Topic Filters
A QoS profile may contain several writer, reader and topic QoSs. Connext DDS will select a QoS based on the evaluation of a filter expression on the topic name. The filter expression is specified as an attribute in the XML QoS definition. For example:
<qos_profile name="StrictReliableCommunicationProfile"> <datawriter_qos topic_filter="A*"> <history> <kind>KEEP_ALL_HISTORY_QOS</kind> </history> <reliability> <kind>RELIABLE_RELIABILITY_QOS</kind> </reliability> </datawriter_qos> <datawriter_qos topic_filter="B*"> <history> <kind>KEEP_ALL_HISTORY_QOS</kind> </history> <reliability> <kind>RELIABLE_RELIABILITY_QOS</kind> </reliability> <resource_limits> <max_samples>128</max_samples> <max_samples_per_instance>128 </max_samples_per_instance> <initial_samples>128</initial_samples> <max_instances>1</max_instances> <initial_instances>1</initial_instances> </resource_limits> </datawriter_qos> ... </qos_profile>
If topic_filter is not specified in a QoS, Connext DDS will assume the filter '*'. The QoSs with an explicit topic_filter attribute definition will be evaluated in order; they have precedence over a QoS without a topic_filter expression.
The topic_filter attribute is only used with the following APIs:
DomainParticipantFactory:
- get_<entity>_qos_from_profile_w_topic_name() (where <entity> may be topic, datareader, or datareader; see Getting QoS Values from a QoS Profile)
DomainParticipant:
- create_datawriter_with_profile() (see Creating DataWriters)
- create_datareader_with_profile() (see Creating DataReaders
- create_topic_with_profile() (see Creating Topics)
Publisher:
- create_datawriter_with_profile() (see Creating DataWriters)
Subscriber:
- create_datareader_with_profile() (see Creating DataReaders)
Topic:
- set_qos_with_profile() (see Setting Topic QosPolicies)
DataWriter:
- set_qos_with_profile() (see Changing QoS Settings After the Publisher Has Been Created)
DataReader:
- set_qos_with_profile() (see Setting DataReader QosPolicies)
Other APIs will ignore QoSs with a topic_filter value different than "*". A QoS Profile with QoSs using topic_filter can also inherits from other QoS Profiles. In this case, inheritance will consider the value of the topic_filter expression.
Example 1:
<qos_library name=”Library”>
<qos_profile name="BaseProfile">
<datawriter_qos>
...
</datawriter_qos>
<datawriter_qos topic_filter="T1*">
...
</datawriter_qos>
<datawriter_qos topic_filter="T2*">
...
</datawriter_qos>
</qos_profile>
<qos_profile name="DerivedProfile" base_name="BaseProfile">
<datawriter_qos topic_filter="T11">
...
</datawriter_qos>
<datawriter_qos topic_filter="T21">
...
</datawriter_qos>
<datawriter_qos topic_filter="T31">
...
</datawriter_qos>
</qos_profile>
</qos_library>
The datawriter_qos with topic_filter T11 in DerivedProfile will inherit its values from the datawriter_qos with topic_filter T1* in BaseProfile. The datawriter_qos with topic_filter T21 in DerivedProfile will inherit its values from the datawriter_qos with topic_filter T2* in BaseProfile. The datawriter_qos with topic_filter T31 in DerivedProfile will inherit its values from the datawriter_qos without topic_filter in BaseProfile.
Example 2:
<qos_library name=”Library”>
<qos_profile name="BaseProfile">
<datawriter_qos topic_filter="T1*">
...
</datawriter_qos>
<datawriter_qos name="T2DataWriterQoS" topic_filter="T2*">
...
</datawriter_qos>
</qos_profile>
<qos_profile name="DerivedProfile" base_name="BaseProfile">
<datawriter_qos topic_filter="T11"
base_name="BaseProfile::T2DataWriterQoS">
...
</datawriter_qos>
<datawriter_qos topic_filter="T21">
...
</datawriter_qos>
</qos_profile>
</qos_library>
Although the topic_filter expressions do not match, the datawriter_qos with topic_filter T11 in DerivedProfile will inherit its values from the datawriter_qos with topic_filter T2* in BaseProfile. topic_filter is not used with inheritance from QoS to QoS. The datawriter_qos with topic_filter T21 in DerivedProfile will inherit its values from the datawriter_qos with topic_filter T2* in BaseProfile.
Example 3:
<qos_library name=”Library”> <datawriter_qos name="BaseQos" topic_filter="T1"> ... </datawriter_qos> <datawriter_qos name="DerivedQos" base_name="BaseQos" topic_filter="T2"> ... </datawriter_qos> </qos_library>
In the case of a single QoS profile, although the topic_filter expressions do not match, the datawriter_qos named DerivedQos with topic_filter T2 will inherit its values from the datawriter_qos named BaseQos with topic_filter T1.
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