Doubt in ROS-2 design document about topics

asked 2020-12-28 09:39:32 -0500

Aakashp
41 ●9 ●12 ●17

updated 2020-12-28 10:19:25 -0500

In the ROS-2 design page "https://design.ros2.org/articles/ros_...", 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.

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@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.

gvdhoorn ( 2020-12-29 04:28:12 -0500 )edit

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answered 2020-12-28 12:22:05 -0500

tfoote

58457 ●128 ●550 ●526 http://www.ros.org/

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.

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Thanks @tfoote for clearing my doubt.

Aakashp ( 2020-12-29 03:57:02 -0500 )edit

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answered 2020-12-28 12:26:48 -0500

jacobperron

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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

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Thanks @jacobperron for your answer, it is helpful.

Aakashp ( 2020-12-29 03:56:17 -0500 )edit

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Doubt in ROS-2 design document about topics

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