Specifies whether it is allowed for multiple DDSDataWriter (s) to write the same instance of the data and if so, how these modifications should be arbitrated. More...

Public Attributes
DDS_OwnershipQosPolicyKind	kind
	The kind of ownership.

Detailed Description

Specifies whether it is allowed for multiple DDSDataWriter (s) to write the same instance of the data and if so, how these modifications should be arbitrated.

Entity:: DDSTopic, DDSDataReader, DDSDataWriter

Status:: DDS_OFFERED_INCOMPATIBLE_QOS_STATUS, DDS_REQUESTED_INCOMPATIBLE_QOS_STATUS

Properties:: RxO = YES
Changeable = UNTIL ENABLE

See Also: OWNERSHIP_STRENGTH

Usage

Along with the OWNERSHIP_STRENGTH, this QoS policy specifies if DDSDataReader entities can receive updates to the same instance (identified by its key) from multiple DDSDataWriter entities at the same time.

There are two kinds of ownership, selected by the setting of the kind: SHARED and EXCLUSIVE.

SHARED ownership

DDS_SHARED_OWNERSHIP_QOS indicates that RTI Connext does not enforce unique ownership for each instance. In this case, multiple writers can update the same data type instance. The subscriber to the DDSTopic will be able to access modifications from all DDSDataWriter objects, subject to the settings of other QoS that may filter particular samples (e.g. the TIME_BASED_FILTER or HISTORY policy). In any case, there is no "filtering" of modifications made based on the identity of the DDSDataWriter that causes the modification.

EXCLUSIVE ownership

DDS_EXCLUSIVE_OWNERSHIP_QOS indicates that each instance of a data type can only be modified by one DDSDataWriter. In other words, at any point in time, a single DDSDataWriter owns each instance and is the only one whose modifications will be visible to the DDSDataReader objects. The owner is determined by selecting the DDSDataWriter with the highest value of the DDS_OwnershipStrengthQosPolicy::value that is currently alive, as defined by the LIVELINESS policy, and has not violated its DEADLINE contract with regards to the data instance.

Ownership can therefore change as a result of:

a DDSDataWriter in the system with a higher value of the strength that modifies the instance,
a change in the strength value of the DDSDataWriter that owns the instance, and
a change in the liveliness of the DDSDataWriter that owns the instance.
a deadline with regards to the instance that is missed by the DDSDataWriter that owns the instance.

The behavior of the system is as if the determination was made independently by each DDSDataReader. Each DDSDataReader may detect the change of ownership at a different time. It is not a requirement that at a particular point in time all the DDSDataReader objects for that DDSTopic have a consistent picture of who owns each instance.

It is also not a requirement that the DDSDataWriter objects are aware of whether they own a particular instance. There is no error or notification given to a DDSDataWriter that modifies an instance it does not currently own.

The requirements are chosen to (a) preserve the decoupling of publishers and subscriber, and (b) allow the policy to be implemented efficiently.

It is possible that multiple DDSDataWriter objects with the same strength modify the same instance. If this occurs RTI Connext will pick one of the DDSDataWriter objects as the owner. It is not specified how the owner is selected. However, the algorithm used to select the owner guarantees that all DDSDataReader objects will make the same choice of the particular DDSDataWriter that is the owner. It also guarantees that the owner remains the same until there is a change in strength, liveliness, the owner misses a deadline on the instance, or a new DDSDataWriter with higher same strength, or a new DDSDataWriter with same strength that should be deemed the owner according to the policy of the Service, modifies the instance.

Exclusive ownership is on an instance-by-instance basis. That is, a subscriber can receive values written by a lower strength DDSDataWriter as long as they affect instances whose values have not been set by the higher-strength DDSDataWriter.

Compatibility

The value of the DDS_OwnershipQosPolicyKind offered must exactly match the one requested or else they are considered incompatible.

Relationship between registration, liveliness and ownership \endif <P> The need for registering/unregistering instances stems from two use cases: \li Ownership resolution on redundant systems <P> \li Detection of loss in topological connectivity <P> These two use cases also illustrate the semantic differences between the ::FooDataWriter::unregister_instance and ::FooDataWriter::dispose. <P> \ifnot BROKEN_SECTIONS @subsection Ownership_redundant_systems Ownership Resolution on Redundant Systems

It is expected that users may use DDS to set up redundant systems where multiple DDSDataWriter entities are "capable" of writing the same instance. In this situation, the DDSDataWriter entities are configured such that:

Either both are writing the instance "constantly"

Or else they use some mechanism to classify each other as "primary" and "secondary", such that the primary is the only one writing, and the secondary monitors the primary and only writes when it detects that the primary "writer" is no longer writing.

Both cases above use the DDS_EXCLUSIVE_OWNERSHIP_QOS and arbitrate themselves by means of the DDS_OwnershipStrengthQosPolicy. Regardless of the scheme, the desired behavior from the DDSDataReader point of view is that DDSDataReader normally receives data from the primary unless the "primary" writer stops writing, in which case the DDSDataReader starts to receive data from the secondary DDSDataWriter.

This approach requires some mechanism to detect that a DDSDataWriter (the primary) is no longer "writing" the data as it should. There are several reasons why this may happen and all must be detected (but not necessarily distinguished):

[crash] The writing process is no longer running (e.g. the whole application has crashed)

[connectivity loss] Connectivity to the writing application has been lost (e.g. network disconnection)

[application fault] The application logic that was writing the data is faulty and has stopped calling FooDataWriter::write.

Arbitrating from a DDSDataWriter to one of a higher strength is simple and the decision can be taken autonomously by the DDSDataReader. Switching ownership from a higher strength DDSDataWriter to one of a lower strength DDSDataWriter requires that the DDSDataReader can make a determination that the stronger DDSDataWriter is "no longer writing the instance".

Member Data Documentation

DDS_OwnershipQosPolicyKind DDS_OwnershipQosPolicy::kind

The kind of ownership.