demos/composition

• What Is This?
• Build
• Run
• • Manual Composition
  • DlOpen Composition
  • Linktime Composition
  • Composition Using Launch Actions
• Verify
• • Manual Composition
  • DlOpen Composition
  • Linktime Composition
• FAQ
• References

What Is This?

This demo provides examples of three different ways to use the rclcpp_components API to compose multiple nodes in a single process.

This ROS 2 package consists of the following demo applications:

1. dlopen_composition
2. linktime_composition
3. manual_composition

Build

Run the commands below to build the ROS 2 package:

colcon build --packages-up-to composition

Run

Manual Composition

Running manual_composition compiles an executable that runs the following 4 components:

• Talker: A ROS 2 component that publishes a string
• Listener: A ROS 2 component that prints the received string from Talker
• Server: A ROS 2 component that adds two integers and outputs its result to Client
• Client: A ROS 2 component that sends two integers to Server and prints the received result from Server

ros2 run composition manual_composition

DlOpen Composition

This runs dlopen_composition which is an alternative to run-time composition by creating a generic container process and explicitly passing the libraries to load without using ROS interfaces.

The process will open each library and create one instance of each “rclcpp::Node” class in the library.

ros2 run composition dlopen_composition `ros2 pkg prefix composition`/lib/libtalker_component.so `ros2 pkg prefix composition`/lib/liblistener_component.so

Linktime Composition

Similar to previous, this runs linktime_composition which links all classes from libraries that are registered under the library_path with the linker.

ros2 run composition linktime_composition

Composition Using Launch Actions

Rather than using the command line tool to run each composition, we can automate this action with ros2 launch functionality:

ros2 launch composition composition_demo_launch.py

Verify

Manual Composition

When executed correctly, strings should be printed to terminal similar to what is shown below:

[INFO] [1674528188.026468320] [talker]: Publishing: 'Hello World: 1'
[INFO] [1674528188.027043857] [listener]: I heard: [Hello World: 1]
[INFO] [1674528189.026414368] [talker]: Publishing: 'Hello World: 2'
[INFO] [1674528189.026742015] [listener]: I heard: [Hello World: 2]
[INFO] [1674528189.032512995] [Server]: Incoming request: [a: 2, b: 3]
[INFO] [1674528189.032815843] [Client]: Got result: [5]
[INFO] [1674528190.026455807] [talker]: Publishing: 'Hello World: 3'
[INFO] [1674528190.026795770] [listener]: I heard: [Hello World: 3]
[INFO] [1674528191.026457639] [talker]: Publishing: 'Hello World: 4'
[INFO] [1674528191.026801926] [listener]: I heard: [Hello World: 4]
[INFO] [1674528191.032377264] [Server]: Incoming request: [a: 2, b: 3]
[INFO] [1674528191.032604427] [Client]: Got result: [5]
[INFO] [1674528192.026428269] [talker]: Publishing: 'Hello World: 5'
[INFO] [1674528192.026537974] [listener]: I heard: [Hello World: 5]
[INFO] [1674528193.026437034] [talker]: Publishing: 'Hello World: 6'
[INFO] [1674528193.026767708] [listener]: I heard: [Hello World: 6]
[INFO] [1674528193.032377748] [Server]: Incoming request: [a: 2, b: 3]
[INFO] [1674528193.032603036] [Client]: Got result: [5]
#...

⚠️

Note that manually-composed components will not be reflected in the ros2 component list command line tool output.

DlOpen Composition

When executed correctly, strings should be printed to terminal similar to what is shown below:

INFO] [1674529118.496557668] [dlopen_composition]: Load library /opt/ros/rolling/lib/libtalker_component.so
[INFO] [1674529118.496774575] [dlopen_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Talker>
[INFO] [1674529118.503388909] [dlopen_composition]: Load library /opt/ros/rolling/lib/liblistener_component.so
[INFO] [1674529118.503739855] [dlopen_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Listener>
[INFO] [1674529119.503505873] [talker]: Publishing: 'Hello World: 1'
[INFO] [1674529119.503770137] [listener]: I heard: [Hello World: 1]
[INFO] [1674529120.503572362] [talker]: Publishing: 'Hello World: 2'
[INFO] [1674529120.503888374] [listener]: I heard: [Hello World: 2]
[INFO] [1674529121.503503459] [talker]: Publishing: 'Hello World: 3'
[INFO] [1674529121.503628269] [listener]: I heard: [Hello World: 3]
[INFO] [1674529122.503557862] [talker]: Publishing: 'Hello World: 4'
[INFO] [1674529122.503894772] [listener]: I heard: [Hello World: 4]
[INFO] [1674529123.503574524] [talker]: Publishing: 'Hello World: 5'
[INFO] [1674529123.503884894] [listener]: I heard: [Hello World: 5]
#...

⚠️

Note that dlopen-composed components will not be reflected in the ros2 component list command line tool output.

Linktime Composition

When executed correctly, strings should be printed to terminal similar to what is shown below:

[INFO] [1674528568.091949637] [linktime_composition]: Load library
[INFO] [1674528568.091995119] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Client>
[INFO] [1674528568.098833910] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Listener>
[INFO] [1674528568.100669644] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Server>
[INFO] [1674528568.102665704] [linktime_composition]: Instantiate class rclcpp_components::NodeFactoryTemplate<composition::Talker>
[INFO] [1674528569.104717098] [talker]: Publishing: 'Hello World: 1'
[INFO] [1674528569.105206993] [listener]: I heard: [Hello World: 1]
[INFO] [1674528570.099206827] [Server]: Incoming request: [a: 2, b: 3]
[INFO] [1674528570.099376432] [Client]: Got result: [5]
[INFO] [1674528570.104656875] [talker]: Publishing: 'Hello World: 2'
[INFO] [1674528570.105069514] [listener]: I heard: [Hello World: 2]
[INFO] [1674528571.104710545] [talker]: Publishing: 'Hello World: 3'
[INFO] [1674528571.105150094] [listener]: I heard: [Hello World: 3]
[INFO] [1674528572.099350955] [Server]: Incoming request: [a: 2, b: 3]
[INFO] [1674528572.099628903] [Client]: Got result: [5]
[INFO] [1674528572.104631322] [talker]: Publishing: 'Hello World: 4'
[INFO] [1674528572.104911174] [listener]: I heard: [Hello World: 4]
[INFO] [1674528573.104596009] [talker]: Publishing: 'Hello World: 5'
[INFO] [1674528573.104751214] [listener]: I heard: [Hello World: 5]
#...

⚠️

Note that linktime-composed components will not be reflected in the ros2 component list command line tool output.

FAQ

Q: Why use node composition?

A: Node composition avoids the overhead of marshalling and unmarshaling messages by allowing nodes to be instantiated within the same process.

References

1. Composing multiple nodes in a single process
2. About Composition