public String getRefreshCommandForComponent(ServiceComponentHost sch, String propertyName) {
    if (sch.isClientComponent()) {
      return REFRESH_CONFIGS;
    }
    String keyName = findKey(propertyName);
    Map<String, String> componentCommandMap = propertyComponentCommandMap.get(keyName);
    if (componentCommandMap != null) {
      String commandForComponent = componentCommandMap.get(sch.getServiceComponentName());
      if (commandForComponent != null) {
        return commandForComponent;
      } else if(componentCommandMap.size() == 1) {
        return REFRESH_CONFIGS;
      }
    }
    return null;
  }
If no command is defined for a component then the default command will be REFRESH_CONFIGS in case of a client component or
if there's only one command defined for an another component. This is because if RELOAD_CONFIGS is defined for NAMENODE then
presumably other dependent components will need just a refresh.
ambari-server/src/main/java/org/apache/ambari/server/state/RefreshCommandConfiguration.java