Configures the ability of a DDS_DataWriter to publish samples in response to a DDS_TopicQuery. More...

Data Fields
DDS_Boolean	enable
	Allows this writer to dispatch TopicQueries. More...

struct DDS_Duration_t	publication_period
	Sets the periodic interval at which samples are published. More...

DDS_Long	samples_per_period
	Sets the maximum number of samples to publish in each publication_period. More...

Detailed Description

Configures the ability of a DDS_DataWriter to publish samples in response to a DDS_TopicQuery.

Enables the ability of a DDS_DataWriter to publish historical samples upon reception of a DDS_TopicQuery and how often they are published.

Since a TopicQuery selects previously written samples, the DataWriter must have a DDS_DurabilityQosPolicy::kind different from DDS_VOLATILE_DURABILITY_QOS. Also, DDS_ReliabilityQosPolicy::kind must be set to DDS_RELIABLE_RELIABILITY_QOS.

Only samples that fall within the DDS_DurabilityQosPolicy::writer_depth for an instance are evaluated against the TopicQuery filter. While the DataWriter is waiting for acknowledgements from one or more DataReaders, there may temporarily be more than DDS_DurabilityQosPolicy::writer_depth samples per instance in the DataWriter's queue if the DDS_HistoryQosPolicy::depth is set to a higher value than DDS_DurabilityQosPolicy::writer_depth. Those additional samples past DDS_DurabilityQosPolicy::writer_depth are not eligible to be sent in response to the TopicQuery.

A TopicQuery may select multiple samples at once. The writer will publish them periodically, independently from newly written samples. DDS_TopicQueryDispatchQosPolicy::publication_period configures the frequency of that period and DDS_TopicQueryDispatchQosPolicy::samples_per_period configures the maximum number of samples to publish each period.

If the DataWriter blocks during the publication of one of these samples, it will stop and try again the next period. (See FooDataWriter_write for the conditions that may cause the write operation to block.)

All the DataWriters that belong to a single DDS_Publisher and enable TopicQueries share the same event thread, but each DataWriter schedules separate events. To configure that thread, see DDS_AsynchronousPublisherQosPolicy::topic_query_publication_thread.

If the DataWriter is dispatching more than one TopicQuery at the same time, the configuration of this periodic event applies to all of them. For example, if a DataWriter receives two TopicQueries around the same time, the period is 1 second, the number of samples per period is 10, the first TopicQuery selects 5 samples, and the second one selects 8, the DataWriter will immediately attempt to publish all 5 for the first TopicQuery and 5 for the second one. After one second, it will publish the remaining 3 samples.