@Override
    public String perform(String[] args, String data, Map<String, Map<String, String>> replacementsMap) {
      if (data != null) {
        return data.toLowerCase();
      }

      return "";
    }
ToLowerFunction is a Function implementation that converts a String to lowercase
ambari-server/src/main/java/org/apache/ambari/server/state/kerberos/VariableReplacementHelper.java