title: Automatically scale Axon's tracking event processors
body: <p>I am using Axon framework 4.0.3 with Spring Boot to have event sourcing, and have one tracking processor which is configured to have multiple segments/threads to process events concurrently:</p>
  
  <pre><code>axon.eventhandling.processors[my_processor].initial-segment-count = 6
  axon.eventhandling.processors[my_processor].thread-count = 3
  </code></pre>
  
  <p>It is meant to have 2 nodes of <code>my_processor</code> using 3 threads each. </p>
  
  <p>However, the problem with this solution is that it's not scalable. I have to know from the very beginning how many nodes and threads I must have, as it's not possible to change it later: if I increase the <code>initial-segment-count</code> and restart the processor, nothing happens. Even worse if I decrease the segment count: events that were meant for the "removed" segments are never processed! </p>
  
  <p>Ideally it should be able to specify just the number of threads that each node should use. After that, when new nodes are added to the processor, the number of segments should scale up accordingly. Similarly if I remove nodes, the number of segments should scale down. Is this possible with Axon, or is it not designed to be scaled this way at all?</p>