数据分发服务 (DDS) 内置主题

文章目录

1 数据分发服务 (DDS) 内置主题
2 什么是内置主题？
3 内置主题中包含哪些信息？
- 3.1 DCPSParticipant
- - 3.1.1 ParticipantBuiltinTopicData 成员表
- 3.2 DCPSTopic
- - 3.2.1 TopicBuiltinTopicData 成员表
- 3.3 DCPSPublication
- - 3.3.1 PublicationBuiltinTopicData 成员表
- 3.4 DCPSSubscription
- - 3.4.1 SubscriptionBuiltinTopicData 成员表

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1 数据分发服务 (DDS) 内置主题

DDS 内置主题是数据分发服务用来帮助应用程序相互发现的一种特殊主题。中间件自动处理这些主题。应用程序还可以访问它们以获取有关域的信息。

DDS 中间件在运行过程中需要跟踪大量数据。DDS 应用程序及其实体可以在 DDS 域中出现和消失。这些实体可以是 DomainPartiticpants、DataWriters、DataReaders 和 Topics。DDS 中间件需要能够跟踪这一点。使用 DDS 的应用程序可能想了解这些更改。这使他们能够采取适当的行动。

DDS 规范定义了一些内置于 DDS 实现的主题。DDS 使用这些内置主题来帮助应用程序相互发现。订阅内置主题使应用程序可以访问有关域的信息。这些信息包括：

在域中注册的主题
哪些数据读取器和写入器建立了连接或断开或连接
域中实体的 QoS 设置

The build-in topics do not communicate all the QoS policy settings, only the ones that determine connectivity（例如，具有 RxO 功能）。

2 什么是内置主题？

不同的 DDS 实体位于不同的内置主题中，如下所示，内置主题包含：

DCPSParticipant
DCPSTopic
DCPSPublication
DCPSSubscription

DCPSParticipant 主题用于描述域参与者的每个实例。
Each Topic entity is described by an instance of the DCPSTopic topic.
Information about Publishers and DataWriters comes in a joint built-in topic called DCPSPublication。This contains one instance for each DataWriter which embeds the relevant information from the Publisher that contains it.。要获取有关系统中writers 的信息，您应该订阅 DCPSPublication 主题。
订阅者和数据读取者也来自一个名为 DCPSSubscription 的联合内置主题。 This contains one instance for each DataReader which also contains the information from the Subscriber that contains it.

3 内置主题中包含哪些信息？

3.1 DCPSParticipant

DCPSParticipant 主题使用 ParticipantBuiltinTopicData 数据类型来跟踪域中的 DomainParticipants。每个 ParticipantBuiltinTopic 数据样本代表一个 DomainParticipant。
如果创建了一个新的 DomainParticipant，那么就会创建一个新的 ParticipantBuiltinTopicData 实例。如果 DomainParticipant 被删除，则 ParticipantBuiltinTopicData 实例被释放。如果 DomainParticipant 修改其 UserDataQosPolicy，an updated ParticipantBuiltinTopicData sample is written。

3.1.1 ParticipantBuiltinTopicData 成员表

下表显示了 DCPSParticipant 主题中包含的信息。

姓名	类型	描述
key	内置TopicKey_t	Globally unique identifier of the participant
user_data	UserDataQosPolicy	通过 QosPolicy 附加到 participant 的用户定义数据

3.2 DCPSTopic

DCPSTopic 主题通过使用 TopicBuiltinTopicData 数据类型来传达主题的存在。域中的 TopicBuiltinTopicData 样本是该域中主题的表示。当一个新的主题被添加到域中时，一个新的 TopicBuiltinTopic 数据实例被创建。

当主题被参与者删除时，该实例不会被释放，因为主题生命周期与域的生命周期相关联，而不是与参与者的生命周期相关联。由于 DDS 的性质，主题（及其样本）的生命周期与发布者和订阅者无关。主题（和内置主题）在 DDS 中是永存的，因为使用它们的应用程序可以随时加入或离开。即使所有引入主题的应用程序都消失了，主题仍将保留在系统中。如果整个域消失，它们就会消失。

如果一个 Topic 修改了它的一个或多个 QoSPolicy 值，一个新的 TopicBuiltinTopicData 样本将被写入。

DCPSTopicTopic 中发布的信息对数据分发服务至关重要，因此无法通过配置文件中的 Domain/BuiltinTopics 元素禁用它。

3.2.1 TopicBuiltinTopicData 成员表

下表显示了 TopicBuiltinTopicData 主题中包含的信息。

key	BuiltinTopicKey_t	Global unique identifier of the Topic
name	String	Name of the Topic
type_name	String	Type name of the Topic (i.e. the fully scoped IDL name)
durability	DurabilityQosPolicy	QosPolicy attached to the Topic
durability_service	DurabilityServiceQosPolicy	QosPolicy attached to the Topic
deadline	DeadlineQosPolicy	QosPolicy attached to the Topic
latency_budget	LatencyBudgetQosPolicy	QosPolicy attached to the Topic
liveliness	LivelinessQosPolicy	QosPolicy attached to the Topic
reliability	ReliabilityQosPolicy	QosPolicy attached to the Topic
transport_priority	TransportPriorityQosPolicy	QosPolicy attached to the Topic
lifespan	LifespanQosPolicy	QosPolicy attached to the Topic
destination_order	DestinationOrderQosPolicy	QosPolicy attached to the Topic
history	HistoryQosPolicy	QosPolicy attached to the Topic
resource_limits	ResourceLimitsQosPolicy	QosPolicy attached to the Topic
ownership	OwnershipQosPolicy	QosPolicy attached to the Topic
topic_data	TopicDataQosPolicy	QosPolicy attached to the Topic

