Publishing and subscribing on custom messages

[aakashsehgal]

Summary.
I am trying to use roslibpy to publish and subscribe custom messages to/ from a ROS machine.

I try to publish on a custom topic using a custom_message but it doesnt work.

Expected Result

I had expected that when I use rostopic list command on my ROS machine, then it should have showed me /custom_topic in the list but unfortunately it doesnt.

Am I missing something here

Actual Result

It shows me the default 2 topics i.e. /rosout and /rosout_agg

Reproduction Steps

This is my sample code : 

`from __future__ import print_function
import time
import roslibpy
from roslibpy import Message, Ros, Topic

def run_topic_pub():
    print('entering run_topic_pub')
    ros = roslibpy.Ros(host='172.17.0.2', port=9090)
    publisher = Topic(ros, '/custom_topic', 'custom_message/float32')


    def start_sending():
        while True:
            if not ros.is_connected:
                break
            publisher.publish(Message({'data': '1'}))
            time.sleep(0.1)
        publisher.unadvertise()

    
    ros.on_ready(lambda: print('Is ROS connected ?', ros.is_connected))
    ros.on_ready(start_sending, run_in_thread=True)
    ros.get_topics(print)

    ros.run_forever()

if __name__ == '__main__':

    print('main')

    run_topic_pub()
`

System Information

Linux (ubuntu 16.04), ROS Melodic

[aakashsehgal]

I looked at the documentation more carefully and found that we need to define the topic first. So i did the following before creating a publisher :

message = roslibpy.Message() topic = roslibpy.Topic(ros,custom_topic,message) publisher = Topic(ros,/custom_topic, std_msgs/ String)

and it works.

However I am still not able to add a custom message something like :

message = roslibpy.Message('custom_message/float32') topic = roslibpy.Topic(ros,/custom_topic,message) publisher = Topic(ros,'/custom_topic,'custom_message/float32'')

How can i basically create a custom message definition ?

[gonzalocasas]

@aakashsehgal If what you want is really just a float32, you can use std_msgs/float32 (http://docs.ros.org/lunar/api/std_msgs/html/msg/Float32.html) instead of a custom one, but I assume that was just used as example. If you need a custom message type, you need to create it and compile as usual in ROS. As far as I know, there's no way for ROS to dynamically build a message type.

[aakashsehgal]

@gonzalocasas yes true that was just an example.
I have already created and compiled ROS with this custom message. So basically it is available in ROS. But since I am using the rosbridge to connect to a non ROS machine, how can i use the same custom message definition in roslibpy ?

[gonzalocasas]

@aakashsehgal that should work, but you need to stop rosbridge re-run the source of your catkin workspace and restart rosbridge. After that, the new message type should just work.

[aakashsehgal]

@gonzalocasas thank you for the information. I will try this out day after tomorrow and let you know.
E.g if my message type is custom_msg/ float 32 then would it work In the similar way as mentioned above ?

[gonzalocasas]

@aakashsehgal yep, it should work fine.

[aakashsehgal]

@gonzalocasas : Somehow it doesnt work. The rosbridge gives an error : Unable to load the manifest for package <pkg_name>
Could you please refer here

[aakashsehgal]

I have the following message structure :

Msg1.msg
int a
int b
float c
float d
float e

Msg2.msg
int x
Msg1[] y

I have 2 ros nodes, where in the first one publishes and the second one subscribes : both on topic Msg2.
Just for a test, now I am publishing the message using the 1st node in ROS and subscribing using ROS_BRIDGE. How can I use it in the ROSBridge_Python_API : I am doing something like this :

topic = Topic(ros, /custom_topic, 'Msg2[a,b,c,d,e], Msg2/int')

But i guess I am doing something wrong here.

[gonzalocasas]

@aakashsehgal you are definitely missing the package in the topic subscription. Msg2 on its own is not the name of your message type, you need to use the full name, including the package. So, let's assume you created a package called custom_stuff, on ROS side, you have your Msg1 and Msg2 definitions in the package, you compiled, you sourced the environment, you re-launched rosbridge from a console that has sourced this, and only then, you can subscribe to your message with something like topic = Topic(ros, '/custom_topic', 'custom_stuff/Msg2'). (Notice that the type definition in your topic subscription is only a message name (i.e. package_name/message_type_name), not a type definition inlined, that will never work.

Also, you might want to test this things directly on ROS, because this is not really specific to roslibpy, if you'd try to do the same thing on rospy or even directly from the ros command line, it wouldn't work either.

