DDS_PresentationQosPolicy Struct Reference
[PRESENTATION]

Specifies how the samples representing changes to data instances are presented to a subscribing application. More...

List of all members.

Public Attributes

DDS_PresentationQosPolicyAccessScopeKind access_scope
 Determines the largest scope spanning the entities for which the order and coherency of changes can be preserved.
DDS_Boolean coherent_access
 Specifies support for coherent access. Controls whether coherent access is supported within the scope access_scope.
DDS_Boolean ordered_access
 Specifies support for ordered access to the samples received at the subscription end. Controls whether ordered access is supported within the scope access_scope.


Detailed Description

Specifies how the samples representing changes to data instances are presented to a subscribing application.

This QoS policy controls the extent to which changes to data instances can be made dependent on each other and also the kind of dependencies that can be propagated and maintained by RTI Connext. Specifically, this policy affects the application's ability to:

Entity:
DDSPublisher, DDSSubscriber
Status:
DDS_OFFERED_INCOMPATIBLE_QOS_STATUS, DDS_REQUESTED_INCOMPATIBLE_QOS_STATUS
Properties:
RxO = YES
Changeable = UNTIL ENABLE

Usage

A DDSDataReader will usually receive data in the order that it was sent by a DDSDataWriter, and the data is presented to the DDSDataReader as soon as the application receives the next expected value. However, sometimes, you may want a set of data for the same topic to be presented to the DDSDataReader only after all of the elements of the set have been received. Or you may want the data to be presented in a different order than that in which it was received. Specifically for keyed data, you may want the middleware to present the data in keyed -- or instance -- order, such that samples pertaining to the same instance are presented together.

The Presentation QoS policy allows you to specify different scopes of presentation: within a topic, across instances of a single topic, and even across multiple topics used by different writers of a publisher. It also controls whether or not a set of changes within the scope is delivered at the same time or can be delivered as soon as each element is received.

If coherent_access is set, then the access_scope controls the maximum extent of coherent changes. The behavior is as follows:

If ordered_access is set, then the access_scope controls the maximum extent for which order will be preserved by RTI Connext.

Note that this QoS policy controls the scope at which related changes are made available to the subscriber. This means the subscriber can access the changes in a coherent manner and in the proper order; however, it does not necessarily imply that the DDSSubscriber will indeed access the changes in the correct order. For that to occur, the application at the subscriber end must use the proper logic in reading the DDSDataReader objects.

For DDS_GROUP_PRESENTATION_QOS the subscribing application must use the APIs DDSSubscriber::begin_access, DDSSubscriber::end_access and DDSSubscriber::get_datareaders to access the changes in the proper order.

Compatibility

The value offered is considered compatible with the value requested if and only if the following conditions are met:


Member Data Documentation

Determines the largest scope spanning the entities for which the order and coherency of changes can be preserved.

[default] DDS_INSTANCE_PRESENTATION_QOS

Specifies support for coherent access. Controls whether coherent access is supported within the scope access_scope.

That is, the ability to group a set of changes as a unit on the publishing end such that they are received as a unit at the subscribing end.

[default] DDS_BOOLEAN_FALSE

Specifies support for ordered access to the samples received at the subscription end. Controls whether ordered access is supported within the scope access_scope.

That is, the ability of the subscriber to see changes in the same order as they occurred on the publishing end.

[default] DDS_BOOLEAN_FALSE


RTI Connext C++ API Version 4.5f Copyright © 17 Mar 2012 Real-Time Innovations, Inc