|
RTI Connext C API Version 7.4.0
|
DDS_TopicQuery and associated elements. More...
Data Structures | |
| struct | DDS_TopicQuerySelection |
| <<extension>> Specifies the data query that defines a DDS_TopicQuery More... | |
| struct | DDS_TopicQueryData |
| <<extension>> Provides information about a DDS_TopicQuery More... | |
Typedefs | |
| typedef struct DDS_TopicQuerySelection | DDS_TopicQuerySelection |
| <<extension>> Specifies the data query that defines a DDS_TopicQuery More... | |
| typedef struct DDS_TopicQueryImpl | DDS_TopicQuery |
| <<extension>> Allows a DDS_DataReader to query the sample cache of its matching DDS_DataWriter. More... | |
| typedef struct DDS_TopicQueryData | DDS_TopicQueryData |
| <<extension>> Provides information about a DDS_TopicQuery More... | |
Enumerations | |
| enum | DDS_TopicQuerySelectionKind { DDS_TOPIC_QUERY_SELECTION_KIND_HISTORY_SNAPSHOT , DDS_TOPIC_QUERY_SELECTION_KIND_CONTINUOUS } |
| Kinds of TopicQuerySelection. More... | |
Functions | |
| DDS_Boolean | DDS_TopicQueryHelper_topic_query_data_from_service_request (struct DDS_TopicQueryData *topic_query_data, const struct DDS_ServiceRequest *service_request) |
| Retrieves the DDS_TopicQueryData data contained in the specified DDS_ServiceRequest. More... | |
| DDS_ReturnCode_t | DDS_TopicQuery_get_guid (DDS_TopicQuery *self, DDS_GUID_t *guid) |
| Get the TopicQuery's GUID. More... | |
Variables | |
| const struct DDS_TopicQuerySelection | DDS_TOPIC_QUERY_SELECTION_USE_READER_CONTENT_FILTER |
| Special value for creating a DDS_TopicQuery that applies the same filter as the DataReader's DDS_ContentFilteredTopic. More... | |
| const struct DDS_TopicQuerySelection | DDS_TOPIC_QUERY_SELECTION_SELECT_ALL |
| Special value for creating a DDS_TopicQuery that selects all the samples in a DDS_DataWriter cache. More... | |
DDS_TopicQuery and associated elements.
TopicQueries allow a DDS_DataReader to query the sample cache of its matching DDS_DataWriter. A user can create a DDS_TopicQuery with the DDS_DataReader_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 DDS_DataReader_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's queue at the time the DataWriter received the TopicQuery. These TopicQueries are snapshot TopicQueries. However, a TopicQuery can be created in "continuous" mode; in this case, a DataWriter will continue delivering samples that pass a continuous TopicQuery filter until the DataReader application explicitly deletes it. These TopicQueries are continuous TopicQueries.
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 DDS_DataReader_create_querycondition_w_params and DDS_DataReader_create_readcondition_w_params APIs.
Each TopicQuery is identified by a GUID that can be accessed using the DDS_TopicQuery_get_guid method.
A snapshot TopicQuery queries the samples that are in a DataWriter's queue when the DataWriter receives the TopicQuery.
When a DataWriter receives a snapshot TopicQuery, it registers the last sequence number available in the DataWriter queue (known as the cut-off SN). Then, the DataWriter applies the TopicQuery expression to all the samples up to the cut-off SN. If a sample with a SN smaller than or equal to the cut-off SN passes the filter expression, it will be included in the TopicQuery response.
When a DataWriter responds to a snapshot TopicQuery, it doesn't create any copies of the samples that are part of the response. As a result, a sample that passes the TopicQuery expression and should have been included in the response may be removed from the DataWriter queue before it reaches the DataReader that issued the TopicQuery. This can result in incomplete TopicQueries, which may or may not be necessary for your application to detect.
For instance, your application may issue a TopicQuery on a keyed topic using a filter on key-only fields and expect to receive the latest value for an instance that passes the filter. However, if the latest value for the instance is replaced with a newer value before the sample is delivered to the DataReader, it is important to detect this situation and re-issue the TopicQuery.
On the other hand, some applications may not care about which samples are returned in response to a TopicQuery, and are only interested in a snapshot of the DataWriter's queue. In such cases, the concept of incomplete TopicQueries can be ignored.
The application can detect incomplete TopicQueries by checking the DDS_INCOMPLETE_SNAPSHOT_TOPIC_QUERY bit in the DDS_SampleInfo::flag field of the last sample received for a snapshot TopicQuery.
To reduce the probability of receiving incomplete responses, DataWriters configured with DDS_KEEP_LAST_HISTORY_QOS recalculate the cut-off SN for a snapshot TopicQuery if a sample is replaced in the DataWriter queue due to KEEP_LAST replacement before the complete TopicQuery response is published.
Snapshot TopicQueries are not automatically deleted once the DataWriters that respond to the TopicQuery have finished delivering the samples included in the response. It is the responsibility of the user to delete the TopicQuery when it is no longer needed. To detect the end of a snapshot TopicQuery response, check for the DDS_INTERMEDIATE_TOPIC_QUERY_SAMPLE flag in the DDS_SampleInfo::flag field of the samples included in the response. For more information on how to use this flag, see Reading TopicQuery Samples.
A TopicQuery response is the set of samples published by a DataWriter in response to a TopicQuery. Because multiple DataWriters may respond to a TopicQuery, a DataReader may receive multiple responses for a single TopicQuery.
