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Messages lost in ROS ELECTRIC

asked 2012-05-17 22:10:06 -0500

McMurdo gravatar image

Why do certain message packets get lost? Is this normal? I have attached a image for reference. Clearly, packets 0,1,2 were lost.

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This problem is similar to another one, which also detailed solutions: http://answers.ros.org/question/11167/how-do-i-publish-exactly-one-message/

Benoit Larochelle gravatar imageBenoit Larochelle ( 2012-11-14 01:01:30 -0500 )edit

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answered 2012-05-18 03:00:58 -0500

Lorenz gravatar image

You lose only the first few messages. This probably doesn't have anything to do with buffer sizes but that subscribing takes a while. Your publisher node always publishes something although the connection to your subscriber is not completely established yet. This behavior directly follows from the way ros works.

Normally nodes are written in a way that this doesn't matter. However, roscpp provides callbacks when nodes subscribe or unsubscribe (connect_cb and disconnect_cb as shown here) and you can get the number of subscribed nodes from the publisher object. That way, the publisher can start publishing only when the subscriber is present.

Latched topics are used more commonly though. With a latched topic, the subscriber always receives the newest message that has been published right after the connection to the publisher is established.

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This definitely seems to be the case, not the queue stuff I mention in my answer. :)

Miguel Prada gravatar imageMiguel Prada ( 2012-05-20 07:12:37 -0500 )edit
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answered 2012-05-17 23:01:28 -0500

Miguel Prada gravatar image

Depending on the queue size you define on your publisher (or subscriber), and the rate at which you are publishing (or processing the incoming messages) it is perfectly possible that the publisher (or subscriber) is dropping messages.

Try to increase the publisher and subscriber queue sizes and see if this still happens.

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As an addition: the underlying TCP stack might drop packages as well depending on the quality of your network and traffic.

Lorenz gravatar imageLorenz ( 2012-05-17 23:02:59 -0500 )edit

Quality of network? I am doing both the processes on the same computer!

McMurdo gravatar imageMcMurdo ( 2012-05-18 02:49:51 -0500 )edit

Ok. You didn't mention that in your question.

Lorenz gravatar imageLorenz ( 2012-05-18 02:51:17 -0500 )edit

I am sorry, I thought it was apparent from the terminal outputs. Thanks a lot for helping me out.

McMurdo gravatar imageMcMurdo ( 2012-05-20 19:45:37 -0500 )edit

I solved it! Super Answer, @Lorenz

McMurdo gravatar imageMcMurdo ( 2012-05-20 21:10:50 -0500 )edit

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Asked: 2012-05-17 22:10:06 -0500

Seen: 1,070 times

Last updated: May 18 '12