DDS_PublishModeQosPolicy Struct Reference
[PUBLISH_MODE]

Specifies how RTI Data Distribution Service sends application data on the network. This QoS policy can be used to tell RTI Data Distribution Service to use its own thread to send data, instead of the user thread. More...

Data Fields
DDS_PublishModeQosPolicyKind	kind
	Publishing mode.
char *	flow_controller_name
	Name of the associated flow controller.

---

Detailed Description

Specifies how RTI Data Distribution Service sends application data on the network. This QoS policy can be used to tell RTI Data Distribution Service to use its own thread to send data, instead of the user thread.

The publishing mode of a DDS_DataWriter determines whether data is written synchronously in the context of the user thread when calling FooDataWriter_write or asynchronously in the context of a separate thread internal to the middleware.

Each DDS_Publisher spawns a single asynchronous publishing thread (DDS_AsynchronousPublisherQosPolicy::thread) to serve all its asynchronous DDS_DataWriter instances.

See also:

DDS_AsynchronousPublisherQosPolicy

DDS_HistoryQosPolicy

DDS_FlowController

Entity:

DDS_DataWriter

Properties:

RxO = N/A
Changeable = NO

Usage

The fastest way for RTI Data Distribution Service to send data is for the user thread to execute the middleware code that actually sends the data itself. However, there are times when user applications may need or want an internal middleware thread to send the data instead. For instance, to send large data reliably, you must use an asynchronous thread.

When data is written asynchronously, a DDS_FlowController, identified by flow_controller_name, can be used to shape the network traffic. Shaping a data flow usually means limiting the maximum data rates at which the middleware will send data for a DDS_DataWriter. The flow controller will buffer any excess data and only send it when the send rate drops below the maximum rate. The flow controller's properties determine when the asynchronous publishing thread is allowed to send data and how much.

Asynchronous publishing may increase latency, but offers the following advantages:

- The FooDataWriter_write call does not make any network calls and is therefore faster and more deterministic. This becomes important when the user thread is executing time-critical code.

- When data is written in bursts or when sending large data types as multiple fragments, a flow controller can throttle the send rate of the asynchronous publishing thread to avoid flooding the network.

- Asynchronously written samples for the same destination will be coalesced into a single network packet which reduces bandwidth consumption.

The maximum number of samples that will be coalesced depends on NDDS_Transport_Property_t::gather_send_buffer_count_max (each sample requires at least 2-4 gather-send buffers). Performance can be improved by increasing NDDS_Transport_Property_t::gather_send_buffer_count_max. Note that the maximum value is operating system dependent.

The middleware must queue samples until they can be sent by the asynchronous publishing thread (as determined by the corresponding DDS_FlowController). The number of samples that will be queued is determined by the DDS_HistoryQosPolicy. When using DDS_KEEP_LAST_HISTORY_QOS, only the most recent DDS_HistoryQosPolicy::depth samples are kept in the queue. Once unsent samples are removed from the queue, they are no longer available to the asynchronous publishing thread and will therefore never be sent.

---

Field Documentation

DDS_PublishModeQosPolicyKind DDS_PublishModeQosPolicy::kind

Publishing mode.

[default] DDS_SYNCHRONOUS_PUBLISH_MODE_QOS

char* DDS_PublishModeQosPolicy::flow_controller_name

Name of the associated flow controller.

NULL value or zero-length string refers to DDS_DEFAULT_FLOW_CONTROLLER_NAME.

Unless flow_controller_name points to a built-in flow controller, finalizing the DDS_DataWriterQos will also free the string pointed to by flow_controller_name. Therefore, use DDS_String_dup before passing the string to flow_controller_name, or reset flow_controller_name to NULL before finalizing the QoS.

Please refer to Conventions for more information.

See also:

DDS_DomainParticipant_create_flowcontroller

DDS_DEFAULT_FLOW_CONTROLLER_NAME

DDS_FIXED_RATE_FLOW_CONTROLLER_NAME

DDS_ON_DEMAND_FLOW_CONTROLLER_NAME

[default] DDS_DEFAULT_FLOW_CONTROLLER_NAME