Setting Topic QosPolicies

A Topic’s QosPolicies control its behavior, or more specifically, the behavior of the DataWriters and DataReaders of the Topic. You can think of the policies as the ‘properties’ for the Topic. The DDS_TopicQos structure has the following format:

DDS_TopicQos struct  {
	DDS_TopicDataQosPolicy		topic_data; 
	DDS_DurabilityQosPolicy		durability; 
	DDS_DurabilityServiceQosPolicy	durability_service; 
	DDS_DeadlineQosPolicy		deadline; 
	DDS_LatencyBudgetQosPolicy	latency_budget; 
	DDS_LivelinessQosPolicy		liveliness; 
	DDS_ReliabilityQosPolicy	reliability; 
	DDS_DestinationOrderQosPolicy	destination_order; 
	DDS_HistoryQosPolicy		history; 
	DDS_ResourceLimitsQosPolicy	resource_limits; 
	DDS_TransportPriorityQosPolicy	transport_priority;
	DDS_LifespanQosPolicy		lifespan; 
	DDS_OwnershipQosPolicy		ownership; 
} DDS_TopicQos;

Topic QosPolicies summarizes the meaning of each policy (arranged alphabetically). For information on why you would want to change a particular QosPolicy, see the section noted in the Reference column. For defaults and valid ranges, please refer to the API Reference HTML documentation for each policy.

Configuring QoS Settings when the Topic is Created

As described in Creating Topics, there are different ways to create a Topic, depending on how you want to specify its QoS (with or without a QoS profile).

In Figure 2, we saw an example of how to create a Topic with default QosPolicies by using the special constant, DDS_TOPIC_QOS_DEFAULT, which indicates that the default QoS values for a Topic should be used. The default Topic QoS values are configured in the DomainParticipant; you can change them with the DomainParticipant’s set_default_topic_qos() or set_default_topic_qos_with_profile() operations (see Getting and Setting Default QoS for Child Entities).

To create a Topic with non-default QoS values, without using a QoS profile, use the DomainParticipant’s get_default_topic_qos() operation to initialize a DDS_TopicQos structure. Then change the policies from their default values before passing the QoS structure to create_topic().

You can also create a Topic and specify its QoS settings via a QoS profile. To do so, call create_topic_with_profile().

If you want to use a QoS profile, but then make some changes to the QoS before creating the Topic, call get_topic_qos_from_profile(), modify the QoS and use the modified QoS when calling create_topic().

Comparing QoS Values

The equals() operation compares two Topic’s DDS_TopicQoS structures for equality. It takes two parameters for the two Topics’ QoS structures to be compared, then returns TRUE is they are equal (all values are the same) or FALSE if they are not equal.

Changing QoS Settings After the Topic Has Been Created

There are two ways to change an existing Topic’s QoS after it is has been created—again depending on whether or not you are using a QoS Profile.

To change QoS programmatically (that is, without using a QoS Profile), see the example code in Figure 3. It retrieves the current values by calling the Topic’s get_qos() operation. Then it modifies the value and calls set_qos() to apply the new value. Note, however, that some QosPolicies cannot be changed after the Topic has been enabled—this restriction is noted in the descriptions of the individual QosPolicies.

You can also change a Topic’s (and all other Entities’) QoS by using a QoS Profile. For an example, see Figure 4. For more information, see Configuring QoS with XML.

Figure 3 Changing the QoS of an Existing Topic (without a QoS Profile)

DDS_TopicQos topic_qos;1For the C API, use DDS_TopicQos_INITIALIZER or DDS_TopicQos_initialize(). See Special QosPolicy Handling Considerations for C on page 1
// Get current QoS. topic points to an existing DDSTopic.
if (topic->get_qos(topic_qos) != DDS_RETCODE_OK) {
    // handle error
}
// Next, make changes.
// New ownership kind will be Exclusive
topic_qos.ownership.kind = DDS_EXCLUSIVE_OWNERSHIP_QOS;
// Set the new QoS
if (topic->set_qos(topic_qos) != DDS_RETCODE_OK ) {
    // handle error
}

retcode = topic->set_qos_with_profile(
     “FooProfileLibrary”,”FooProfile”);
if (retcode != DDS_RETCODE_OK) {
// handle error
}

© 2017 RTI