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ROS 2 Nodes, Topics, and Services.

Heading Breakdown​

ROS 2 Nodes, Topics, and Services are the atomic units of the ROS graph. Nodes are the executable processes that perform computation—the "neurons" of the robot. Topics are the named buses over which nodes exchange messages—the "axons" carrying signals. Services are the synchronous request-response interfaces—the "reflexes" where you ask for a specific outcome and wait for it. Understanding these is critical because they define the computational graph of the robot. Real usage involves a "Camera Node" publishing images to an "Image Topic," which a "Vision Node" subscribes to. An example is the /cmd_vel topic, the universal interface for telling a mobile base how fast to move. This is key for upgradable high-DoF humanoids because it decouples the sender from the receiver; you can replace the joystick teleoperation node with an AI path planner node, and the wheel controller doesn't know the difference.

Training Focus: Modular Architecture​

We focus on decoupling. A well-architected humanoid system has dozens of nodes. We train you to define clear boundaries.

  • Single Responsibility Principle: A node should do one thing well (e.g., "Face Detection," not "Face Detection and Arm Control").
  • Interface Stability: Topics serve as the contract between modules.

Detailed Content​

Nodes​

A node in ROS 2 is an object that interacts with the ROS graph.

  • Lifecycle: Nodes can be managed (Unconfigured, Inactive, Active, Finalized).
  • Composition: Multiple nodes can run in a single process to save memory.

Topics (Publish-Subscribe)​

The primary way data moves.

  • Many-to-Many: One publisher can talk to ten subscribers.
  • Asynchronous: The publisher doesn't wait for the subscriber to process the data.

Services (Request-Response)​

For "transactions."

  • Synchronous: The client blocks until the server replies.
  • Example: spawn_entity in Gazebo.

Industry Vocab​

  • Callback: A function that runs when a message arrives.
  • Spin: The loop that keeps the node alive and checking for callbacks.
  • Service Definition (.srv): The file defining the request and response structure.

Code Example: Typed Publisher​

# Defensive Node with Typed Interfaces
from rclpy.node import Node
from std_msgs.msg import String

class SupervisorNode(Node):
    def __init__(self):
        super().__init__('supervisor_node')
        # Explicit type declaration prevents serialization errors
        self.publisher_ = self.create_publisher(String, 'system_status', 10)
        self.timer = self.create_timer(1.0, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = f'System Nominal: {self.i}'
        self.publisher_.publish(msg)
        self.get_logger().info(f'Publishing: "{msg.data}"')
        self.i += 1

Real-World Use Case: Unitree G1​

In the G1, the Battery Node publishes the voltage to the /battery_state topic. The Safety Node subscribes to this. If the voltage drops below 20V, the Safety Node triggers a service call to the Motion Manager to enter_crouch_mode, ensuring the robot sits down safely before power failure.