Memahami Nodes, Topics, Services, dan Actions di ROS 2 | Understanding Nodes, Topics, Services, and Actions in ROS 2

Konsep Dasar ROS 2

Learning Objectives

Setelah materi ini, kamu akan memahami: (1) Nodes sebagai unit eksekusi, (2) Topics untuk komunikasi publish/subscribe, (3) Services untuk request/response, (4) Actions untuk task dengan feedback.

Nodes

Node adalah unit eksekusi terkecil di ROS 2. Setiap node menjalankan satu tugas spesifik dan berkomunikasi dengan node lain.

Interactive Demo: ROS 2 Node Graph

Publisher

Subscriber

Both

Click on nodes to see their connections. Hover over lines to see topic names.

Membuat Node Sederhana

# minimal_node.py
import rclpy
from rclpy.node import Node

class MinimalNode(Node):
    def __init__(self):
        super().__init__('minimal_node')
        self.get_logger().info('Hello from MinimalNode!')

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

if __name__ == '__main__':
    main()

CLI Commands untuk Nodes

# List semua nodes yang sedang berjalan
ros2 node list

# Info tentang node tertentu
ros2 node info /minimal_node

Topics

Topics adalah channel untuk komunikasi publish/subscribe - one-way, continuous data streaming.

Interactive Demo: ROS 2 Topic Communication

Publisher

camera_node

/camera/image

Subscriber

detector_node

$ ros2 topic echo /camera/image

Waiting for messages...

View Code

# Publisher
self.publisher = self.create_publisher(
    String, '/camera/image', 10
)
self.publisher.publish(msg)

# Subscriber  
self.subscription = self.create_subscription(
    String, '/camera/image',
    self.callback, 10
)

Contoh: Publisher Node

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

class PublisherNode(Node):
    def __init__(self):
        super().__init__('publisher_node')
        self.publisher_ = self.create_publisher(String, 'chatter', 10)
        self.timer = self.create_timer(0.5, self.timer_callback)
        self.count = 0

    def timer_callback(self):
        msg = String()
        msg.data = f'Hello BASCORRO #{self.count}'
        self.publisher_.publish(msg)
        self.count += 1

Contoh: Subscriber Node

class SubscriberNode(Node):
    def __init__(self):
        super().__init__('subscriber_node')
        self.subscription = self.create_subscription(
            String,
            'chatter',
            self.listener_callback,
            10
        )

    def listener_callback(self, msg):
        self.get_logger().info(f'Received: "{msg.data}"')

CLI Commands untuk Topics

ros2 topic list                    # List semua topics
ros2 topic echo /chatter           # Lihat data yang di-publish
ros2 topic info /chatter           # Info tentang topic
ros2 topic hz /chatter             # Check publish rate

Services

Services adalah komunikasi request/response - synchronous, two-way communication. Cocok untuk operasi yang membutuhkan konfirmasi.

Contoh: Service Server

from std_srvs.srv import SetBool

class ServiceServer(Node):
    def __init__(self):
        super().__init__('service_server')
        self.srv = self.create_service(
            SetBool, 'set_led', self.set_led_callback
        )
        self.led_on = False

    def set_led_callback(self, request, response):
        self.led_on = request.data
        response.success = True
        response.message = f'LED is now {"ON" if self.led_on else "OFF"}'
        return response

CLI Commands untuk Services

ros2 service list
ros2 service call /set_led std_srvs/srv/SetBool "data: true"

Actions

Actions adalah untuk task yang membutuhkan waktu lama dengan feedback dan kemampuan cancel.

Use Case	Contoh
Task lama	Walking to goal, executing motion sequence
Butuh feedback	Navigation progress, motion percentage
Bisa di-cancel	Stop walking, abort kick

CLI Commands untuk Actions

ros2 action list
ros2 action info /navigate_to_pose

Perbandingan

Aspek	Topics	Services	Actions
Pola	Pub/Sub	Request/Response	Goal/Feedback/Result
Arah	One-way	Two-way	Two-way + Feedback
Blocking	Non-blocking	Blocking	Non-blocking
Use Case	Sensor data, continuous	Get/Set params	Long tasks
Cancel	N/A	N/A	Yes

Latihan

Challenge: Ball Position Publisher

Buat sebuah node yang:

Publish posisi bola (x, y) setiap 100ms ke topic /ball_position
Posisi bola berubah secara random dalam range 0-10

Hints: Gunakan geometry_msgs/msg/Point dan random.uniform(0, 10)

Lihat Solusi

import rclpy
from rclpy.node import Node
from geometry_msgs.msg import Point
import random

class BallPublisher(Node):
    def __init__(self):
        super().__init__('ball_publisher')
        self.publisher_ = self.create_publisher(Point, 'ball_position', 10)
        self.timer = self.create_timer(0.1, self.publish_ball)

    def publish_ball(self):
        msg = Point()
        msg.x = random.uniform(0, 10)
        msg.y = random.uniform(0, 10)
        msg.z = 0.0
        self.publisher_.publish(msg)

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

Quiz

1. Kapan menggunakan Topic vs Service?

Topic: Untuk data continuous yang perlu di-broadcast ke banyak subscriber (sensor data, camera images, robot state).

Service: Untuk operasi request-response satu kali (get parameter, trigger action).

2. Apa keuntungan Actions dibanding Service?

Actions menyediakan: (1) Feedback progress selama eksekusi, (2) Kemampuan cancel task, (3) Non-blocking execution.

Konsep Dasar ROS 2 | ROS 2 Core Concepts