Basic Guide to build and run ROS 2 Nodes (Python & C++)

Nodes are subprograms in an application, responsible for only one thing. Nodes communicate with each other through topics, services, and parameters. Like OOP, nodes reduce code complexity, and provide low fault tolerance. Even further, nodes can be written in many different programming languages including Python and C++. Nodes should have a single purpose while communicating each nodes.

In ros2, a package is an independent unit in an application. packages contain nodes, enabling inter-package communication.

flowchart TB

  %% -------------------------
  %% Package 1
  %% -------------------------
  subgraph Pkg1["sensing_pkg"]
    direction TB
    S1["Node: sensor_reader"]
    S2["Node: imu_reader"]
  end

  %% -------------------------
  %% Package 2
  %% -------------------------
  subgraph Pkg2["processing_pkg"]
    direction TB
    P1["Node: data_filter"]
    P2["Node: state_estimator"]
  end

  %% -------------------------
  %% Package 3
  %% -------------------------
  subgraph Pkg3["control_pkg"]
    direction TB
    C1["Node: controller"]
  end

  %% -------------------------
  %% Package 4
  %% -------------------------
  subgraph Pkg4["output_pkg"]
    direction TB
    O1["Node: actuator_driver"]
    O2["Node: logger"]
  end

  %% -------------------------
  %% Node-to-node communications
  %% -------------------------
  S1 -- "/sensor_data" --> P1
  S2 -- "/imu_data" --> P1

  P1 -- "/filtered_data" --> P2
  P2 -- "/state" --> C1

  C1 -- "/control_cmd" --> O1
  C1 -- "/status" --> O2

Write code for nodes. (Python)

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node


class MyNode(Node):
    def __init__(self):
        super().__init__("py_test")  # Create a node
        self.get_logger().info("Hello world")  # Logging with the node
        # Run timer_callback every 1 second.
        self.create_timer(1.0, self.timer_callback)

    def timer_callback(self):
        self.get_logger().info("Hello")


def main(args=None):
    rclpy.init(args=args)

    node = MyNode()
    rclpy.spin(node)  # makes the node keep running

    rclpy.shutdown()

if __name__ == "__main__":
    main()

Install a node to a package.

from setuptools import find_packages, setup

package_name = 'my_py_pkg'

setup(
    name=package_name,
    version='0.0.0',
    packages=find_packages(exclude=['test']),
    data_files=[
        ('share/ament_index/resource_index/packages',
            ['resource/' + package_name]),
        ('share/' + package_name, ['package.xml']),
    ],
    install_requires=['setuptools'],
    zip_safe=True,
    maintainer='jj',
    maintainer_email='jj@todo.todo',
    description='TODO: Package description',
    license='TODO: License declaration',
    extras_require={
        'test': [
            'pytest',
        ],
    },
    ''' Where I've changed '''
    entry_points={
        'console_scripts': [
            "py_node = my_py_pkg.my_first_node:main"  # node_name = path_to_py_file:function_name
        ],
    },
)

setup.py

and run below so that you can

colcon build --packages-select my_py_pkg  # Build with a node
source ./install/setup.bash  # Run setup.bash whenever finished building.
ros2 run my_py_pkg py_node

You should see output similar to the following.

[INFO] [1764994343.482507922] [py_test]: Hello world

NOTE: py_test is a “node name” and py_node is an “execution name”. Node name is defined in the node Python code, and excution name is defined in the setup.py file

NOTE: Remember that below commands should be run every time the code for the node is fixed.

colcon build --packages-select my_py_pkg
source ./install/setup.bash
ros2 run my_py_pkg my_node

Write code for nodes. (C++)

#include "rclcpp/rclcpp.hpp"

class MyNode : public rclcpp::Node{
public:
    MyNode() : Node("cpp_test"), counter_(0)
    {
        RCLCPP_INFO(this->get_logger(), "Hello world");
        timer_ = this->create_wall_timer(
            std::chrono::seconds(1),
            std::bind(&MyNode::timerCallback, this)
        );
    }
private:
    void timerCallback(){
        RCLCPP_INFO(this->get_logger(), "Hello %d", counter_);
        counter_++;
    }
    rclcpp::TimerBase::SharedPtr timer_;
    int counter_;
};

int main(int argc, char **argv){
    rclcpp::init(argc, argv);

    auto node = std::make_shared<MyNode>();
    rclcpp::spin(node);

    rclcpp::shutdown();
    return 0;
}

cmake_minimum_required(VERSION 3.8)
project(my_cpp_pkg)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)

# Where I've changed
add_executable(cpp_node src/my_first_node.cpp)
ament_target_dependencies(cpp_node rclcpp)

install(TARGETS
  cpp_node
  DESTINATION lib/${PROJECT_NAME}
)

ament_package()

my_cpp_pkg/CMakeLists.txt