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Chapter 5: Publishers & Subscribers

Topic Communication​

Topics are the primary way nodes exchange data in ROS 2. They use a publish-subscribe pattern for asynchronous, one-to-many communication.

Message Types​

Common message types:

from std_msgs.msg import String, Int32, Float64, Bool
from geometry_msgs.msg import Twist, Pose, Point
from sensor_msgs.msg import Image, LaserScan, Imu

Advanced Publisher​

from geometry_msgs.msg import Twist

class RobotController(Node):
    def __init__(self):
        super().__init__('robot_controller')
        self.cmd_vel_pub = self.create_publisher(Twist, 'cmd_vel', 10)
        self.timer = self.create_timer(0.1, self.move_robot)  # 10Hz

    def move_robot(self):
        msg = Twist()
        msg.linear.x = 0.5  # Forward 0.5 m/s
        msg.angular.z = 0.2  # Turn 0.2 rad/s
        self.cmd_vel_pub.publish(msg)

Advanced Subscriber​

from sensor_msgs.msg import LaserScan

class ObstacleDetector(Node):
    def __init__(self):
        super().__init__('obstacle_detector')
        self.scan_sub = self.create_subscription(
            LaserScan, 'scan', self.scan_callback, 10)

    def scan_callback(self, msg):
        min_distance = min(msg.ranges)
        if min_distance < 0.5:  # Obstacle within 0.5m
            self.get_logger().warn(f'Obstacle detected: {min_distance}m')

QoS Policies​

from rclpy.qos import QoSProfile, ReliabilityPolicy, DurabilityPolicy

# Sensor data: best effort, transient local
sensor_qos = QoSProfile(
    reliability=ReliabilityPolicy.BEST_EFFORT,
    durability=DurabilityPolicy.TRANSIENT_LOCAL,
    depth=10
)

# Control commands: reliable
control_qos = QoSProfile(
    reliability=ReliabilityPolicy.RELIABLE,
    depth=10
)

self.sub = self.create_subscription(LaserScan, 'scan', callback, sensor_qos)

Custom Messages​

  1. Create message file msg/CustomMsg.msg:
string name
int32 id
float64[] values

  1. Update CMakeLists.txt and package.xml

  2. Use in code:

from my_package.msg import CustomMsg

msg = CustomMsg()
msg.name = "sensor_1"
msg.id = 42
msg.values = [1.0, 2.0, 3.0]

CLI Tools​

# List topics
ros2 topic list

# Echo topic
ros2 topic echo /cmd_vel

# Info
ros2 topic info /scan

# Hz
ros2 topic hz /scan

# Pub from CLI
ros2 topic pub /cmd_vel geometry_msgs/Twist "{linear: {x: 0.5}}"

Exercise: Create node that subscribes to /scan and publishes /cmd_vel to avoid obstacles.

Next: Services