docs: def process(self, input_data, topic=None):
          """
          Invokes each handler in sequence.
          Publishes final output data.
  
          Params:
              input_data:  message received by stream
              topic:       name of plugin or stream message received from,
                           if applicable
          """
          for handler in self.handlers:
              output = handler.handle(input_data)
              input_data = output
  
          self.publish(input_data)