@Override
  public UpgradeCheckResult perform(UpgradeCheckRequest request)
      throws AmbariException {
    UpgradeCheckResult result = new UpgradeCheckResult(this);

    RepositoryVersion repositoryVersion = request.getTargetRepositoryVersion();

    Map<String, Map<String, ThreeWayValue>> changes = m_mergeHelper.getConflicts(
        request.getClusterName(), new StackId(repositoryVersion.getStackId()));

    Set<String> failedTypes = new HashSet<>();

    for (Entry<String, Map<String, ThreeWayValue>> entry : changes.entrySet()) {
      for (Entry<String, ThreeWayValue> configEntry : entry.getValue().entrySet()) {

        ThreeWayValue twv = configEntry.getValue();
        if (null == twv.oldStackValue) { // !!! did not exist and in the map means changed
          failedTypes.add(entry.getKey());

          result.getFailedOn().add(entry.getKey() + "/" + configEntry.getKey());

          MergeDetail md = new MergeDetail();
          md.type = entry.getKey();
          md.property = configEntry.getKey();
          md.current = twv.savedValue;
          md.new_stack_value = twv.newStackValue;
          md.result_value = md.current;
          result.getFailedDetail().add(md);

        } else if (!twv.oldStackValue.equals(twv.savedValue)) {  // !!! value customized
          if (null == twv.newStackValue || // !!! not in new stack
              !twv.oldStackValue.equals(twv.newStackValue)) { // !!! or the default value changed
            failedTypes.add(entry.getKey());

            result.getFailedOn().add(entry.getKey() + "/" + configEntry.getKey());

            MergeDetail md = new MergeDetail();
            md.type = entry.getKey();
            md.property = configEntry.getKey();
            md.current = twv.savedValue;
            md.new_stack_value = twv.newStackValue;
            md.result_value = md.current;
            result.getFailedDetail().add(md);
          }
        }
      }
    }

    if (result.getFailedOn().size() > 0) {
      result.setStatus(UpgradeCheckStatus.WARNING);
      String failReason = getFailReason(result, request);

      result.setFailReason(String.format(failReason, StringUtils.join(
          failedTypes, ", ")));

    } else {
      result.setStatus(UpgradeCheckStatus.PASS);
    }

    return result;
  }
The following logic determines if a warning is generated for config merge
issues:
<ul>
  <li>A value that has been customized from HDP 2.2.x.x no longer exists in HDP 2.3.x.x</li>
  <li>A value that has been customized from HDP 2.2.x.x has changed its default value between HDP 2.2.x.x and HDP 2.3.x.x</li>
</ul>
ambari-server/src/main/java/org/apache/ambari/server/checks/ConfigurationMergeCheck.java