ViewKey QoS

This QosPolicy is used to define a set of keys on a DataReaderView.

NOTE: This is an OpenSplice-specific QosPolicy, it is not part of the DDS Specification.

Attributes

Value	Meaning	Concerns	RxO	Changeable
A boolean: use_key_list	Controls whether the alternative key list is applied to the DataReaderView.	DataReaderView	N/A	No
A sequence of strings: key_list	A sequence of strings with one or more names of Topic fields acting as alternative keys.

This QosPolicy is used to set the key list of a DataReaderView. A DataReaderView allows a different view, defined by this key list, on the data set of the DataReader from which it is created.

Operations that operate on instances or instance handles, such as lookup_instance or get_key_value, respect the alternative key-list and work as expected. However, since the mapping of writer instances to reader instances is no longer trivial (one writer instance may now map to more than one matching reader instance and vice versa), a writer instance will no longer be able to fully determine the lifecycle of its matching reader instance, nor the value its view_state and instance_state.

In fact, the view sample will always copy the view_state and instance_state values from the reader sample to which it is slaved. If both samples preserve a 1 – 1 correspondence with respect to their originating instances (this may sometimes be the case even when an alternative keylist is provided, i.e. when one reader instance never maps to more than one view instance and vice versa) then the resulting instance_state and view_state still have a valid semantical meaning. If this 1 – 1 correspondence cannot be guaranteed, the resulting instance_state and view_state are semantically meaningless and should not be used to derive any conclusion regarding the lifecycle of a view instance.

By default, the ViewKeyQosPolicy is disabled because use_key_list is set to FALSE.

This QosPolicy is applicable to a DataReaderView only, and cannot be changed after the DataReaderView is created.