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RTI Connext C API
Version 6.1.1
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Specifies how the samples representing changes to data instances are presented to a subscribing application. More...
Data Fields | |
| 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 DDS_DataReader 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 DDS_DataReader will usually receive data in the order that it was sent by a DDS_DataWriter, and the data is presented to the DDS_DataReader 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 sent. 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 is a request-offered QoS policy that allows you to specify different scopes of presentation. 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 DDS_Publisher_begin_coherent_changes and DDS_Publisher_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 set on the DDS_Subscriber controls the maximum extent of coherent changes. The behavior is as follows:
access_scope is set to DDS_INSTANCE_PRESENTATION_QOS (the default), the behavior is the same as DDS_TOPIC_PRESENTATION_QOS. access_scope is set to DDS_TOPIC_PRESENTATION_QOS, then coherent changes (indicated by their enclosure within calls to DDS_Publisher_begin_coherent_changes and DDS_Publisher_end_coherent_changes) will be made available as a unit to each remote DDS_DataReader independently. That is, changes made to instances within each individual DDS_DataWriter will be presented as a unit. They will not be grouped with changes made to instances belonging to a different DDS_DataWriter. If ordered_access is set, then the access_scope set on the DDS_Subscriber controls the maximum extent to which order will be preserved by RTI Connext.
access_scope is set to DDS_INSTANCE_PRESENTATION_QOS (the lowest level), then the relative order of DDS samples sent by a DDS_DataWriter is only preserved on a per-instance basis. If two DDS samples refer to the same instance (identified by Topic and a particular value for the key), then the order in which they are stored in the DDS_DataReader queue is consistent with the order in which the changes occurred. However, if the two DDS samples belong to different instances, the order in which they are presented may or may not match the order in which the changes occurred. This is the case even if it is the same application thread making the changes using the same DDS_DataWriter. access_scope is set to DDS_TOPIC_PRESENTATION_QOS, changes (creations, deletions, modifications) made by a single DDS_DataWriter to one or more instances are presented to a DDS_DataReader in the same order in which they occur. Changes made to instances though different DDS_DataWriter entities are not required to be presented in the order in which they occur. This is the case even if the changes are made by a single application thread using DDS_DataWriter objects attached to the same DDS_Publisher. If access_scope is set to DDS_HIGHEST_OFFERED_PRESENTATION_QOS on the DDS_Subscriber, then the DDS_Subscriber will use the access_scope offered by the remote DDS_Publisher.
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 DDS_Subscriber 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 DDS_DataReader objects.
For DDS_GROUP_PRESENTATION_QOS the subscribing application must use the APIs DDS_Subscriber_begin_access, DDS_Subscriber_end_access and DDS_Subscriber_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 that belong to the same DDS_Publisher.
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 DDS_DataWriter 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 DDS_DataReader 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