DDS_ReliabilityQosPolicy Struct Reference

Indicates the level of reliability offered/requested by RTI Data Distribution Service. More...

List of all members.

Public Attributes

DDS_ReliabilityQosPolicyKind kind
 Kind of reliability.
struct DDS_Duration_t max_blocking_time
 The maximum time a writer may block on a write() call.

Detailed Description

Indicates the level of reliability offered/requested by RTI Data Distribution Service.

DDSTopic, DDSDataReader, DDSDataWriter
Changeable = UNTIL ENABLE


This policy indicates the level of reliability requested by a DDSDataReader or offered by a DDSDataWriter.

The reliability of a connection between a DataWriter and DataReader is entirely user configurable. It can be done on a per DataWriter/DataReader connection. A connection may be configured to be "best effort" which means that RTI Data Distribution Service will not use any resources to monitor or guarantee that the data sent by a DataWriter is received by a DataReader.

For some use cases, such as the periodic update of sensor values to a GUI displaying the value to a person, DDS_BEST_EFFORT_RELIABILITY_QOS delivery is often good enough. It is certainly the fastest, most efficient, and least resource-intensive (CPU and network bandwidth) method of getting the newest/latest value for a topic from DataWriters to DataReaders. But there is no guarantee that the data sent will be received. It may be lost due to a variety of factors, including data loss by the physical transport such as wireless RF or even Ethernet.

However, there are data streams (topics) in which you want an absolute guarantee that all data sent by a DataWriter is received reliably by DataReaders. This means that RTI Data Distribution Service must check whether or not data was received, and repair any data that was lost by resending a copy of the data as many times as it takes for the DataReader to receive the data. RTI Data Distribution Service uses a reliability protocol configured and tuned by these QoS policies: DDS_HistoryQosPolicy, DDS_DataWriterProtocolQosPolicy, DDS_DataReaderProtocolQosPolicy, and DDS_ResourceLimitsQosPolicy.

The Reliability QoS policy is simply a switch to turn on the reliability protocol for a DataWriter/DataReader connection. The level of reliability provided by RTI Data Distribution Service is determined by the configuration of the aforementioned QoS policies.

You can configure RTI Data Distribution Service to deliver all data in the order they were sent (also known as absolute or strict reliability). Or, as a tradeoff for less memory, CPU, and network usage, you can choose a reduced level of reliability where only the last N values are guaranteed to be delivered reliably to DataReaders (where N is user-configurable). In the reduced level of reliability, there are no guarantees that the data sent before the last N are received. Only the last N data packets are monitored and repaired if necessary.

These levels are ordered, DDS_BEST_EFFORT_RELIABILITY_QOS < DDS_RELIABLE_RELIABILITY_QOS. A DDSDataWriter offering one level is implicitly offering all levels below.

Note: To send large data reliably, you will also need to set DDS_ASYNCHRONOUS_PUBLISH_MODE_QOS. Large in this context means that the data cannot be sent as a single packet by the transport (for example, data larger than 63K when using UDP/IP).

The setting of this policy has a dependency on the setting of the RESOURCE_LIMITS policy. In case the reliability kind is set to DDS_RELIABLE_RELIABILITY_QOS the write operation on the DDSDataWriter may block if the modification would cause data to be lost or else cause one of the limits in specified in the RESOURCE_LIMITS to be exceeded. Under these circumstances, the RELIABILITY max_blocking_time configures the maximum duration the write operation may block.

If the DDS_ReliabilityQosPolicy::kind is set to DDS_RELIABLE_RELIABILITY_QOS, data samples originating from a single DDSDataWriter cannot be made available to the DDSDataReader if there are previous data samples that have not been received yet due to a communication error. In other words, RTI Data Distribution Service will repair the error and resend data samples as needed in order to reconstruct a correct snapshot of the DDSDataWriter history before it is accessible by the DDSDataReader.

If the DDS_ReliabilityQosPolicy::kind is set to DDS_BEST_EFFORT_RELIABILITY_QOS, the service will not re-transmit missing data samples. However, for data samples originating from any one DataWriter the service will ensure they are stored in the DDSDataReader history in the same order they originated in the DDSDataWriter. In other words, the DDSDataReader may miss some data samples, but it will never see the value of a data object change from a newer value to an older value.

See also:



The value offered is considered compatible with the value requested if and only if the inequality offered kind >= requested kind evaluates to 'TRUE'. For the purposes of this inequality, the values of DDS_ReliabilityQosPolicy::kind are considered ordered such that DDS_BEST_EFFORT_RELIABILITY_QOS < DDS_RELIABLE_RELIABILITY_QOS.

Member Data Documentation

The maximum time a writer may block on a write() call.

This setting applies only to the case where DDS_ReliabilityQosPolicy::kind = DDS_RELIABLE_RELIABILITY_QOS. FooDataWriter::write is allowered to block if the DDSDataWriter does not have space to store the value written. Only applies to DDSDataWriter.

[default] 100 milliseconds

[range] [0,1 year] or DDS_DURATION_INFINITE

See also:

RTI Data Distribution Service C++ API Version 4.5e Copyright © 23 Oct 2011 Real-Time Innovations, Inc