public void mergeProcessing(UpgradePack pack) {
    List<ProcessingService> list = pack.processing;
    if (list == null) {
      return;
    }
    if (processing == null) {
      processing = list;
      return;
    }
    processing.addAll(list);

    // new processing has been created, so rebuild the mappings
    initializeProcessingComponentMappings();
  }
Merges the processing section of the upgrade xml with
the processing section from a service's upgrade xml.
These are added to the end of the current list of services.

@param pack
             the service's upgrade pack
ambari-server/src/main/java/org/apache/ambari/server/stack/upgrade/UpgradePack.java