DDS_AvailabilityQosPolicy Struct Reference
[AVAILABILITY]

Configures the availability of data. More...

List of all members.

Public Attributes
struct DDS_Duration_t	max_data_availability_waiting_time
	Defines how much time to wait before delivering a sample to the application without having received some of the previous samples.
struct DDS_Duration_t	max_endpoint_availability_waiting_time
	Defines how much time to wait to discover DataWriters providing samples for the same data source (virtual GUID).
struct DDS_EndpointGroupSeq	required_matched_endpoint_groups
	A sequence of endpoint groups.

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Detailed Description

Configures the availability of data.

Entity:

DDSDataReader

Properties:

RxO = NO
Changeable = NO

Usage

This QoS policy is used in the context of the Collaborative DataWriters feature.

Collaborative DataWriters

The Collaborative DataWriters feature allows having multiple DataWriters publishing samples from a common logical data source. The DataReaders will combine the samples coming from the DataWriters in order to reconstruct the correct order at the source.

This QoS policy allows you to configure the ordering and combination process in the DataReader and can be used to support two different use cases:

• Ordered delivery of samples with RTI Persistence Service: When a late-joining DataReader configured with DDS_DurabilityQosPolicy set to DDS_PERSISTENT_DURABILITY_QOS or DDS_TRANSIENT_DURABILITY_QOS joins a DDS domain, it will start receiving historical samples from multiple DataWriters. For example, if the original DataWriter is still alive, the newly created DataReader will receive samples from the original DataWriter and one or more RTI Persistence Service DataWriters (PRSTDataWriters). This policy can be used to configure the sample ordering process on the DataReader.

• Ordered delivery of samples with Group Ordered Access: This policy can also be used to configure the sample ordering process when the Subscriber is configured with DDS_PresentationQosPolicy access_scope set to DDS_GROUP_PRESENTATION_QOS. In this case, the Subscriber must deliver in order the samples published by a group of DataWriters that belong to the same Publisher and have access_scope set to DDS_GROUP_PRESENTATION_QOS.

Each sample published in a DDS domain for a given logical data source is uniquely identified by a pair (virtual GUID, virtual sequence number). Samples from the same data source (same virtual GUID) can be published by different DataWriters. A DataReader will deliver a sample (VGUIDn, VSNm) to the application if one of the following conditions is satisfied:

• (VGUIDn, VSNm-1) has already been delivered to the application.
• All the known DataWriters publishing VGUIDn have announced that they do not have (VGUIDn, VSNm-1).
• None of the known DataWriters publishing GUIDn have announced potential availability of (VGUIDn, VSNm-1) and both timeouts in this QoS policy have expired.

A DataWriter announces potential availability of samples by using virtual heartbeats (HBs).

When DDS_PresentationQosPolicy::access_scope is set to DDS_TOPIC_PRESENTATION_QOS or DDS_INSTANCE_PRESENTATION_QOS, the virtual HB contains information about the samples contained in the DDSDataWriter history.

When DDS_PresentationQosPolicy::access_scope is set to DDS_GROUP_PRESENTATION_QOS, the virtual HB contains information about all DataWriters in the DDSPublisher.

The frequency at which virtual HBs are sent is controlled by the protocol parameters DDS_RtpsReliableWriterProtocol_t::virtual_heartbeat_period and DDS_RtpsReliableWriterProtocol_t::samples_per_virtual_heartbeat.

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Member Data Documentation

struct DDS_Duration_t DDS_AvailabilityQosPolicy::max_data_availability_waiting_time [read]

Defines how much time to wait before delivering a sample to the application without having received some of the previous samples.

A sample identified by (GUIDn, SNm) will be delivered to the application if this timeout expires for the sample and the following two conditions are satisfied:

• None of the known DataWriters publishing GUIDn have announced potential availability of (GUIDn, SNm-1).
• The DataWriters for all the endpoint groups specified in required_matched_endpoint_groups have been discovered or max_endpoint_availability_waiting_time has expired.

[default] DDS_DURATION_AUTO (DDS_DURATION_INFINITE for DDS_GROUP_PRESENTATION_QOS. Otherwise, 0 seconds)

[range] [0, DDS_DURATION_INFINITE], DDS_DURATION_AUTO

struct DDS_Duration_t DDS_AvailabilityQosPolicy::max_endpoint_availability_waiting_time [read]

Defines how much time to wait to discover DataWriters providing samples for the same data source (virtual GUID).

The set of endpoint groups that are required to provide samples for a data source can be configured using required_matched_endpoint_groups.

A non-consecutive sample identified by (GUIDn, SNm) cannot be delivered to the application unless DataWriters for all the endpoint groups in required_matched_endpoint_groups are discovered or this timeout expires.

[default] DDS_DURATION_AUTO (DDS_DURATION_INFINITE for DDS_GROUP_PRESENTATION_QOS. Otherwise, 0 seconds)

[range] [0, DDS_DURATION_INFINITE], DDS_DURATION_AUTO

struct DDS_EndpointGroupSeq DDS_AvailabilityQosPolicy::required_matched_endpoint_groups [read]

A sequence of endpoint groups.

In the context of Collaborative DataWriters, it specifies the set of endpoint groups that are expected to provide samples for the same data source.

The quorum count in a group represents the number of DataWriters that must be discovered for that group before the DataReader is allowed to provide non consecutive samples to the application.

A DataWriter becomes a member of an endpoint group by configuring the role_name in DDS_DataWriterQos::publication_name.

[default] Empty sequence