DDS_DeadlineQosPolicy Struct Reference

Expresses the maximum duration (deadline) within which an instance is expected to be updated. More...

Data Fields
struct DDS_Duration_t	period
	Duration of the deadline period.

Detailed Description

Expresses the maximum duration (deadline) within which an instance is expected to be updated.

A DDS_DataReader expects a new sample updating the value of each instance at least once every period. That is, period specifies the maximum expected elapsed time between arriving data samples.

A DDS_DataWriter indicates that the application commits to write a new value (using the DDS_DataWriter) for each instance managed by the DDS_DataWriter at least once every period.

This QoS can be used during system integration to ensure that applications have been coded to meet design specifications.

It can also be used during run time to detect when systems are performing outside of design specifications. Receiving applications can take appropriate actions to prevent total system failure when data is not received in time. For topics on which data is not expected to be periodic, period should be set to an infinite value.

Entity:

DDS_Topic, DDS_DataReader, DDS_DataWriter

Status:

DDS_OFFERED_DEADLINE_MISSED_STATUS, DDS_REQUESTED_DEADLINE_MISSED_STATUS, DDS_OFFERED_INCOMPATIBLE_QOS_STATUS, DDS_REQUESTED_INCOMPATIBLE_QOS_STATUS

Properties:

RxO = YES
Changeable = YES

Usage

This policy is useful for cases where a DDS_Topic is expected to have each instance updated periodically. On the publishing side this setting establishes a contract that the application must meet. On the subscribing side the setting establishes a minimum requirement for the remote publishers that are expected to supply the data values.

When RTI Connext 'matches' a DDS_DataWriter and a DDS_DataReader it checks whether the settings are compatible (i.e., offered deadline <= requested deadline); if they are not, the two entities are informed (via the DDS_Listener or DDS_Condition mechanism) of the incompatibility of the QoS settings and communication will not occur.

Assuming that the reader and writer ends have compatible settings, the fulfilment of this contract is monitored by RTI Connext and the application is informed of any violations by means of the proper DDS_Listener or DDS_Condition.

Compatibility

The value offered is considered compatible with the value requested if and only if the inequality offered period <= requested period holds.

Consistency

The setting of the DEADLINE policy must be set consistently with that of the TIME_BASED_FILTER.

For these two policies to be consistent the settings must be such that deadline period >= minimum_separation.

An attempt to set these policies in an inconsistent manner will result in DDS_RETCODE_INCONSISTENT_POLICY in set_qos (abstract), or the DDS_Entity will not be created.

For a DDS_DataReader, the DEADLINE policy and DDS_TimeBasedFilterQosPolicy may interact such that even though the DDS_DataWriter is writing samples fast enough to fulfill its commitment to its own deadline, the DDS_DataReader may see violations of its deadline. This happens because RTI Connext will drop any samples received within the DDS_TimeBasedFilterQosPolicy::minimum_separation. To avoid triggering the DDS_DataReader's deadline, even though the matched DDS_DataWriter is meeting its own deadline, set the two QoS parameters so that:

reader deadline >= reader minimum_separation + writer deadline

See DDS_TimeBasedFilterQosPolicy for more information about the interactions between deadlines and time-based filters.

See Also

DDS_TimeBasedFilterQosPolicy

Field Documentation

struct DDS_Duration_t DDS_DeadlineQosPolicy::period

Duration of the deadline period.

[default] DDS_DURATION_INFINITE

[range] [1 nanosec, 1 year] or DDS_DURATION_INFINITE, >= DDS_TimeBasedFilterQosPolicy::minimum_separation