanding a nuance of MQTT, or more generally, pub/sub?  </p>