FlowController Use Cases

Working with flow controllers. More...

Working with flow controllers.

Creating a flow controller

• Set up participant
• Create a flow controller

   DDS.FlowController controller = null;
   DDS.FlowControllerProperty_t property = new DDS.FlowControllerProperty_t();
   
   try {
      participant.get_default_flowcontroller_property(property);
  } catch (DDS.Exception) {
       System.Console.WriteLine(
           "***Error: failed to get default flow controller property");
   }
   
   // optionally modify flow controller property values
   
   controller = participant.create_flowcontroller(
       "my flow controller name", property);
   
   if (controller == null) {
       System.Console.WriteLine(
           "***Error: failed to create flow controller");
   }

Flow controlling a data writer

• Set up participant
• Create flow controller
• Create an asynchronous data writer, FooDataWriter, of user data type Foo:

  DataWriterQos writer_qos = new DataWriterQos();
  
  // MyWriterListener is user defined and 
  // extends DataWriterListener
  MyWriterListener writer_listener = new MyWriterListener(); // or = null
  
  try {
      publisher.get_default_datawriter_qos(writer_qos);
  } catch (DDS.Exception) {
      // ... error 
  }
  
  // Change the writer QoS to publish asnychronously
  writer_qos.publish_mode.kind =
      PublishModeQosPolicyKind.ASYNCHRONOUS_PUBLISH_MODE_QOS;
  
  // Setup to use the previously created flow controller
  writer_qos.publish_mode.flow_controller_name = 
      "my flow controller name";
  
  // Samples queued for asynchronous write are subject to the History Qos policy
  writer_qos.history.kind = HistoryQosPolicyKind.KEEP_ALL_HISTORY_QOS;
  
  FooDataWriter writer = (FooDataWriter) publisher.create_datawriter(
                                             topic,
                                             writer_qos, 
                                             writer_listener,
                           STATUS_MASK_ALL);
  if (writer == null) {
      // ... error 
  }
  
  // Send data asynchronously...
  
  // Wait for asynchronous send completes, if desired
  try {
      writer.wait_for_asynchronous_publishing(timout);
  } catch (DDS.Exception) {
      System.Console.WriteLine(
          "***Error: failed to wait for asynchronous publishing");
  }

Using the built-in flow controllers

RTI Connext provides several built-in flow controllers.

The DDS::FlowController::DEFAULT_FLOW_CONTROLLER_NAME built-in flow controller provides the basic asynchronous writer behavior. When calling DDS::TypedDataWriter::write, the call signals the DDS::Publisher asynchronous publishing thread (DDS::PublisherQos::asynchronous_publisher) to send the actual data. As with any ASYNCHRONOUS_PUBLISH_MODE_QOS DDS::DataWriter, the DDS::TypedDataWriter::write call returns immediately afterwards. The data is sent immediately in the context of the DDS::Publisher asynchronous publishing thread.

When using the DDS::FlowController::FIXED_RATE_FLOW_CONTROLLER_NAME flow controller, data is also sent in the context of the DDS::Publisher asynchronous publishing thread, but at a regular fixed interval. The thread accumulates samples from different DDS::DataWriter instances and generates data on the wire only once per DDS::FlowControllerTokenBucketProperty_t::period.

In contrast, the DDS::FlowController::ON_DEMAND_FLOW_CONTROLLER_NAME flow controller permits flow only when DDS::FlowController::trigger_flow is called. The data is still sent in the context of the DDS::Publisher asynchronous publishing thread. The thread accumulates samples from different DDS::DataWriter instances (across any DDS::Publisher) and sends all data since the previous trigger.

The properties of the built-in DDS::FlowController instances can be adjusted.

• Set up participant
• Lookup built-in flow controller

  FlowController controller = null;
  
  controller = participant.lookup_flowcontroller(
      FlowController.DEFAULT_FLOW_CONTROLLER_NAME);
  
  // This should never happen, built-in flow controllers are always created
  if (controller == null) {
      System.Console.WriteLine("***Error: failed to lookup flow controller");
  }

• Change property of built-in flow controller, if desired

  DDS.FlowControllerProperty_t property = new DDS.FlowControllerProperty_t();
  
  // Get the property of the flow controller
  try {
      controller.get_property(property);
  } catch (DDS.Exception) {
      System.Console.WriteLine(
          "***Error: failed to get flow controller property");
  }
  
  // Change the property value as desired
  property.token_bucket.period.sec = 2;
  property.token_bucket.period.nanosec = 0;
  
  // Update the flow controller property
  try {
     controller.set_property(property);
  } catch (DDS.Exception) {
      System.Console.WriteLine(
          "***Error: failed to set flow controller property");
  }

• Create a data writer using the correct flow controller name

Shaping the network traffic for a particular transport

• Set up participant
• Create the transports
• Create a separate flow controller for each transport
• Configure DDS::DataWriter instances to only use a single transport
• Associate all data writers using the same transport to the corresponding flow controller
• For each transport, the corresponding flow controller limits the network traffic based on the token bucket properties

Coalescing multiple samples in a single network packet

• Set up participant
• Create a flow controller with a desired token bucket period
• Associate the data writer with the flow controller
• Multiple samples written within the specified period will be coalesced into a single network packet (provided that tokens_added_per_period and bytes_per_token permit).