在同一台计算机上的两个进程之间使用ROSBridge

Question

I have a C++ ROS node using ROSBridge to send the ROS Topics as JSON strings via TCP WebSocket over port 9090. On the same computer, I have a C# Unity program that needs to access the JSON and send some of its own. If I put this program on a different computer there is no problem--I can access port 9090 and see the JSON. But if it is on the same computer as the ROSBridge Host, then there is a conflict with using the same port. So the two applications cannot communicate with each other on the same PC but they can on separate PCs.

So what can I do? Must the ports match and thus always conflict or maybe I can somehow make Process A send on 9090 but receive on 9091 and Process B just the opposite or something? Any other options? In essence, I'm trying to get Unity to participate in the ROS communications, but it does not have native support for that and thus the workaround with the ROSBridge JSON strings.

Answer 1

According to http://wiki.ros.org/rosbridge_suite/Tutorials/RunningRosbridge you can set the port that Rosbridge talks on in the .launch file.

If you are using the example rosbridge_websocket.launch file, the second line of the file includes a port number in the form of <arg name="port" default="9090" /> . You can change 9090 to change the default port it uses.

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According to the roslaunch documentation at http://wiki.ros.org/roslaunch/Commandline%20Tools you can also pass arguments to roslaunch on the command line:

The rosbridge_websocket.launch file sets the port from the argument "port", so if you run roslaunch like this it will use port 8080:

roslaunch rosbridge_server rosbridge_websocket.launch port:=8080

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You can see the full rosbridge_websocket.launch file at https://github.com/RobotWebTools/rosbridge_suite/blob/develop/rosbridge_server/launch/rosbridge_websocket.launch

Looking over that file for the line <param name="port" value="$(arg port)"/> is how I know that it uses the "port" argument to set the port.

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If you post your .launch files I can give your more advice on how to edit them if this answer is not clear.

Answer 2

It sounds like my understanding about ports was lacking. The host for the ROSBridge is being set up to use port 9090 so that anyone wishing to connect to it will request the host's port 9090 for the connection but will still use whatever port it desires on its own side. In both applications, I can state that the "Host" port is 9090 without conflict because the client application will still open a random other port for its own use to connect to the host port. Therefore, the trouble I am experiencing is something other than "port sharing" since that is not what is happening. Only one is hosting on 9090.