Read communication statuses are treated slightly differently than the others because they don't change independently. In other words, at least two changes will appear at the same time (DDS.DATA_ON_READERS_STATUS and DDS.DATA_AVAILABLE_STATUS) and even several of the last kind may be part of the set. This 'grouping' has to be communicated to the application.

For each plain communication status, there is a corresponding structure to hold the status value. These values contain the information related to the change of status, as well as information related to the statuses themselves (e.g., contains cumulative counts).

Changes in Status

Associated with each one of an DDS.Entity's communication status is a logical StatusChangedFlag. This flag indicates whether that particular communication status has changed since the last time the status was read by the application. The way the status changes is slightly different for the Plain Communication Status and the Read Communication status.

Changes in plain communication status

For the plain communication status, the StatusChangedFlag flag is initially set to FALSE. It becomes TRUE whenever the plain communication status changes and it is reset to DDS.BOOLEAN_FALSE each time the application accesses the plain communication status via the proper get_<plain communication status>() operation on the DDS.Entity.

The communication status is also reset to FALSE whenever the associated listener operation is called as the listener implicitly accesses the status which is passed as a parameter to the operation. The fact that the status is reset prior to calling the listener means that if the application calls the get_<plain communication status> from inside the listener it will see the status already reset.

An exception to this rule is when the associated listener is the 'nil' listener. The 'nil' listener is treated as a NOOP and the act of calling the 'nil' listener does not reset the communication status.

For example, the value of the StatusChangedFlag associated with the DDS.REQUESTED_DEADLINE_MISSED_STATUS will become TRUE each time new deadline occurs (which increases DDS.RequestedDeadlineMissedStatus.total_count). The value changes to FALSE when the application accesses the status via the corresponding DDS.DataReader.get_requested_deadline_missed_status method on the proper Entity

Changes in read communication status

For the read communication status, the StatusChangedFlag flag is initially set to FALSE. The StatusChangedFlag becomes TRUE when either a data-sample arrives or else the DDS.ViewStateKind, DDS.SampleStateKind, or DDS.InstanceStateKind of any existing sample changes for any reason other than a call to FooDataReader.read, FooDataReader.take or their variants. Specifically any of the following events will cause the StatusChangedFlag to become TRUE:

• The arrival of new data.
• A change in the DDS.InstanceStateKind of a contained instance. This can be caused by either:
  • The arrival of the notification that an instance has been disposed by:
    • the DDS.DataWriter that owns it if DDSOwnershipQosModule QoS kind= DDS.EXCLUSIVE_OWNERSHIP_QOS
    • or by any DDS.DataWriter if DDSOwnershipQosModule QoS kind= DDS.SHARED_OWNERSHIP_QOS
  • The loss of liveliness of the DDS.DataWriter of an instance for which there is no other DDS.DataWriter.
  • The arrival of the notification that an instance has been unregistered by the only DDS.DataWriter that is known to be writing the instance.

Depending on the kind of StatusChangedFlag, the flag transitions to FALSE again as follows:

• The DDS.DATA_AVAILABLE_STATUS StatusChangedFlag becomes FALSE when either the corresponding listener operation (on_data_available) is called or the read or take operation (or their variants) is called on the associated DDS.DataReader.
• The DDS.DATA_ON_READERS_STATUS StatusChangedFlag becomes FALSE when any of the following events occurs:
  • The corresponding listener operation (on_data_on_readers) is called.
  • The on_data_available listener operation is called on any DDS.DataReader belonging to the DDS.Subscriber.
  • The read or take operation (or their variants) is called on any DDS.DataReader belonging to the DDS.Subscriber.

\image html DDSDataAvailableStatus.png

See also: DDS.Listener
See also: DDS.WaitSet, DDS.Condition