Subscribing to a non-existing topic

asked 2012-07-16 03:01:48 -0500

nlauzier
37 ●3 ●4 ●8

updated 2012-07-16 04:53:03 -0500

Hi,

Is it possible for a node to subscribe to a topic which doesn't exist yet? This would be useful if the node publishing on this topic has to be started after the listener.

To put this into context, I have three nodes (A, B, C) and two topics (T1 and T2). Node A publishes on T1. Node B subscribes to T1 and publishes on T2. Finally, Node C only subscribes on T2.

As I have experienced, I must run node A before node B as B will not be able to subscribe on T1 because it doesn't exist until some node publishes on it. The same happens for B and C and therefore I must run the nodes in the proper order (A then B then C). This is not a problem in my case, but if there was a third topic (T3) on which C would publish and A would listen, that would create a problem.

There must be a cleaner way to solve this problem but I haven't found one in ROS documentation yet. Any idea?

Thanks!

Update: reinitializing roscore solved the problem. I'm not sure what was the cause of the problem, though.

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By the way, I am using rospy.

nlauzier ( 2012-07-16 04:34:26 -0500 )edit

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answered 2012-07-16 03:05:22 -0500

cagatay

1850 ●12 ●20 ●31

yes you can subscribe to an unpublished topic but you may not retrieve any data in your callback till topic is published

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You are right, if I run "rostopic list -v", I can see that the topic exists and that it has one subscriber. However, when I run the node publishing on that topic, the callback function of the listener is never called.

nlauzier ( 2012-07-16 03:15:10 -0500 )edit

And if I run the same listener node twice (one before and one after the publisher is run), only the one run after the publisher will get its callback function called.

nlauzier ( 2012-07-16 03:16:54 -0500 )edit

In that case something is wrong. Verify that both nodes are running and connected. Do they have different names?

dornhege ( 2012-07-16 04:07:21 -0500 )edit

Running the same node twice deletes the first one from the ROS graph, so that behavior is correct and expected.

joq ( 2012-07-16 04:35:38 -0500 )edit

ok, now I'm not sure why but restarting roscore solved my problem. Is it possible that modifying my code or node/topic names needs a reinitialisation of roscore?

nlauzier ( 2012-07-16 04:51:44 -0500 )edit

It's possible that you left a connection open, then restarted roscore (which doesn't know about the old node still running) and then reconnected? Also refer to @joq's comment.

dornhege ( 2012-07-16 05:23:52 -0500 )edit

If you need multiple nodes, use different names from launch or cmd line or make the names Anonymous.

dornhege ( 2012-07-16 05:24:22 -0500 )edit

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Subscribing to a non-existing topic

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