DDSTopicQuery and associated elements. More...

Classes
struct	DDS_TopicQuerySelection
	<<extension>> Specifies the data query that defines a DDSTopicQuery More...

struct	DDS_TopicQueryData
	<<extension>> Provides information about a DDSTopicQuery More...

class	DDSTopicQuery
	<<extension>> Allows a DDSDataReader to query the sample cache of its matching DDSDataWriter. More...

class	DDSTopicQueryHelper
	Helpers to provide utility operations related to DDSTopicQuery. More...

Typedefs
typedef struct DDS_TopicQuerySelection	DDS_TopicQuerySelection
	<<extension>> Specifies the data query that defines a DDSTopicQuery More...

typedef struct DDS_TopicQueryData	DDS_TopicQueryData
	<<extension>> Provides information about a DDSTopicQuery More...

Enumerations
enum	DDS_TopicQuerySelectionKind { DDS_TOPIC_QUERY_SELECTION_KIND_HISTORY_SNAPSHOT , DDS_TOPIC_QUERY_SELECTION_KIND_CONTINUOUS }
	Kinds of TopicQuerySelection. More...

Variables
const struct DDS_TopicQuerySelection	DDS_TOPIC_QUERY_SELECTION_USE_READER_CONTENT_FILTER
	Special value for creating a DDSTopicQuery that applies the same filter as the DataReader's DDSContentFilteredTopic. More...

const struct DDS_TopicQuerySelection	DDS_TOPIC_QUERY_SELECTION_SELECT_ALL
	Special value for creating a DDSTopicQuery that selects all the samples in a DDSDataWriter cache. More...

Detailed Description

DDSTopicQuery and associated elements.

TopicQueries allow a DDSDataReader to query the sample cache of its matching DDSDataWriter. A user can create a DDSTopicQuery with the DDSDataReader::create_topic_query API. When a DataReader
creates a TopicQuery, DDS will propagate it to other DomainParticipants and their DataWriters. When a DataWriter matching with the DataReader that created the TopicQuery receives it, it will send the cached samples that pass the TopicQuery's filter.

To configure how to dispatch a TopicQuery, the DDS_DataWriterQos includes the DDS_TopicQueryDispatchQosPolicy policy. By default, a DataWriter ignores TopicQueries unless they are explicitly enabled using this policy.

The delivery of TopicQuery samples occurs in a separate RTPS channel. This allows DataReaders to receive TopicQuery samples and live samples in parallel. This is a key difference with respect to the Durability QoS policy.

Late-joining DataWriters will also discover existing TopicQueries. To delete a TopicQuery you must use DDSDataReader::delete_topic_query.

After deleting a TopicQuery, new DataWriters won't discover it and existing DataWriters currently publishing cached samples may stop before delivering all of them.

By default, a TopicQuery queries the samples that were in the DataWriter cache at the time the DataWriter receives the TopicQuery. However a TopicQuery can be created in a "continuous" mode. A DataWriter will continue delivering samples that pass a continuous TopicQuery filter until the DataReader application explicitly deletes it.

The samples received in response to a TopicQuery are stored in the associated DataReader's cache. Any of the read/take operations can retrieve TopicQuery samples. The field DDS_SampleInfo::topic_query_guid associates each sample to its TopicQuery. If the read sample is not in response to a TopicQuery then this field will be DDS_GUID_UNKNOWN. Note that the same data may be received several times, depending on how many TopicQueries the DataReader creates and their TopicQuerySelection.

You can choose to read or take only TopicQuery samples, only live samples, or both. To support this ReadConditions and QueryConditions provide the DDSDataReader::create_querycondition_w_params and DDSDataReader::create_readcondition_w_params APIs.

Each TopicQuery is identified by a GUID that can be accessed using the DDSTopicQuery::get_guid method.

Debugging Topic Queries

There are a number of ways in which to gain more insight into what is happening in an application that is creating Topic Queries.

The Built-in ServiceRequest DataReader

TopicQueries are communicated to publishing applications through a built-in DDS_ServiceRequest channel. The ServiceRequest channel is designed to be generic so that it can be used for many different purposes, one of which is TopicQueries.

When a DataReader creates a TopicQuery, a DDS_ServiceRequest message is sent containing the TopicQuery information. Just as there are built-in DataReaders for DDS_ParticipantBuiltinTopicData, DDS_SubscriptionBuiltinTopicData, and DDS_PublicationBuiltinTopicData, there is a fourth built-in DataReader for ServiceRequests.

The new built-in DataReader can be retrieved using the built-in subscriber and DDSSubscriber::lookup_datareader. The topic name is DDS_SERVICE_REQUEST_TOPIC_NAME. Installing a listener with the DDSDataReaderListener::on_data_available callback implemented will allow a publishing application to be notified whenever a TopicQuery has been received from a subscribing application.

