@Override
  public StageWrapperBuilder getBuilder() {
    return new ParallelClientGroupingBuilder(this);
  }
Grouping for clients:
<ul>
  <li>The first client will be upgraded.</li>
  <li>Run a service check on the first client.</li>
  <li>Run the remaining clients in parallel.</li>
</ul>
ambari-server/src/main/java/org/apache/ambari/server/stack/upgrade/ParallelClientGrouping.java