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ROS publishers & subscribers still transmitting data after roscore closed

asked 2019-07-26 13:38:50 -0500

ratelle gravatar image


I am working on a scale city project where we will have some cars running along a track connected to a watchtower. We have decided to use ROS for the project and are running Kinetic on Ubuntu 16.04 on RPi 3 B+'s. Today we were testing how ROS behaves in the event of internet failure and are a bit stumped:

We have one node on a Pi publishing sensor data. On my laptop, I am running roscore and subscribing to said data. On the pi, we set: ROS_MASTER_URI=http://(my computer's name).local:11311/ ROS_IP=(hostname -I)

We then launch roscore on my system. Then, we run the simple publishing script on the RPi (very, very simple -- similar to the publisher example code). Next, I run the subscriber on my computer. Data is printed as expected. However -- when I kill roscore, the data keeps transmitting and printing. If I reboot roscore, it continues to transmit.

This is extremely convenient for us, but why exactly is this happening? Shouldn't I expect connection between the two machines to cease once roscore has closed?

Thank you.

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answered 2019-07-26 23:12:24 -0500

updated 2019-07-26 23:14:42 -0500

As far as I know the subscriber and publisher nodes only communicate with roscore when declaring that they are subscribing or publishing a certain topic, service, parameter etc... After that, the nodes communicate with each other directly.

You can think of roscore as a sort of DNS. Once the nodes have discovered each other, they don't need to talk to roscore any more.

You can easily verify this behavior by opening 3 terminals. On the first one you run roscore, on the second one you run rostopic pub /my_int std_msgs/Int32 "data: 42" -r 1 (this will publish an integer with value 42 every second on a topic named /my_int) and on the third terminal run rostopic echo /my_int.

You will see the following output on the third terminal:

data: 42
data: 42
data: 42

After that, if you kill roscore on the first terminal you will see that the publisher and the subscriber are unaffected and messages data: 42 will keep piling up on the third terminal.

The problem is if roscore dies and a new node wants to register itself as a publisher or as a subscriber it will not be able to and will give you an error.

For your application you might be interested in looking into ROS2. No roscore is needed any more.

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What the OP decribes would seem to come down to an misunderstanding of how TCPROS works: it's not like MQTT where the broker is always needed. Nodes communicate peer-to-peer as soon as the initial connection has been setup, as exemplified by the series of commands shown by @Martin Peris.

gvdhoorn gravatar image gvdhoorn  ( 2019-07-28 12:01:23 -0500 )edit

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Asked: 2019-07-26 13:38:50 -0500

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Last updated: Jul 26 '19