ROS Nodes and ROS Topics

ROS Node

A node is an executable process that performs one specific computation (e.g. camera driver, motor controller, path planner). A robot system is built from many small nodes that talk to each other, which keeps the software modular and reusable.

• Each node registers with the ROS Master using a unique name.
• Nodes can be written in Python (rospy) or C++ (roscpp).
• Useful commands: rosnode list, rosnode info /talker, rosrun pkg node.

ROS Topic

A topic is a named, one-way communication channel (publish/subscribe) over which nodes exchange messages of a fixed type.

• A publisher sends messages to a topic.
• A subscriber receives messages from that topic.
• Many publishers and many subscribers are allowed; they do not know about each other (decoupled).
• Useful commands: rostopic list, rostopic echo /chatter, rostopic info /chatter.

#!/usr/bin/env python
import rospy
from std_msgs.msg import String


def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10)
    while not rospy.is_shutdown():
        pub.publish('hello robot')
        rate.sleep()


if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

Node vs Topic

Point	Node	Topic
What it is	Process doing a task	Named message channel
Role	Publisher / subscriber	Carries data between nodes
Example	/talker	/chatter

Exam tip

Draw the talker → topic → listener diagram and write the two short code snippets; mention the Master only does name lookup, data flows directly node to node.