topic question

topic question

asked 2017-08-25 13:34:12 -0600

moonspacedancer
123 ●10 ●13 ●18 http://Hedron.technology/

updated 2017-08-25 23:16:36 -0600

Ed Venator

1185 ●8 ●19 ●23

ubuntu 16.04 ROS kinetic

as per http://wiki.ros.org/ROS/Tutorials/Und... , i have listed my verbose details on topics in use rn:

~$ rostopic list -v

Published topics:
 * /turtle1/color_sensor [turtlesim/Color] 1 publisher
 * /turtle1/cmd_vel [geometry_msgs/Twist] 1 publisher
 * /rosout [rosgraph_msgs/Log] 4 publishers
 * /rosout_agg [rosgraph_msgs/Log] 1 publisher
 * /turtle1/pose [turtlesim/Pose] 1 publisher

Subscribed topics:
 * /turtle1/cmd_vel [geometry_msgs/Twist] 2 subscribers
 * /rosout [rosgraph_msgs/Log] 1 subscriber
 * /statistics [rosgraph_msgs/TopicStatistics] 1 subscriber

then I tried $ rostopic list -p to see the list of publishers.

/rosout
/rosout_agg
/turtle1/cmd_vel
/turtle1/color_sensor
/turtle1/pose

I did not expect to see the names of topics here because I did not understand that the publishers and subscribers themselves are actually the topics. I would have thought that the pubs and subs would just be nodes. Can someone explain why this is?
are all of these topics only topics or can a topic also be a node or a message?
just to check my understanding, is this right? a topic recieves a message from a node and gives it to a different node? does it sometimes give it to a different topic like could it be a recursive path through many things or like a function in algebra where you may have one message and one topic goes to more than one node ?

Comments

This question would be easier to read (and answer) if you use the Preformat Text (101010) button on the commands that you ran and the terminal output.

jayess ( 2017-08-25 14:10:01 -0600 )edit

@binarybabe: have you read the Concepts page on the wiki? That should answer some of your questions. Also, I'll reiterate my advise from #q269540 to pick up a book or two about ROS.

gvdhoorn ( 2017-08-25 15:34:29 -0600 )edit

rosout is a node and /rosout is also a topic. that clears a lot up for me actually. thank you

moonspacedancer ( 2017-08-26 19:02:22 -0600 )edit

I'm not understanding this. Is this an answer to your question or a comment?

jayess ( 2017-08-26 22:05:58 -0600 )edit

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answered 2017-08-26 22:25:14 -0600

jayess
6155 ●26 ●85 ●92

updated 2017-08-26 22:28:23 -0600

Well, based on your answer (comment?) a node is not also a topic (if I understand you correctly). There is a node named /rosout and a topic with the name /rosout. If you run rostopic info /rosout you'll see that the node /rosout is not publishing to the topic /rosout:

Type: rosgraph_msgs/Log

Publishers: 
 * /turtlesim (http://jayess:55350/)

Subscribers: 
 * /rosout (http://jayess:38176/)

I'm not sure what node(s) you were running for your question so I am using the turtlesim_node here because it looks to be pretty similar.

With turtlesim_node running, if i run rostopic list -v I get the following output:

Published topics:
 * /turtle1/color_sensor [turtlesim/Color] 1 publisher
 * /rosout [rosgraph_msgs/Log] 1 publisher
 * /rosout_agg [rosgraph_msgs/Log] 1 publisher
 * /turtle1/pose [turtlesim/Pose] 1 publisher

Subscribed topics:
 * /turtle1/cmd_vel [geometry_msgs/Twist] 1 subscriber
 * /rosout [rosgraph_msgs/Log] 1 subscriber

This shows the names of the topics such as /turtle1/color_sensor, the message type turtlesim/Color, and how many publishers 1 publisher listed under the section labeled Published topics. It's similar for the subscribers.

Now, if I run rostopic list, I get the following output:

/rosout
/rosout_agg
/turtle1/cmd_vel
/turtle1/color_sensor
/turtle1/pose

and if I run rostopic list -p, I get the following output:

/rosout
/rosout_agg
/turtle1/color_sensor
/turtle1/pose

The difference between the two commands (as far as I know) is that rostopic list shows all topics and rostopic list -p shows only the topics that are currently being published. Notice with the last command that the /turtle1/cmd_vel isn't shown. This is because nothing is publishing to it. The turtlesim_node is subscribed to it, that's why it's listed with rostopic list. You can prove this by running rostopic info /turtle1/cmd_vel:

Type: geometry_msgs/Twist

Publishers: None

Subscribers: 
 * /turtlesim (http://jayess:55350/)

So, don't get confused when nodes and topics have the same name, they are completely different entities but can have the same name without affecting anything (except to sometimes confuse people). Like @gvdhoorn suggested, you should pick up some books on ROS (and read more of the wiki) to help clarify some of these concepts.

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Thank you!! very in depth answer. I see now that the rosout comes as a node and as a topic and that's what was confusing me

moonspacedancer ( 2017-08-28 14:22:57 -0600 )edit

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