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Using ROS 2 launch to launch composable nodes

In the Composition tutorial, you learned about composable nodes and how to use them from the command-line. In the Launch tutorials, you learned about launch files and how to use them to manage multiple nodes.

This guide will combine the above two topics and teach you how to write launch files for composable nodes.

Setup

See the installation instructions for details on installing ROS 2.

If you’ve installed ROS 2 from packages, ensure that you have ros-galactic-image-tools installed. If you downloaded the archive or built ROS 2 from source, it will already be part of the installation.

Launch file examples

Below is a launch file that launches composable nodes in Python, XML, and YAML. The launch files all do the following:

  • Instantiate a cam2image composable node with remappings, custom parameters, and extra arguments

  • Instantiate a showimage composable node with remappings, custom parameters, and extra arguments

import launch
from launch_ros.actions import ComposableNodeContainer
from launch_ros.descriptions import ComposableNode


def generate_launch_description():
    """Generate launch description with multiple components."""
    container = ComposableNodeContainer(
            name='image_container',
            namespace='',
            package='rclcpp_components',
            executable='component_container',
            composable_node_descriptions=[
                ComposableNode(
                    package='image_tools',
                    plugin='image_tools::Cam2Image',
                    name='cam2image',
                    remappings=[('/image', '/burgerimage')],
                    parameters=[{'width': 320, 'height': 240, 'burger_mode': True, 'history': 'keep_last'}],
                    extra_arguments=[{'use_intra_process_comms': True}]),
                ComposableNode(
                    package='image_tools',
                    plugin='image_tools::ShowImage',
                    name='showimage',
                    remappings=[('/image', '/burgerimage')],
                    parameters=[{'history': 'keep_last'}],
                    extra_arguments=[{'use_intra_process_comms': True}])
            ],
            output='both',
    )

    return launch.LaunchDescription([container])

Loading composable nodes into an existing container

Containers can sometimes be launched by other launch files or from a commandline. In that case, you need to add your components to an existing container. For this, you may use LoadComposableNodes to load components into a given container. The below example launches the same nodes as above.

from launch import LaunchDescription
from launch_ros.actions import LoadComposableNodes, Node
from launch_ros.descriptions import ComposableNode

def generate_launch_description():
    container = Node(
        name='image_container',
        package='rclcpp_components',
        executable='component_container',
        output='both',
    )

    load_composable_nodes = LoadComposableNodes(
        target_container='image_container',
        composable_node_descriptions=[
            ComposableNode(
                 package='image_tools',
                plugin='image_tools::Cam2Image',
                name='cam2image',
                remappings=[('/image', '/burgerimage')],
                parameters=[{'width': 320, 'height': 240, 'burger_mode': True, 'history': 'keep_last'}],
                extra_arguments=[{'use_intra_process_comms': True}],
            ),
            ComposableNode(
                package='image_tools',
                plugin='image_tools::ShowImage',
                name='showimage',
                remappings=[('/image', '/burgerimage')],
                parameters=[{'history': 'keep_last'}],
                extra_arguments=[{'use_intra_process_comms': True}]
            ),
        ],
    )

    return LaunchDescription([container, load_composable_nodes])

Using the Launch files from the command-line

Any of the launch files above can be run with ros2 launch. Copy the data into a local file, and then run:

ros2 launch <path_to_launch_file>

Intra-process communications

All of the above examples use an extra argument to setup intra-process communication between the nodes. For more information on what intra-process communications are, see the intra-process comms tutorial.