RTI Connext Java API
Version 5.0.0
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Specifies how the samples representing changes to data instances are presented to a subscribing application. More...
Public Attributes | |
PresentationQosPolicyAccessScopeKind | access_scope |
Determines the largest scope spanning the entities for which the order and coherency of changes can be preserved. | |
boolean | coherent_access |
Specifies support for coherent access. Controls whether coherent access is supported within the scope access_scope . | |
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 . | |
Public Attributes inherited from QosPolicy | |
final QosPolicyId_t | id |
The ID of this QoS policy. | |
final String | policy_name |
The name of this QoS policy. | |
Additional Inherited Members | |
Public Member Functions inherited from Struct | |
abstract boolean | equals (Object obj) |
abstract int | hashCode () |
String | toString () |
Protected Member Functions inherited from Struct | |
Struct () | |
abstract void | pull_from_nativeI (long native_status) |
abstract void | push_to_nativeI (long native_status) |
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 com.rti.dds.subscription.DataReader will usually receive data in the order that it was sent by a com.rti.dds.publication.DataWriter, and the data is presented to the com.rti.dds.subscription.DataReader as soon as the application receives the next expected value. However, sometimes, you may want a set of data for the same com.rti.dds.publication.DataWriter to be presented to the com.rti.dds.subscription.DataReader 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 com.rti.dds.publication.DataWriter, across instances of a single com.rti.dds.publication.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 com.rti.dds.publication.Publisher.begin_coherent_changes and com.rti.dds.publication.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
controls the maximum extent of coherent changes. The behavior is as follows:
access_scope
is set to com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.INSTANCE_PRESENTATION_QOS (the default), the use of com.rti.dds.publication.Publisher.begin_coherent_changes and com.rti.dds.publication.Publisher.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 com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.TOPIC_PRESENTATION_QOS, then coherent changes (indicated by their enclosure within calls to com.rti.dds.publication.Publisher.begin_coherent_changes and com.rti.dds.publication.Publisher.end_coherent_changes) will be made available as such to each remote com.rti.dds.subscription.DataReader independently. That is, changes made to instances within each individual com.rti.dds.publication.DataWriter will be available as coherent with respect to other changes to instances in that same com.rti.dds.publication.DataWriter, but will not be grouped with changes made to instances belonging to a different com.rti.dds.publication.DataWriter. 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 com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.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 com.rti.dds.publication.DataWriter. access_scope
is set to com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.TOPIC_PRESENTATION_QOS, changes (creations, deletions, modifications) made by a single com.rti.dds.publication.DataWriter are made available to subscribers in the same order they occur. Changes made to instances though different com.rti.dds.publication.DataWriter 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 com.rti.dds.publication.DataWriter objects attached to the same com.rti.dds.publication.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 com.rti.dds.subscription.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 com.rti.dds.subscription.DataReader objects.
For com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.GROUP_PRESENTATION_QOS the subscribing application must use the APIs com.rti.dds.subscription.Subscriber.begin_access, com.rti.dds.subscription.Subscriber.end_access and com.rti.dds.subscription.Subscriber.get_datareaders to access the changes in the proper order.
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 com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.HIGHEST_OFFERED_PRESENTATION_QOS. For the purposes of this inequality, the values of access_scope
are considered ordered such that com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.INSTANCE_PRESENTATION_QOS < com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.TOPIC_PRESENTATION_QOS < com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.GROUP_PRESENTATION_QOS. coherent_access
is com.rti.dds.infrastructure.false, or else both offered and requested coherent_access
are com.rti.dds.infrastructure.true. ordered_access
is com.rti.dds.infrastructure.false, or else both offered and requested ordered_access
are com.rti.dds.infrastructure.true.PresentationQosPolicyAccessScopeKind access_scope |
Determines the largest scope spanning the entities for which the order and coherency of changes can be preserved.
[default] com.rti.dds.infrastructure.PresentationQosPolicyAccessScopeKind.PresentationQosPolicyAccessScopeKind.INSTANCE_PRESENTATION_QOS
boolean 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 com.rti.dds.infrastructure.ReliabilityQosPolicyKind.ReliabilityQosPolicyKind.RELIABLE_RELIABILITY_QOS).
[default] com.rti.dds.infrastructure.false
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
.
That is, the ability of the subscriber to see changes in the same order as they occurred on the publishing end.
[default] com.rti.dds.infrastructure.false