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In the ROS-2 design page "https://design.ros2.org/articles/ros_middleware_interface.html", it is written by @Dirk Thomas as

The DDS DataReader and DataWriter as well as DDS topics are not exposed through the ROS API.

My question is: How can we say that the DDS topics are not exposed through the ROS API, when using "ros2 topic list", we do access the topics ? Does these topics do not belong to DDS implementation ? What was the actual concept that was meant to be conveyed by the quoted text above ?

I request to help me in clearing this doubt.


Originally posted by Aakashp on ROS Answers with karma: 41 on 2020-12-28

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Comment by gvdhoorn on 2020-12-29:
@Aakashp: please don't close questions to mark them as resolved. Instead, accept the best answer by clicking the checkmark to the left of the answer.

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2 Answers 2

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What you see in the ros2 topic list are the ROS topics and not the DDS topics. Under the hood it is implemented with DDS by default but the topics are not exactly the same. And you don't have access to the raw DDS DataReader and DataWriter objects.


Originally posted by tfoote with karma: 58457 on 2020-12-28

This answer was ACCEPTED on the original site

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Comment by Aakashp on 2020-12-29:
Thanks @tfoote for clearing my doubt.

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ROS topics and DDS topics are different things and there is a mapping between the two, as described by the design document Topic and Service name mapping to DDS.

You can check out the design document for more info, but some key points from the design:

  • ROS topics have different naming constraints compared to DDS topics
  • Some name mangling occurs when mapping a ROS topic name to a DDS topic name

Originally posted by jacobperron with karma: 1870 on 2020-12-28

This answer was NOT ACCEPTED on the original site

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Comment by Aakashp on 2020-12-29:
Thanks @jacobperron for your answer, it is helpful.

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