Does the order of a launch file node influence the order of execution?

asked 2021-08-21 10:24:24 -0500

Kansai
170 ●47 ●57 ●65

I have a launch file like

<launch>

<arg name="input_folder" default="input" />
<node type= "dosomething.py" name="dosomething" pkg="a_package" output="screen" cwd="node" required="true" />

<arg name="output_folder" />
<node name="rosbag" type="record" pkg="rosbsag"
args="-O $(arg output_folder) /message">
</node>
</launch>

dosomething publishes messages. My problem is that it seems that rosbag record does not record the first published messages

when I launch the file though I can see that rosbag starts after dosomething has started, but I think that still it starts before the first message is published

However it seems it is not recorded. How can I solve this and make sure all published messages are recorded?

answered 2021-08-21 13:36:57 -0500

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

updated 2021-08-21 13:37:20 -0500

Does the order of a launch file node influence the order of execution?

No, it doesn't.

(or at least not significantly)

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The why is it not recording the first message? And how to record it?

Kansai ( 2021-08-21 19:41:55 -0500 )edit

The why is it not recording the first message?

Likely because setting up subscriptions takes time, and any messages published during connection establishment will not be seen by the new subscriber (ie: rosbag in this case).

There are quite a few Q&As on this topic here on ROS Answers. Use Google and append site:answers.ros.org to your query. Keywords which will likely find those Q&As: first message lost.

gvdhoorn ( 2021-08-22 01:55:09 -0500 )edit

I stopped using rosbag record and wrote my own script. After some experiments I can see that the order of launch file node do have a significant influence in the result

Kansai ( 2021-08-22 07:55:25 -0500 )edit

It may be more efficient to use the rosbag API instead of replaying a .bag. That would make things much more predictable.

gvdhoorn ( 2021-08-22 08:41:47 -0500 )edit

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