docs: def getService(self, serviceIdentifier):
          """
          Return the requested service instance.
  
          :param serviceIdentifier: <str> service identifier
          :return: <object> service instance
          """
          if serviceIdentifier in self._services:
              return self._services[serviceIdentifier]
          else:
              message = "Application - getService() - " \
                        "Service with identifier {} does not exist." \
                        .format(serviceIdentifier)
              raise Exception(message)