[gonzalocasas]

I'm going to close this because I think it's not a bug in roslibpy, but feel free to keep discussing it here anyway. And if you think it is actually a bug in roslibpy, then re-open pls.

[deep-saxena]

I am also trying to publish to a ROS topic using roslibpy. I have used the similar code for publishing as given in the summary above and have also added the logger using:

import logging

logging.basicConfig(level=logging.DEBUG, format='[%(thread)03d] %(asctime)-15s [%(levelname)s] %(message)s')
LOGGER = logging.getLogger('test')

I am facing issue as the /custom_topic is not showing when I run the rostopic list command after running this script. ros_bridge node is already running at port 9090 using the command roslaunch rosbridge_server rosbridge_websocket.launch.

Looking at the logs it seems that the connection gets closed as soon as we send data over the websocket.

[140283134949120] 2018-10-08 23:27:29,112 [DEBUG] Started to connect...
[140283134949120] 2018-10-08 23:27:29,116 [DEBUG] Server connected: tcp4:127.0.0.1:9090
[140283134949120] 2018-10-08 23:27:29,116 [INFO] Connection to ROS MASTER ready.
Is ROS connected ? True
[140283046196992] 2018-10-08 23:27:29,119 [DEBUG] Sending ROS message|

{"topic": "/custom_topic", "queue_size": 100, "latch": false, "type": "std_msgs/String", "id": "advertise:/custom_topic:1", "op": "advertise"}

[140283046196992] 2018-10-08 23:27:29,119 [DEBUG] Sending ROS message|

{"topic": "/custom_topic", "msg": {"data": "Hello"}, "latch": false, "id": "publish:/custom_topic:2", "op": "publish"}

[140283134949120] 2018-10-08 23:27:29,120 [INFO] WebSocket connection closed: Code=1009, Reason=message too big
[140283134949120] 2018-10-08 23:27:29,120 [DEBUG] Lost connection. Reason: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionLost'>: Connection to the other side was lost in a non-clean fashion.
]
[140283134949120] 2018-10-08 23:27:29,120 [INFO] <twisted.internet.tcp.Connector instance at 0x7f9631a73cb0> will retry in 2 seconds
[140283134949120] 2018-10-08 23:27:29,120 [INFO] Stopping factory <roslibpy.comm.comm_autobahn.AutobahnRosBridgeClientFactory object at 0x7f9631cf4cd0>
[140283134949120] 2018-10-08 23:27:31,432 [INFO] Starting factory <roslibpy.comm.comm_autobahn.AutobahnRosBridgeClientFactory object at 0x7f9631cf4cd0>
[140283134949120] 2018-10-08 23:27:31,433 [DEBUG] Started to connect...
[140283134949120] 2018-10-08 23:27:31,442 [DEBUG] Server connected: tcp4:127.0.0.1:9090
[140283134949120] 2018-10-08 23:27:31,442 [INFO] Connection to ROS MASTER ready.

The websocket close code 1009 specifies "endpoint won't process large frames". Can you help me with what could be the possible reason for this.

[aakashsehgal]

@deep-saxena After compiling your package, source the environment and then launch the rosbridge in the same terminal. This should help.

[aakashsehgal]

@gonzalocasas : I solved the problem. I was basically trying to find the wrong package. And yes, you mentioned it correctly, I had to use publisher = Topic(ros, '/custom_topic', 'ros_tutorials/Msg2'). Now I can subscribe easily.
Could you here please guide me as to how can I publish a message using the same API.
I you look at my message definition above, iin Msg2.msg I have an array of Msg1. How can I do it in an efficient way ?
publisher.publish(Message)({'x' : '10'}, 'Msg1.a':'20')). Something like this wherein I can loop through all the members of the array.

[gonzalocasas]

@aakashsehgal:

I solved the problem

cool!

how can I publish a message using the same API

It's relatively straight-forward, you don't need to add any additional type information, simply supply a message (which is basically a python dictionary) that matches to the expected format.

To exemplify it with a simple message type, if you want to publish a message of type std_msgs/String (see here), you would publish it as follows:

publisher.publish(Message({'data': 'hello world'}))

For your case of ros_tutorials/Msg2, I think the message should be:

publisher.publish(Message({'x': 123, 'y': [{'a': 1, 'b': 2, 'c': 3.14, 'e': 7.23, 'f': 2.353}]}))

(notice y is a list, so you can pass as many Msg1-matching dictionaries as you want).