5.4.8. 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.
5.4.8.1. Setup¶
See the installation instructions for details on installing ROS 2.
If you’ve installed ROS 2 from packages, ensure that you have ros-jazzy-image-tools installed.
If you downloaded the archive or built ROS 2 from source, it will already be part of the installation.
5.4.8.2. Launch file examples¶
Below is a launch file that launches composable nodes in XML, YAML, and Python. 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
<?xml version="1.0" encoding="UTF-8"?>
<launch>
<node_container pkg="rclcpp_components" exec="component_container" name="image_container">
<composable_node pkg="image_tools" plugin="image_tools::Cam2Image" name="cam2image">
<remap from="/image" to="/burgerimage" />
<param name="width" value="320" />
<param name="height" value="240" />
<param name="burger_mode" value="true" />
<param name="history" value="keep_last" />
<extra_arg name="use_intra_process_comms" value="true" />
</composable_node>
<composable_node pkg="image_tools" plugin="image_tools::ShowImage" name="showimage">
<remap from="/image" to="/burgerimage" />
<param name="history" value="keep_last" />
<extra_arg name="use_intra_process_comms" value="true" />
</composable_node>
</node_container>
</launch>
%YAML 1.2
---
launch:
- node_container:
pkg: rclcpp_components
exec: component_container
name: image_container
composable_node:
- pkg: image_tools
plugin: image_tools::Cam2Image
name: cam2image
remap:
- from: /image
to: /burgerimage
param:
- name: width
value: 320
- name: height
value: 240
- name: burger_mode
value: true
- name: history
value: keep_last
extra_arg:
- name: use_intra_process_comms
value: true
- pkg: image_tools
plugin: image_tools::ShowImage
name: showimage
remap:
- from: /image
to: /burgerimage
param:
- name: history
value: keep_last
extra_arg:
- name: use_intra_process_comms
value: true
import launch
from launch_ros.actions import ComposableNodeContainer
from launch_ros.descriptions import ComposableNode
def generate_launch_description():
"""Generate launch description with multiple components."""
return launch.LaunchDescription([
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',
),
])
5.4.8.3. 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.
<?xml version="1.0" encoding="UTF-8"?>
<launch>
<node pkg="rclcpp_components" exec="component_container" name="image_container" />
<load_composable_node target="image_container">
<composable_node pkg="image_tools" plugin="image_tools::Cam2Image" name="cam2image">
<remap from="/image" to="/burgerimage" />
<param name="width" value="320" />
<param name="height" value="240" />
<param name="burger_mode" value="true" />
<param name="history" value="keep_last" />
<extra_arg name="use_intra_process_comms" value="true" />
</composable_node>
<composable_node pkg="image_tools" plugin="image_tools::ShowImage" name="showimage" namespace="">
<remap from="/image" to="/burgerimage" />
<param name="history" value="keep_last" />
<extra_arg name="use_intra_process_comms" value="true" />
</composable_node>
</load_composable_node>
</launch>
%YAML 1.2
---
launch:
- node_container:
pkg: rclcpp_components
exec: component_container
name: image_container
- load_composable_node:
target: "image_container"
composable_node:
- pkg: image_tools
plugin: image_tools::Cam2Image
name: cam2image
remap:
- from: /image
to: /burgerimage
param:
- name: width
value: 320
- name: height
value: 240
- name: burger_mode
value: true
- name: history
value: keep_last
extra_arg:
- name: use_intra_process_comms
value: true
- pkg: image_tools
plugin: image_tools::ShowImage
name: showimage
remap:
- from: /image
to: /burgerimage
param:
- name: history
value: keep_last
extra_arg:
- name: use_intra_process_comms
value: true
from launch import LaunchDescription
from launch_ros.actions import LoadComposableNodes, Node
from launch_ros.descriptions import ComposableNode
def generate_launch_description():
return LaunchDescription([
Node(
name='image_container',
package='rclcpp_components',
executable='component_container',
output='both',
),
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}]
),
],
)
])
5.4.8.4. 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>
5.4.8.5. 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.
5.4.8.6. XML, YAML, or Python: Which should I use?¶
See the discussion in Using XML, YAML, and Python for ROS 2 Launch Files for more information.