Composing multiple nodes in a single process

Goal: Compose multiple nodes into a single process.

Tutorial level: Intermediate

Time: 20 minutes

Run the demos

The demos use executables from rclcpp_components, ros2component, and composition packages, and can be run with the following commands.

Discover available components

To see what components are registered and available in the workspace, execute the following in a shell:

ros2 component types

The terminal will return the list of all available components:

(... components of other packages here)
composition
  composition::Talker
  composition::Listener
  composition::NodeLikeListener
  composition::Server
  composition::Client
(... components of other packages here)

Run-time composition using ROS services with a publisher and subscriber

In the first shell, start the component container:

ros2 run rclcpp_components component_container

Open the second shell and verify that the container is running via ros2 command line tools:

ros2 component list

You should see a name of the component:

/ComponentManager

In the second shell load the talker component (see talker source code):

ros2 component load /ComponentManager composition composition::Talker

The command will return the unique ID of the loaded component as well as the node name:

Loaded component 1 into '/ComponentManager' container node as '/talker'

Now the first shell should show a message that the component was loaded as well as repeated message for publishing a message.

Run another command in the second shell to load the listener component (see listener source code):

ros2 component load /ComponentManager composition composition::Listener

Terminal will return:

Loaded component 2 into '/ComponentManager' container node as '/listener'

The ros2 command line utility can now be used to inspect the state of the container:

ros2 component list

You will see the following result:

/ComponentManager
   1  /talker
   2  /listener

Now the first shell should show repeated output for each received message.

Run-time composition using ROS services with a server and client

The example with a server and a client is very similar.

In the first shell:

ros2 run rclcpp_components component_container

In the second shell (see server and client source code):

ros2 component load /ComponentManager composition composition::Server
ros2 component load /ComponentManager composition composition::Client

In this case the client sends a request to the server, the server processes the request and replies with a response, and the client prints the received response.

Compile-time composition using ROS services

This demos shows that the same shared libraries can be reused to compile a single executable running multiple components. The executable contains all four components from above: talker and listener as well as server and client.

In the shell call (see source code):

ros2 run composition manual_composition

This should show repeated messages from both pairs, the talker and the listener as well as the server and the client.

Note

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

Run-time composition using dlopen

This demo presents 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 source code).

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

Now the shell should show repeated output for each sent and received message.

Note

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

Composition using launch actions

While the command line tools are useful for debugging and diagnosing component configurations, it is frequently more convenient to start a set of components at the same time. To automate this action, we can use the functionality in ros2 launch.

ros2 launch composition composition_demo.launch.py

Advanced Topics

Now that we have seen the basic operation of components, we can discuss a few more advanced topics.

Unloading components

In the first shell, start the component container:

ros2 run rclcpp_components component_container

Verify that the container is running via ros2 command line tools:

ros2 component list

You should see a name of the component:

/ComponentManager

In the second shell load both the talker and listener as we have before:

ros2 component load /ComponentManager composition composition::Talker
ros2 component load /ComponentManager composition composition::Listener

Use the unique ID to unload the node from the component container.

ros2 component unload /ComponentManager 1 2

The terminal should return:

Unloaded component 1 from '/ComponentManager' container
Unloaded component 2 from '/ComponentManager' container

In the first shell, verify that the repeated messages from talker and listener have stopped.

Remapping container name and namespace

The component manager name and namespace can be remapped via standard command line arguments:

ros2 run rclcpp_components component_container --ros-args -r __node:=MyContainer -r __ns:=/ns

In a second shell, components can be loaded by using the updated container name:

ros2 component load /ns/MyContainer composition composition::Listener

Note

Namespace remappings of the container do not affect loaded components.

Remap component names and namespaces

Component names and namespaces may be adjusted via arguments to the load command.

In the first shell, start the component container:

ros2 run rclcpp_components component_container

Some examples of how to remap names and namespaces.

Remap node name:

ros2 component load /ComponentManager composition composition::Talker --node-name talker2

Remap namespace:

ros2 component load /ComponentManager composition composition::Talker --node-namespace /ns

Remap both:

ros2 component load /ComponentManager composition composition::Talker --node-name talker3 --node-namespace /ns2

Now use ros2 command line utility:

ros2 component list

In the console you should see corresponding entries:

/ComponentManager
   1  /talker2
   2  /ns/talker
   3  /ns2/talker3

Note

Namespace remappings of the container do not affect loaded components.

Passing parameter values into components

The ros2 component load command-line supports passing arbitrary parameters to the node as it is constructed. This functionality can be used as follows:

ros2 component load /ComponentManager image_tools image_tools::Cam2Image -p burger_mode:=true

Passing additional arguments into components

The ros2 component load command-line supports passing particular options to the component manager for use when constructing the node. As of now, the only command-line option that is supported is to instantiate a node using intra-process communication. This functionality can be used as follows:

ros2 component load /ComponentManager composition composition::Talker -e use_intra_process_comms:=true

Composable nodes as shared libraries

If you want to export a composable node as a shared library from a package and use that node in another package that does link-time composition, add code to the CMake file which imports the actual targets in downstream packages.

Then install the generated file and export the generated file.

A practical example can be seen here: ROS Discourse - Ament best practice for sharing libraries