code: @Override
      public RegisterEventTopicResult registerEventTopic(RegisterEventTopicRequest request) {
          request = beforeClientExecution(request);
          return executeRegisterEventTopic(request);
      }
docs: <p>
  Associates a directory with an SNS topic. This establishes the directory as a publisher to the specified SNS
  topic. You can then receive email or text (SMS) messages when the status of your directory changes. You get
  notified if your directory goes from an Active status to an Impaired or Inoperable status. You also receive a
  notification when the directory returns to an Active status.
  </p>
  
  @param registerEventTopicRequest
  Registers a new event topic.
  @return Result of the RegisterEventTopic operation returned by the service.
  @throws EntityDoesNotExistException
  The specified entity could not be found.
  @throws InvalidParameterException
  One or more parameters are not valid.
  @throws ClientException
  A client exception has occurred.
  @throws ServiceException
  An exception has occurred in AWS Directory Service.
  @sample AWSDirectoryService.RegisterEventTopic
  @see <a href="http://docs.aws.amazon.com/goto/WebAPI/ds-2015-04-16/RegisterEventTopic" target="_top">AWS API
  Documentation</a>