Section 2: Topics - Publish and Subscribe

Estimated Reading Time: 8 minutes Bloom's Level: Understand, Apply Key Learning: One-to-many communication with decoupled nodes

---

The Problem with Direct Communication

Imagine you have a sensor node and a control node. How should they talk?

❌ Direct Connection (Bad)

Sensor Node → Control Node

Problems:

• Control node must exist before sensor starts
• Sensor knows about control (tight coupling)
• Hard to add a second control node
• If control crashes, sensor can't publish elsewhere

✅ Topic-Based (Good)

Sensor Node → [Topic: /data] ← Control Node
              ↗ Logger Node
              ↗ Visualizer Node

Benefits:

• Nodes don't know about each other (decoupled)
• Multiple nodes can listen (one-to-many)
• Nodes can start/stop independently
• Easy to add new subscribers

---

What is a Topic?

A topic is a named channel for publishing and subscribing to messages.

Think of it like a bulletin board:

• Publisher: Posts a message on the board
• Subscribers: Read messages from the board
• Topic Name: The bulletin board label (e.g., /temperature)
• Message Type: What data is posted (e.g., Float32)

Key characteristics:

• One-to-many: One publisher, many subscribers
• Asynchronous: Publisher doesn't wait for subscribers
• Typed: Every topic has a message type
• Named: Topics have unique names starting with /

---

Publishers and Subscribers

Publisher Node

# Creates a publisher
publisher = self.create_publisher(String, '/greeting', 10)

# Publishes messages
msg = String()
msg.data = 'Hello!'
publisher.publish(msg)

Subscriber Node

# Creates a subscriber with callback
subscription = self.create_subscription(
    String,
    '/greeting',
    self.listener_callback,
    10
)

# Callback handles incoming messages
def listener_callback(self, msg):
    self.get_logger().info(f'Received: {msg.data}')

---

Code Example 2: Publisher/Subscriber System

Location: code-examples/ros2_packages/pub_sub_py/

Generated by: ros2-code-generator skill

Publisher Node

# File: pub_sub_py/pub_sub_py/publisher.py
import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class PublisherNode(Node):
    def __init__(self):
        super().__init__('publisher')

        # Create a publisher for String messages on /greeting topic
        self.publisher = self.create_publisher(String, '/greeting', 10)

        # Timer to publish every 1 second
        self.timer = self.create_timer(1.0, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        # Create and publish message
        msg = String()
        msg.data = f'Hello from publisher: #{self.i}'
        self.publisher.publish(msg)
        self.get_logger().info(f'Publishing: {msg.data}')
        self.i += 1

def main(args=None):
    rclpy.init(args=args)
    node = PublisherNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Subscriber Node

# File: pub_sub_py/pub_sub_py/subscriber.py
import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class SubscriberNode(Node):
    def __init__(self):
        super().__init__('subscriber')

        # Create subscriber that listens to /greeting topic
        # Calls listener_callback when messages arrive
        self.subscription = self.create_subscription(
            String,
            '/greeting',
            self.listener_callback,
            10
        )

    def listener_callback(self, msg):
        # This function runs when a message arrives
        self.get_logger().info(f'Received: {msg.data}')

def main(args=None):
    rclpy.init(args=args)
    node = SubscriberNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

---

Running a Pub/Sub System

Terminal 1: Publisher

ros2 run pub_sub_py publisher

Output:

[INFO] [publisher]: Publishing: Hello from publisher: #0
[INFO] [publisher]: Publishing: Hello from publisher: #1
[INFO] [publisher]: Publishing: Hello from publisher: #2

Terminal 2: Subscriber

ros2 run pub_sub_py subscriber

Output:

[INFO] [subscriber]: Received: Hello from publisher: #0
[INFO] [subscriber]: Received: Hello from publisher: #1
[INFO] [subscriber]: Received: Hello from publisher: #2

Terminal 3: Inspect the Graph

rqt_graph

Visualizes:

[publisher] --[/greeting]--> [subscriber]

---

Topic Naming Conventions

ROS 2 topics follow a naming pattern:

/namespace/subtopic/specific_name

Examples:
/robot/camera/image        (camera images)
/robot/sensors/imu         (IMU data)
/robot/navigation/goal     (navigation goal)
/system/health/cpu_load    (system metrics)

Best Practices:

• Use lowercase with underscores
• Start with /
• Be specific and descriptive
• Group related topics with namespace

---

Message Types

Topics carry typed messages. Common types:

from std_msgs.msg import (
    String,           # Text
    Float32,          # Single floating point
    Int32,            # Integer
    Bool,             # Boolean
    Header,           # Timestamp + frame info
)

from geometry_msgs.msg import (
    Point,            # X, Y, Z coordinates
    Twist,            # Linear and angular velocity
    Pose,             # Position and orientation
)

from sensor_msgs.msg import (
    Image,            # Camera images
    PointCloud2,      # 3D point clouds
    LaserScan,        # LIDAR data
    Imu,              # Inertial measurement unit
)

---

Quality of Service (QoS)

The 10 parameter in create_publisher() and create_subscription() is the queue size (QoS):

# Queue size: 10 messages
self.publisher = self.create_publisher(String, '/topic', 10)

What it means:

• If subscriber is slow, publisher queues up to 10 messages
• Older messages are dropped if queue fills
• Larger queue = more memory, better for slow subscribers
• Smaller queue = faster, better for real-time systems

---

Key Takeaways

✅ Topics enable one-to-many communication ✅ Publishers and subscribers are decoupled ✅ Multiple subscribers can listen to one topic ✅ Publishers don't wait for subscribers ✅ Topics are typed and named ✅ QoS settings control message buffering

---

Try It Yourself

Try modifying the pub/sub example:

• Change the message text in publisher
• Create a second subscriber (run it in a third terminal)
• Use rqt_graph to see multiple subscribers
• Change topic name (must match in both nodes)
• Try ros2 topic echo /greeting in another terminal

---

Next

Now you understand how nodes communicate via topics. The next pattern is services: synchronous request/response communication.

👉 Next: Section 3: Services - Request and Response

Or review:

• Section 1: Nodes
• Exercises
• Summary

---

Section Status: Complete ✅ Skills Used: ros2-code-generator, code-explainer