ROS Topics, Services, and Actions Explained with Clear Examples

It is said that the next ten years will be the era of Robotics with AI (Artificial Intelligence).

“ROS – Robot Operating System The Robot Operating System (ROS) is a set of software libraries and tools that help you build robot applications. From drivers to state-of-the-art algorithms, and with powerful developer tools, ROS has what you need for your next robotics project. And it’s all open source.”

Introduction to ROS and ROS 2

• ROS (Robot Operating System) is a flexible framework for writing robot software. It provides tools, libraries, and conventions to simplify the task of creating complex and robust robot behavior across a wide variety of robotic platforms.
• ROS 2 is the next-generation version of ROS, built to address limitations in real-time performance, security, and multi-platform support. It uses DDS (Data Distribution Service) under the hood for scalable and reliable communication.

For more information, you can visit: ros.org

Core Communication Concepts in ROS

• Topics – Used for streaming data between nodes asynchronously.
• Services – Used for synchronous request/response communication.
• Actions – Used for long-running, goal-oriented tasks with feedback and the ability to cancel.

1. Topics

What Are Topics?

• Topics provide a publish/subscribe communication model.
• Ideal for sensor data, continuous streams, or event broadcasting.
• Many publishers and many subscribers can use the same topic.

Key Characteristics

Pattern	Publish/Subscribe
Direction	One-way (Unidirectional)
Communication	Asynchronous
Use Case	Streaming sensor data (e.g. images, laser scans)

Example

# Publisher  
pub = rospy.Publisher('/chatter', String, queue_size=10)  
pub.publish("Hello World!")  

# Subscriber  
def callback(msg):  
    rospy.loginfo(msg.data)  

sub = rospy.Subscriber('/chatter', String, callback)

2. Services

What Are Services?

• Services use a client-server model for request/response communication.
• Synchronous and blocking — the client waits for the response.
• Best used for tasks that require a definite reply, like configuring a component.

Key Characteristics

Pattern	Request/Response
Direction	Bidirectional
Communication	Synchronous
Use Case	Querying a device status, turning a component on/off

Example

# Client  
rospy.wait_for_service('/add_two_ints')  
add_two_ints = rospy.ServiceProxy('/add_two_ints', AddTwoInts)  
resp = add_two_ints(3, 5)  

# Server  
def handle_add_two_ints(req):  
    return req.a + req.b  

srv = rospy.Service('/add_two_ints', AddTwoInts, handle_add_two_ints)

3. Actions

What Are Actions?

• Actions are used for long-running tasks that can be monitored and cancelled.
• They provide feedback and result messages along with goal status.
• Great for navigation, manipulation, or any goal-driven process.

Key Characteristics

Pattern	Goal/Feedback/Result
Direction	Bidirectional
Communication	Asynchronous with feedback
Use Case	Move a robot arm, navigate to a location

Example

# Client  
client = actionlib.SimpleActionClient('move_base', MoveBaseAction)  
client.wait_for_server()  
goal = MoveBaseGoal()  
client.send_goal(goal)  
client.wait_for_result()  

# Server  
def execute_cb(goal):  
    # Execute goal  
    feedback = MoveBaseFeedback()  
    result = MoveBaseResult()  
    server.set_succeeded(result)  

server = actionlib.SimpleActionServer('move_base', MoveBaseAction, execute_cb, False)  
server.start()

Difference Between Action and Service in ROS

Feature	Service	Action
Pattern	Request / Response	Goal / Feedback / Result
Synchronous?	Yes (blocking)	No (non-blocking)
Duration	Short	Long-running
Feedback Support	No	Yes
Cancelable	No	Yes
Use Case	Quick query or configuration	Long task with progress, like navigation

Analogy

• Service = like calling a function and waiting for the result
• Action = like starting a job in the background and checking progress

Conclusion

• Use Topics for fast, continuous data streams with no need for response.
• Use Services when you need a quick answer to a request.
• Use Actions when you need to send a goal and get progress updates or be able to cancel it later.

ROS 2 Note:

• All three patterns (Topics, Services, Actions) are supported and enhanced in ROS 2 with DDS for better reliability and performance.

Robot Operating System (ROS)

• ROS Topics, Services and Actions Explained with Clear Examples

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