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Composing multiple nodes in a single process

Goal: Compose multiple nodes into a single process.

Tutorial level: Intermediate

Time: 20 minutes

Background 

See the conceptual article.

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::Server
  composition::Client

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

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

ros2 pkg prefix composition

to get the path to where composition is installed. Then call

ros2 run composition dlopen_composition <path_to_composition_install>\bin\talker_component.dll <path_to_composition_install>\bin\listener_component.dll

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 a launch file:

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 (see talker source code). The command will return the unique ID of the loaded component as well as the node name.

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
/talker2
/ns/talker
/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

Composing Non-Node Derived Components 

In ROS 2, components allow for more efficient use of system resources and provide a powerful feature that enables you to create reusable functionality that is not tied to a specific node.

One advantage of using components is that they allow you to create non-node derived functionality as standalone executables or shared libraries that can be loaded into the ROS system as needed.

To create a component that is not derived from a node, follow these guidelines:

Implement a constructor that takes const rclcpp::NodeOptions& as its argument.
Implement the get_node_base_interface() method, which should return a NodeBaseInterface::SharedPtr. You can use the get_node_base_interface() method of a node that you create in your constructor to provide this interface.

Here’s an example of a component that is not derived from a node, which listens to a ROS topic: node_like_listener_component.

For more information on this topic, you can refer to this discussion.