The DDS_ServiceRequest::service_id of a DDS_ServiceRequest corresponding to a DDSTopicQuery will be DDS_TOPIC_QUERY_SERVICE_REQUEST_ID and the DDS_ServiceRequest::instance_id will be equal to the GUID of the DDSTopicQuery (see DDSTopicQuery::get_guid).

The DDS_ServiceRequest::request_body is a sequence of bytes containing more information about the TopcQuery. This information can be retrieved using the DDSTopicQueryHelper::topic_query_data_from_service_request function. The resulting DDS_TopicQueryData contains the DDS_TopicQuerySelection that the DDSTopicQuery was created with, the GUID of the original DataReader that created the TopicQuery, and the topic name of that DataReader. Note: When TopicQueries are propagated through one or more Routing Services, the last DataReader that issued the TopicQuery will be a Routing Service DataReader. The DDS_TopicQueryData::original_related_reader_guid, however, will be that of the first DataReader to have created the TopicQuery.

The on_service_request_accepted DataWriter Listener Callback

It is possible that a DDS_ServiceRequest for a DDSTopicQuery is received but is not immediately dispatched to a DataWriter. This can happen, for example, if a DataWriter was not matching with a DataReader at the time that the TopicQuery was received by the publishing application. The DDSDataWriterListener::on_service_request_accepted callback notifies a DataWriter when a ServiceRequest has been dispatched to that DataWriter. The DDS_ServiceRequestAcceptedStatus provides information about how many ServiceRequests have been accepted by the DataWriter since the last time that the status was read. The status also includes the DDS_ServiceRequestAcceptedStatus::last_request_handle, which is the DDS_InstanceHandle_t of the last ServiceRequest that was accepted. This instance handle can be used to read samples per instance from the built-in ServiceRequest DataReader and correlate which ServiceRequests have been dispatched to which DataWriters.

Reading TopicQuery Samples

Data samples that are received by a DataReader in response to a TopicQuery can be identified with two pieces of information from the corresponding DDS_SampleInfo to the sample. First, if the DDS_SampleInfo::topic_query_guid is not equal to DDS_GUID_UNKNOWN then the sample is in response to the TopicQuery with that GUID (see DDSTopicQuery::get_guid). Second, if the sample is in response to a TopicQuery and the DDS_SampleInfo::flag DDS_INTERMEDIATE_TOPIC_QUERY_SAMPLE flag is set then this is not the last sample in response to the TopicQuery for a DataWriter identified by DDS_SampleInfo::original_publication_virtual_guid. If that flag is not set then there will be no more samples corresponding to that TopicQuery coming from the DataWriter.

Typedef Documentation

◆ DDS_TopicQuerySelection

typedef struct DDS_TopicQuerySelection DDS_TopicQuerySelection

<<extension>> Specifies the data query that defines a DDSTopicQuery

The query format is similar to the expression and parameters of a DDSQueryCondition or a DDSContentFilteredTopic, as described in DDSDomainParticipant::create_contentfilteredtopic_with_filter.

There are two special queries:

See also: Queries and Filters Syntax

◆ DDS_TopicQueryData

typedef struct DDS_TopicQueryData DDS_TopicQueryData

<<extension>> Provides information about a DDSTopicQuery

Contains the information about a TopicQuery that can be retrieved using DDSTopicQueryHelper::topic_query_data_from_service_request.

See also: DDSTopicQueryHelper::topic_query_data_from_service_request

Enumeration Type Documentation

◆ DDS_TopicQuerySelectionKind

enum DDS_TopicQuerySelectionKind

Kinds of TopicQuerySelection.

See also: DDS_TopicQuerySelection

Enumerator

DDS_TOPIC_QUERY_SELECTION_KIND_HISTORY_SNAPSHOT

Indicates that the DDSTopicQuery may only select samples that were in the DataWriter cache upon reception.

[default]

DDS_TOPIC_QUERY_SELECTION_KIND_CONTINUOUS

Indicates that the DDSTopicQuery may continue selecting samples published after its reception.

The subscribing application must explicitly delete the TopicQuery (see DDSDataReader::delete_topic_query) to signal the DataWriters to stop publishing samples for it.

Variable Documentation

◆ DDS_TOPIC_QUERY_SELECTION_USE_READER_CONTENT_FILTER

const struct DDS_TopicQuerySelection DDS_TOPIC_QUERY_SELECTION_USE_READER_CONTENT_FILTER

extern

Special value for creating a DDSTopicQuery that applies the same filter as the DataReader's DDSContentFilteredTopic.

If the DDSDataReader that creates the DDSTopicQuery uses a DDSContentFilteredTopic, this DDS_TopicQuerySelection value indicates that the TopicQuery should use the same filter.

Otherwise it behaves as DDS_TOPIC_QUERY_SELECTION_SELECT_ALL.

◆ DDS_TOPIC_QUERY_SELECTION_SELECT_ALL

const struct DDS_TopicQuerySelection DDS_TOPIC_QUERY_SELECTION_SELECT_ALL