Responses can be proxied by Routing Service DataWriters by setting <topic_query_proxy>/<mode> to PROPAGATION. These DataWriters forward a TopicQuery response, but they are not considered to be responding to the TopicQuery.
Samples that a DataReader receives in response to a TopicQuery have a value for DDS_SampleInfo::topic_query_guid different than DDS_GUID_UNKNOWN.
In addition, the virtual GUID of the DataWriter(s) that responded to a TopicQuery is set in DDS_SampleInfo::source_guid.
Intermediate samples, not the final sample, have the flag DDS_INTERMEDIATE_TOPIC_QUERY_SAMPLE set in DDS_SampleInfo::flag for a given response to a snapshot TopicQuery.
If two DataWriters respond to a snapshot TopicQuery, you should expect two final samples, one per unique DDS_SampleInfo::source_guid responding to the TopicQuery, assuming that at least one sample passes the TopicQuery filter for each DataWriter.
The DDS_SampleInfo::flag field of the last sample received for a snapshot TopicQuery may have the DDS_INCOMPLETE_SNAPSHOT_TOPIC_QUERY bit set if the TopicQuery response is incomplete, which may or may not be necessary for your application to detect. See Snapshot TopicQueries for more information.
There are a number of ways in which to gain more insight into what is happening in an application that is creating Topic Queries.
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 DDS_Subscriber_lookup_datareader. The topic name is DDS_SERVICE_REQUEST_TOPIC_NAME. Installing a listener with the DDS_DataReaderListener::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 DDS_TopicQuery will be DDS_TOPIC_QUERY_SERVICE_REQUEST_ID and the DDS_ServiceRequest::instance_id will be equal to the GUID of the DDS_TopicQuery (see DDS_TopicQuery_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 DDS_TopicQueryHelper_topic_query_data_from_service_request function. The resulting DDS_TopicQueryData contains the DDS_TopicQuerySelection that the DDS_TopicQuery 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.
It is possible that a DDS_ServiceRequest for a DDS_TopicQuery 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 DDS_DataWriterListener::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.
| typedef struct DDS_TopicQuerySelection DDS_TopicQuerySelection |
<<extension>> Specifies the data query that defines a DDS_TopicQuery
The query format is similar to the expression and parameters of a DDS_QueryCondition or a DDS_ContentFilteredTopic, as described in DDS_DomainParticipant_create_contentfilteredtopic_with_filter.
There are two special queries:
| typedef struct DDS_TopicQueryImpl DDS_TopicQuery |
<<extension>> Allows a DDS_DataReader to query the sample cache of its matching DDS_DataWriter.
| typedef struct DDS_TopicQueryData DDS_TopicQueryData |
<<extension>> Provides information about a DDS_TopicQuery
Contains the information about a TopicQuery that can be retrieved using DDS_TopicQueryHelper_topic_query_data_from_service_request.
Kinds of TopicQuerySelection.
| Enumerator | |
|---|---|
| DDS_TOPIC_QUERY_SELECTION_KIND_HISTORY_SNAPSHOT | Indicates that the DDS_TopicQuery may only select samples that were in the DataWriter cache upon reception.
|
| DDS_TOPIC_QUERY_SELECTION_KIND_CONTINUOUS | Indicates that the DDS_TopicQuery may continue selecting samples published after its reception.
|
| DDS_Boolean DDS_TopicQueryHelper_topic_query_data_from_service_request | ( | struct DDS_TopicQueryData * | topic_query_data, |
| const struct DDS_ServiceRequest * | service_request | ||
| ) |
Retrieves the DDS_TopicQueryData data contained in the specified DDS_ServiceRequest.
This operation will extract the content from the request body of the DDS_ServiceRequest and place it into the specified DDS_TopicQueryData object.
The specified DDS_ServiceRequest must be a valid sample associated with the service id DDS_TOPIC_QUERY_SERVICE_REQUEST_ID. Otherwise this operation will return DDS_BOOLEAN_FALSE.
This operation can be called within the context of a DDS_DataWriterListener::on_service_request_accepted to retrieve the DDS_TopicQueryData of a DDS_ServiceRequest that has been received with service id DDS_TOPIC_QUERY_SERVICE_REQUEST_ID
| topic_query_data | <<inout>> A DDS_TopicQueryData object where the content from the service request is extracted. Cannot be NULL. |
| service_request | <<in>> Input DDS_ServiceRequest that contains the DDS_TopicQueryData as part of its request body. Cannot be NULL. |
| DDS_ReturnCode_t DDS_TopicQuery_get_guid | ( | DDS_TopicQuery * | self, |
| DDS_GUID_t * | guid | ||
| ) |
Get the TopicQuery's GUID.
| self | <<in>> Cannot be NULL. |
| guid | <<out>> The GUID of the DDS_TopicQuery. Cannot be NULL. |
|
extern |
Special value for creating a DDS_TopicQuery that applies the same filter as the DataReader's DDS_ContentFilteredTopic.
If the DDS_DataReader that creates the DDS_TopicQuery uses a DDS_ContentFilteredTopic, this DDS_TopicQuerySelection value indicates that the TopicQuery should use the same filter.
Otherwise it behaves as DDS_TOPIC_QUERY_SELECTION_SELECT_ALL.
|
extern |
Special value for creating a DDS_TopicQuery that selects all the samples in a DDS_DataWriter cache.