3.3 DCPSPublication

DCPSPublication 主题跟踪数据写入者的存在。它使用 PublicationBuiltinTopicData 数据类型执行此操作。Every PublicationBuiltinTopicData sample in a Domain is a representation of a DataWriter.

如果您创建一个新的 DataWriter，您将创建一个新的 PublicationBuiltinTopicData 实例。如果您删除 DataWriter，则会释放 PublicationBuiltinTopicData 实例。如果 DataWriter（或它所属的发布者）修改了应用于与其连接的实体的 QoSPolicy，则会写入更新的 PublicationBuiltinTopicData 样本。PublicationBuiltinTopicData 也会在writer 失去或恢复liveliness时更新。

当您在 DataReader 上调用 get_matched_publication_data 操作时，您实际上获得了 PublicationBuiltinTopicData samples ，这些samples 描述了 DataReader 已连接到的 Writer。

3.3.1 PublicationBuiltinTopicData 成员表

Name	Type	Description
key	BuiltinTopicKey_t	Global unique identifier of the DataWriter
participant_key	BuiltinTopicKey_t	Global unique identifier of the Participant to which the DataWriter belongs
topic_name	String	Name of the Topic used by the DataWriter
type_name	String	Type name of the Topic used by the DataWriter
durability	DurabilityQosPolicy	QosPolicy attached to the DataWriter
deadline	DeadlineQosPolicy	QosPolicy attached to the DataWriter
latency_budget	LatencyBudgetQosPolicy	QosPolicy attached to the DataWriter
liveliness	LivelinessQosPolicy	QosPolicy attached to the DataWriter
reliability	ReliabilityQosPolicy	QosPolicy attached to the DataWriter
lifespan	LifespanQosPolicy	QosPolicy attached to the DataWriter
destination_order	DestinationOrderQosPolicy	QosPolicy attached to the DataWriter
user_data	UserDataQosPolicy	QosPolicy attached to the DataWriter
ownership	OwnershipQosPolicy	QosPolicy attached to the DataWriter
ownership_strength	OwnershipStrengthQosPolicy	QosPolicy attached to the DataWriter
presentation	PresentationQosPolicy	QosPolicy attached to the Publisher to which the DataWriter belongs
partition	PartitionQosPolicy	QosPolicy attached to the Publisher to which the DataWriter belongs
topic_data	TopicDataQosPolicy	QosPolicy attached to the Topic used by the DataWriter
group_data	GroupDataQosPolicy	QosPolicy attached to the Topic used by the DataWriter

3.4 DCPSSubscription

DCPSSubscription 主题使用 SubscriptionBuiltinTopicData 数据类型来传达数据读取器的存在。域中的每个 SubscriptionBuiltinTopicData 样本represents 该域中的数据读取器。

当启用新的 DataReader 时，会创建一个新的 SubscriptionBuiltinTopicData 实例。当 DataReader 被删除时，SubscriptionBuiltinTopicData 被释放。如果 DataReader（或其所属的 Subscriber）修改了应用于与其连接的条目的 QoSPolicy，则更新 SubscriptionBuiltinTopicData 样本。

3.4.1 SubscriptionBuiltinTopicData 成员表

Name	Type	Description
key	BuiltinTopicKey_t	Global unique identifier of the DataReader
participant_key	BuiltinTopicKey_t	Global unique identifier of the Participant to which the DataReader belongs
topic_name	String	Name of the Topic used by the DataReader
type_name	String	Type name of the Topic used by the DataReader
durability	DurabilityQosPolicy	QosPolicy attached to the DataReader
deadline	DeadlineQosPolicy	QosPolicy attached to the DataReader
latency_budget	LatencyBudgetQosPolicy	QosPolicy attached to the DataReader
liveliness	LivelinessQosPolicy	QosPolicy attached to the DataReader
reliability	ReliabilityQosPolicy	QosPolicy attached to the DataReader
ownership	LifespanQosPolicy	QosPolicy attached to the DataReader
destination_order	DestinationOrderQosPolicy	QosPolicy attached to the DataReader
user_data	UserDataQosPolicy	QosPolicy attached to the DataReader
time_based_filter	TimeBasedFilterQosPolicy	QosPolicy attached to the DataReader
presentation	PresentationQosPolicy	QosPolicy attached to the Subscriber to which the DataReader belongs
partition	PartitionQosPolicy	QosPolicy attached to the Subscriber to which the DataReader belongs
topic_data	TopicDataQosPolicy	QosPolicy attached to the Topic used by the DataReader
group_data	GroupDataQosPolicy	QosPolicy attached to the Subscriber to which the DataReader belongs