Class model of the DCPS Topic-Definition Module

The Topic-Definition Module is comprised of the following classes:

TopicDescription
Topic
ContentFilteredTopic
MultiTopic
TopicListener
TypeSupport

TopicDescription represents the fact that both publications and subscriptions are tied to a single data-type. Its attribute type_name defines a unique resulting type for the publication or the subscription and therefore creates an implicit association with a TypeSupport. TopicDescription has also a name that allows it to be retrieved locally.

Topic

Topic is identified by its name, which must be unique in the whole Domain. In addition (by virtue of extending TopicDescription) it fully specifies the type of the data that can be communicated when publishing or subscribing to the Topic.

Topic is the only TopicDescription that can be used for publications and therefore associated to a DataWriter. All operations except for the base-class operations set_qos, get_qos, set_listener, get_listener, enable, and get_status_condition may return the value NOT_ENABLED.

Applicable QoS

QoS	Brief
TOPIC_DATA	User data
DURABILITY	Expresses the lifetime of a sample
DURABILITY_SERVICE	Specifies the configuration of the durability service
DEADLINE	Sets the period in which a sample must be sent/received
LATENCY_BUDGET	Sets the maximum delay the sample is in transit
LIVELINESS	Set the method by which an instance is considered alive
OWNERSHIP	Specifies if the DataWriters is shared or exclusive
RELIABILITY	Set the required reliability of a DataReader/DataWriter pair, reliable or best effort
DESTINATION_ORDER	Controls the logical order of the samples by reception time-stamp or source time-stamp
HISTORY	Specify how many generations of the same instance to keep
RESOURCE_LIMITS	Specify the amount of resources the DataReader/DataWriter pair can consume
TRANSPORT_PRIORITY	A hint to the underlying transport for prioritization
LIFESPAN	Specifies the maximum duration of validity of the data written

Default QoS values

QoS	Attribute	Value
TOPIC_DATA	value.length	0
DURABILITY	kind	VOLATILE
DURABILITY_SERVICE	service_cleanup_delay history_kind history_depth max_samples max_instances max_samples_per_instance	0 KEEP_LAST 1 LENGTH_UNLIMITED LENGTH_UNLIMITED LENGTH_UNLIMITED
DEADLINE	period	DURATION_INFINITE
LATENCY_BUDGET	duration	0
LIVELINESS	kind lease_duration	AUTOMATIC DURATION_INFINITE
OWNERSHIP	kind	SHARED
RELIABILITY	kind max_blocking_time synchronous	BEST_EFFORT 100 ms FALSE
DESTINATION_ORDER	kind	BY_RECEPTION_TIMESTAMP
HISTORY	kind depth	KEEP_LAST 1
RESOURCE_LIMITS	max_samples max_instances max_samples_per_instance	LENGTH_UNLIMITED LENGTH_UNLIMITED LENGTH_UNLIMITED
TRANSPORT_PRIORITY	value	0
LIFESPAN	duration	DURATION_INFINITE

Related Status Conditions

Status	Brief
INCONSISTENT_TOPIC	Another topic exists with the same name but different characteristics

Related Listeners

Listener	Brief
on_inconsistent_topic()	Another topic exists with the same name but different characteristics

Content Filtered Topic

ContentFilteredTopic is a specialization of TopicDescription that allows for content-based subscriptions. ContentFilteredTopic describes a more sophisticated subscription that indicates the subscriber does not want to necessarily see all values of each instance published under the Topic. Rather, it wants to see only the values whose contents satisfy certain criteria. This class therefore can be used to request content-based subscriptions.

The selection of the content is done using the filter_expression with parameters expression_parameters:

The filter_expression attribute is a string that specifies the criteria to select the data samples of interest. It is similar to the WHERE part of an SQL clause.
The expression_parameters attribute is a sequence of strings that give values to the ‘parameters’ (i.e., "%n" tokens) in the filter_expression. The number of supplied parameters must fit with the requested values in the filter_expression (i.e., the number of n tokens).