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RTI Connext Traditional C++ API
Version 6.1.0
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Specifies how the samples representing changes to data instances are presented to a subscribing application. More...
Public Attributes | |
| DDS_PresentationQosPolicyAccessScopeKind | access_scope |
| Determines the largest scope spanning the entities for which the order and coherency of changes can be preserved. More... | |
| DDS_Boolean | coherent_access |
Specifies support for coherent access. Controls whether coherent access is supported within the scope access_scope. More... | |
| 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. More... | |
| DDS_Boolean | drop_incomplete_coherent_set |
| <<extension>> Indicates whether or not a DDSDataReader should drop samples from an incomplete coherent set (one for which not all the samples were received). Such samples are reported as lost in the SAMPLE_LOST Status. More... | |
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:
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 DataWriter or different DataWriters to be presented to the DataReader(s) only after all of the elements of the set have been received, but not before. 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 DataWriter, across instances of a single DataWriter, and even across multiple DataWriters 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.
coherent_access controls whether RTI Connext will preserve the groupings of changes made by a publishing application by means of the operations DDSPublisher::begin_coherent_changes and DDSPublisher::end_coherent_changes. ordered_access controls whether RTI Connext will preserve the order of changes. access_scope controls the granularity of the other settings. See below: If coherent_access is set, then the access_scope controls the maximum extent of coherent changes. The behavior is as follows:
access_scope is set to DDS_INSTANCE_PRESENTATION_QOS (the default), the use of DDSPublisher::begin_coherent_changes and DDSPublisher::end_coherent_changes has no effect on how the subscriber can access the data, because with the scope limited to each instance, changes to separate instances are considered independent and thus cannot be grouped into a coherent set. access_scope is set to DDS_TOPIC_PRESENTATION_QOS, then coherent changes (indicated by their enclosure within calls to DDSPublisher::begin_coherent_changes and DDSPublisher::end_coherent_changes) will be made available as such to each remote DDSDataReader independently. That is, changes made to instances within each individual DDSDataWriter will be available as coherent with respect to other changes to instances in that same DDSDataWriter, but will not be grouped with changes made to instances belonging to a different DDSDataWriter. If ordered_access is set, then the access_scope controls the maximum extent for which order will be preserved by RTI Connext.
access_scope is set to DDS_INSTANCE_PRESENTATION_QOS (the lowest level), then changes to each instance are considered unordered relative to changes to any other instance. That means that changes (creations, deletions, modifications) made to two instances are not necessarily seen in the order they occur. This is the case even if it is the same application thread making the changes using the same DDSDataWriter. access_scope is set to DDS_TOPIC_PRESENTATION_QOS, changes (creations, deletions, modifications) made by a single DDSDataWriter are made available to subscribers in the same order they occur. Changes made to instances though different DDSDataWriter entities are not necessarily seen in the order they occur. This is the case, even if the changes are made by a single application thread using DDSDataWriter objects attached to the same DDSPublisher. 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. If you do not use these operations, the data may not be ordered across DataWriters.
The field DDS_SampleInfo::coherent_set_info is set when a sample is part of a coherent set. This field provides information to identify the coherent set that a sample is part of. In addition, DDS_CoherentSetInfo_t::incomplete_coherent_set indicates if a sample is part of an incomplete coherent set (one for which not all samples have been received). Coherent sets for which some of the samples are filtered out by content or time on the DDSDataWriter are considered incomplete.
By default, the samples that are received from an incomplete coherent set are dropped by the DataReader(s) and they are not provided to the application. Such samples are reported as lost in the SAMPLE_LOST Status. By setting DDS_PresentationQosPolicy::drop_incomplete_coherent_set to DDS_BOOLEAN_FALSE, you can change this behavior and, in this case, samples from incomplete coherent sets will be provided to the application. These samples have DDS_CoherentSetInfo_t::incomplete_coherent_set set to DDS_BOOLEAN_TRUE.
The value offered is considered compatible with the value requested if and only if the following conditions are met:
access_scope >= requested access_scope evaluates to 'TRUE' or requested access_scope is DDS_HIGHEST_OFFERED_PRESENTATION_QOS. For the purposes of this inequality, the values of access_scope are considered ordered such that DDS_INSTANCE_PRESENTATION_QOS < DDS_TOPIC_PRESENTATION_QOS < DDS_GROUP_PRESENTATION_QOS. coherent_access is DDS_BOOLEAN_FALSE, or else both offered and requested coherent_access are DDS_BOOLEAN_TRUE. ordered_access is DDS_BOOLEAN_FALSE, or else both offered and requested ordered_access are DDS_BOOLEAN_TRUE.| DDS_PresentationQosPolicyAccessScopeKind DDS_PresentationQosPolicy::access_scope |
Determines the largest scope spanning the entities for which the order and coherency of changes can be preserved.
[default] DDS_INSTANCE_PRESENTATION_QOS
| DDS_Boolean DDS_PresentationQosPolicy::coherent_access |
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.
Note: To use this feature, the DataWriter must be configured for RELIABLE communication (see DDS_RELIABLE_RELIABILITY_QOS).
[default] DDS_BOOLEAN_FALSE
| DDS_Boolean DDS_PresentationQosPolicy::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.
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
| DDS_Boolean DDS_PresentationQosPolicy::drop_incomplete_coherent_set |
<<extension>> Indicates whether or not a DDSDataReader should drop samples from an incomplete coherent set (one for which not all the samples were received). Such samples are reported as lost in the SAMPLE_LOST Status.
Note that a coherent set will be considered incomplete if some of its samples are filtered by content or time on the DataWriter side.
By default, the samples that are received from an incomplete coherent set are dropped (and reported as lost) by the DataReader(s) and they are not provided to the application. By setting this parameter to DDS_BOOLEAN_FALSE, you can change this behavior.
Samples from an incomplete coherent set have DDS_CoherentSetInfo_t::incomplete_coherent_set set to DDS_BOOLEAN_TRUE in DDS_SampleInfo::coherent_set_info.
[default] DDS_BOOLEAN_TRUE