Subscribing to Multiple Nodes Publishing the Same Topic

asked 2014-07-21 16:03:51 -0500

jc2016
150 ●3 ●5 ●14

updated 2014-07-22 18:43:34 -0500

demmeln
4306 ●32 ●45 ●69

Hi all,

I am having an issue that I'm not sure has been addressed. I am writing a node where I am attempting to subscribe to another two nodes that publish to the same topic (i.e. two lasers that are publishing to a topic with the same name). Going back and changing the topic names would be a little bit tricky, so I was wondering if there was a way to differentiate these two (i.e. write a subscriber that takes into account topic name and node-of-origin name when being instantiated.

Thanks

Edit: I'm confused how the syntax works for remapping and was wondering if I could ask for more help:

Originally, I have as a launch file:

<launch>

    <node pkg="leg_detector" type="leg_detector" name="leg_detector_front" args="scan:=scan_front $(find leg_detector)/config/trained_leg_detector.yaml" output="screen"/>

    <node pkg="leg_detector" type="leg_detector" name="leg_detector_rear" args="scan:=scan_rear $(find leg_detector)/config/trained_leg_detector.yaml" output="screen"/>

</launch>

I tried this:

 <launch>

    <node pkg="leg_detector" type="leg_detector" name="leg_detector_front" args="scan:=scan_front $(find leg_detector)/config/trained_leg_detector.yaml" output="screen"/>

    <group>
      <remap from="leg_tracker_measurements" to="leg_tracker_measurements_rear">
      <node pkg="leg_detector" type="leg_detector" name="leg_detector_rear" args="scan:=scan_rear $(find leg_detector)/config/trained_leg_detector.yaml" output="screen"/>
    </group>
  </launch>

But it threw an invalid syntax error

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Comments

'Close' your remap tag. <remap from="leg_tracker_measurements" to="leg_tracker_measurements_rear"/> or <remap from="leg_tracker_measurements" to="leg_tracker_measurements_rear"> </remap>

McMurdo ( 2014-07-23 00:41:22 -0500 )edit

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7

answered 2014-07-21 16:10:15 -0500

demmeln
4306 ●32 ●45 ●69

updated 2014-07-22 18:57:17 -0500

I do not know of a way to distinguish by which node sent a message in the callback on the subscriber side.

I suggest you make your two publishing nodes publish to two different topics and your subscriber then subscribe to both indivially. Remapping topics is your friend here.

Edit:

In response to your syntax problem: Have you read the actual error message? It probably indicates that you forgot to close the <remap> tag. You can put it into a group, but I believe it is clearer to put it directly inside the node tag. Also inside a rules file I would not use the remapping syntax with command line arguments like you do for scan. This is just confusing. Try this:

<launch>
  <node pkg="leg_detector" type="leg_detector" name="leg_detector_front" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen">
    <remap from="scan" to="scan_front" />
    <remap from="leg_tracker_measurements" to="leg_tracker_measurements_front" />
  </node>

  <node pkg="leg_detector" type="leg_detector" name="leg_detector_rear" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen">
    <remap from="scan" to="scan_rear" />
    <remap from="leg_tracker_measurements" to="leg_tracker_measurements_rear" />
  </node>
</launch>

Or alternatively and maybe even better in this case with namespaces, like suggested in another answer below:

<launch>
  <node pkg="leg_detector" type="leg_detector" name="leg_detector_front" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen" ns="front" />
  <node pkg="leg_detector" type="leg_detector" name="leg_detector_rear" args="$(find leg_detector)/config/trained_leg_detector.yaml" output="screen" ns="rear" />
</launch>

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That looks like an awesome feature, thanks so much for the quick reply. So, the way I have it set up I instantiate two instances of the node in the launch file with different names. It seems like I should be able to use the remapping, but does that sound right. Thanks again.

jc2016 ( 2014-07-21 16:50:28 -0500 )edit

I don't have time to check it out yet, but this seems to work, and I'll make it the answer once I figure out if it does. Thanks

jc2016 ( 2014-07-21 16:51:03 -0500 )edit

Sounds right. Use the <remap> tag in your launch file for one (or if you like both) of the nodes. Feel free to update your question with more concrete info if you get stuck.

demmeln ( 2014-07-21 16:56:23 -0500 )edit

2

Although @McMurdo's answer is the precise reply to the question asked, it is important to note that this answer is the right way to do it!

dornhege ( 2014-07-22 05:54:39 -0500 )edit

Alright that worked out well, thanks so much for all of your help!

jc2016 ( 2014-07-23 09:50:39 -0500 )edit

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2

answered 2014-07-22 01:54:03 -0500

BennyRe
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Just for completeness: You could also launch your two nodes in different name spaces.

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So I had already renamed both nodes from which the topics were being published from, if that what you man by name spaces. But I believe I still run into the same issue that the nodes both publish to the same topic.

jc2016 ( 2014-07-22 15:46:56 -0500 )edit

No, the name of the nodes is not what is referred to here. The node tag can have a `ns` attribute (http://wiki.ros.org/roslaunch/XML/node). This causes all topics for this node to have prefix. E.g. if your node publishes to the `data` topic. If you add `ns="foo"` it will then publish to `foo/data`.

demmeln ( 2014-07-22 18:47:52 -0500 )edit

Would you mind looking at the code provided in the edit to the main question? I'm not sure where I'm going wrong there

jc2016 ( 2014-07-22 18:51:07 -0500 )edit

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4

answered 2014-07-22 04:18:34 -0500

McMurdo

1247 ●31 ●42 ●54

updated 2014-07-22 06:12:59 -0500

The real answer is there in the connection header:

http://wiki.ros.org/rospy/Overview/Pu... see the last few lines.

A subscriber can get access to a "connection header", which includes debugging information such as who sent the message, as well information like whether or not a message was latched. This data is stored as the _connection_header field of a received message.

e.g.

print m._connection_header
{'callerid': '/talker_38321_1284999593611',
 'latching': '0',
 'md5sum': '992ce8a1687cec8c8bd883ec73ca41d1',
 'message_definition': 'string data\n\n',
 'topic': '/chatter',
 'type': 'std_msgs/String'}

EDIT: THIS METHOD IS NOT RECOMMENDED. IT IS ONLY FOR DEBUGGING PURPOSES.

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Well, I would not rely on that tbh. The section you link says directly after your quote: "We do not recommend using callerid information beyond debugging purposes, as it can lead to brittle code in an anonymous publish/subscribe architecture." And I don't think this is available with C++?

demmeln ( 2014-07-22 05:55:55 -0500 )edit

1

Yes. The answer is merely precise, not recommended. see @dornhege's comment below.I should edit the answer to point out that it isn't recommended.

McMurdo ( 2014-07-22 06:11:54 -0500 )edit

2

In C++ there is a callback signature that subscribes to MessageEvents that hold the publisher's name, see: http://wiki.ros.org/roscpp/Overview/Publishers%20and%20Subscribers#MessageEvent_.5BROS_1.1.2B-.5D

dornhege ( 2014-07-22 06:18:58 -0500 )edit

Ah, I wasn't aware of the MessageEvent form for callbacks in roscpp. Thanks for pointing that out.

demmeln ( 2014-07-22 06:38:26 -0500 )edit

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