[ROS Q&A] 144 – What is the difference between ROS topics and messages?

In this video we are going to explain and show in a practical way the difference between ROS topics and messages.

We show two nodes running, one publishing to a given topic and another subscribing to this same topic. RQT graph is used to see the big picture of the environment.

Step 0. Create a project in ROS Development Studio(ROSDS)

ROSDS helps you follow our tutorial in a fast pace without dealing without setting up an environment locally. If you haven’t had an account yet, you can create a free account here. Let’s create a new project and call it ros_q_a_topics_messages.

Step 1. Difference between ros topic and ros message

We’ll start by creating a package for the code in catkin_ws

cd catkin_ws/src
catkin_create_pkg my_pkg rospy std_msgs

Let’s also create a scripts folder for our scripts. Then two script called my_subscriber.py and my_publisher.py file in it with the following content.

#!/usr/bin/env python

import rospy
from std_msgs.msg import String

def main():
    pub = rospy.Publisher('my_topic', String, queue_size=10)
    rospy.init_node('my_publisher')

    rate = rospy.Rate(1)
    
    while not rospy.is_shutdown():
        hello_str = 'hello world %s' % rospy.get_time()

        rospy.loginfo(hello_str)

        pub.publish(hello_str)

        rate.sleep()

if __name__ == '__main__':
    try:
        main()
    except rospy.ROSInterruptExeception:
        pass

Basically, a message is data through a channel called topic. In this code, the message is String type and the topic is called my_topic.

#!/usr/bin/env python

import rospy
from std_msgs import String

def main():
    pub = rospy.Publisher('my_topic', String, queue_size = 10)

    rospy.init_node('my_publisher')

    rate = rospy.Rate(1)

    while not rospy.is_shutdown():
        hello_str = "hello world %s" % rospy.get_time()

        rospy.loginfo(hello_str)

        pub.publish(hello_str)

        rate.sleep()

if __name__ == '__main__' :
    try:
        main()
    except rospy.ROSInterruptExeception:
        pass

This script established a subscriber for the message through the topic my_topic.

To run the code, we have to give the scripts permission to execute with chmod +x my_publisher.py and chmod +x my_subscriber.py

Then we can run it with

cd catkin_ws
catkin_make
source devel/setup.bash
rosrun my_pkg my_publisher.py

You should see the publisher is publishing message into the topic.

Open another shell and run

rosrun my_pkg my_subscriber.py

Open the third shell and type rqt_graph , then go to Tools->graphical tool to open the GUI.

You should see the nodes are marked with circles. You can find the /my_publisher and /my_subscriber nodes in the graph.

The topic is marked with the square. In this example, the topic is called my_topic. The two node is communicating though this topic.

Want to learn more?

If you are interested in learning ROS, please check our ROS Basics Python course.

Edit by: Tony Huang

RELATED LINKS

▸ Original question: https://answers.ros.org/question/63511/what-is-the-difference-between-a-topic-and-a-message/
▸ ROS Development Studio (ROSDS)
▸ Robot Ignite Academy
▸ ROS Basics Python
▸ ROS Basics C++

Feedback

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