Can see topic when using ros2 topic list, but can't echo it?

asked 2022-03-22 04:03:29 -0500

Hakaishin
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updated 2022-03-23 03:09:57 -0500

I have two computers(Ubuntu 20.04) in the same network with ROS_DOMAIN_ID=1 and I have a topic called cloud which is a pointcloud from a lidar sensor. The driver is running on the PC=A and I want to show the clouds on PC=B. Now when I ros2 topic echo /cloud on PC=A I receive the msgs as normal. When I ros2 topic list on PC=B I see the cloud topic. I only see it after setting the ROS_DOMAIN_ID=1 to make sure no prior topic with that name is available. So far so good, but now when I try to ros2 topic echo /cloud on PC=B I get nothing. Any idea why this could be? In my understanding the msgs are arriving, as shown by ros2 topic echo /cloud on PC=A and I can see them on PC=B shown by ros2 topic list. Funny enough ros2 topic info /cloud works fine and tells me that the publisher and subscriber count at 1 and 2 respectively. Also running the ros2 run demo_nodes_cpp talker demo on PC=A and ros2 run demo_nodes_cpp listener demo on PC=B works as intended. Is it maybe because the sensor is UDP and the demo listener is tcp?

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Maybe you should check your QoS in both publisher and subscriber on the computers?

ljaniec ( 2022-03-22 17:53:17 -0500 )edit

I checked that the QOS are the same by using ros2 topic echo /cloud --verbose and setting the once which rviz exposes to the same, namely reliability: RELIABLE, DURABILITY: TRANSIENT_LOCAL,

Hakaishin ( 2022-03-23 09:19:42 -0500 )edit

I assume it was the same on the side of the subscriber, it shouldn't prevent anything from working...

ljaniec ( 2022-03-24 11:32:24 -0500 )edit

Well and here we are. I have yet to try in a network without a firewall, there is a possibility that that is a problem. But I doubt it. But I don't have a router atm so I can't try it

Hakaishin ( 2022-03-24 12:06:52 -0500 )edit

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How did you fix that? Because it's a sensor driver, so I can't change the publisher.

Hakaishin ( 2022-03-23 04:05:37 -0500 )edit

I did not need to fix it as when I subscribe to the topic with another node the data is available, it just does not print in the terminal. You could try to subscribe to your /cloud topic using RViz to see if the data is displayed.

mcbed ( 2022-03-23 05:04:21 -0500 )edit

I tried that at first and it doesn't, then I tried echo. Now I'm not sure which is the more reliable tool to check...

Hakaishin ( 2022-03-23 09:21:29 -0500 )edit

I would use minimal subscriber code with simple callback to check if e.g. header of the message display correctly

ljaniec ( 2022-03-24 11:33:49 -0500 )edit

Well isn't echo such a minimal subscriber? I thought that was the point of it, wouldn't I be just rewriting the code in ros2 topic echo?

Hakaishin ( 2022-03-24 12:06:15 -0500 )edit

Overall, yes, ros2 topic echo is quite similar, but I was commenting with @mcbed answer in mind - maybe messages are too big to print in the terminal, in minimal subscriber with printing only a header you could avoid that

ljaniec ( 2022-03-24 12:39:22 -0500 )edit

Facing the same issue and I create a simple Node to subscribe the topic but still cannot get the message. When I try to connect both PC to the same router then I can echo the message. However, when I connect my PC B to the router via WIFI and PC A to the router via Ethernet, then I cannot echo any message.

Any additional hints?

[Using cyclonedds and have changed the config according to the scenario that I described above]

BCJ ( 2023-08-03 22:06:09 -0500 )edit

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Can see topic when using ros2 topic list, but can't echo it?

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Can see topic when using ros2 topic list, but can't echo it?