add explanation about component container types and options

[fujitatomoya]

found on ros2/rclcpp#2541 (comment), it is really hard for user to know the component container options.

we could add the following to the documentation, https://docs.ros.org/en/rolling/Concepts/Intermediate/About-Composition.html

• general explanation about component container, which is the node that loads/unloads the components and execute them.
• component container types and options, such as component_container, component_container_mt with parameter thread_num, and component_container_isolated with --use_multi_threaded_executor option.

[fujitatomoya]

@clalancette @christophebedard @ahcorde what do you think?

[christophebedard]

it should absolutely be documented, but maybe in a tutorial rather than a concepts article? I think this is the only composition tutorial: https://docs.ros.org/en/rolling/Tutorials/Intermediate/Composition.html

[clalancette]

it should absolutely be documented, but maybe in a tutorial rather than a concepts article? I think this is the only composition tutorial: https://docs.ros.org/en/rolling/Tutorials/Intermediate/Composition.html

Yeah, agreed. The general explanation about the component container can go into the Concepts, but then I think we should expand the tutorial instead.

[christophebedard]

The first bullet point in this section of the composition concepts article does mention "generic container process" and links to component_container.cpp: https://docs.ros.org/en/rolling/Concepts/Intermediate/About-Composition.html#using-components. We could just briefly mention here that there are other containers and link to the tutorial.

In the tutorial, component_container is used. We could add a new subsection under Advanced Topics to list the other containers and their options: https://docs.ros.org/en/rolling/Tutorials/Intermediate/Composition.html#advanced-topics.