Launch composable C++ nodes

[sloretz]

Feature request

This is a feature request for launching multiple composable nodes into the same process. The acceptance criteria for this ticket is limited to launching dynamically composable rclcpp non-lifecycle nodes into a container process.

Feature description

launch_ros must pass arguments to the composable nodes where "arguments" means:

• the information that can be passed via the command line of any rcl based node
  • parameters
  • remap rules
  • node name
  • namespace
  • log level
• information that is specific to the client library rclcpp
  • use_intra_process_comms
  • start_parameter_services

An exception is use_global_arguments which should always be false so a composed node is not affected by arguments passed to the container process.

Arguments does not include environment variables like RMW_IMPLEMENTATION which apply to the whole container process.

PRs

• OnProcessStart Add OnProcessStart event handler launch#171
• Reusable type checking for remap rules Add normalize_remap_rule and types launch#173
• Reusable type checking for parameters Add normalize_parameters and evaluate_paramters launch#192
• launch_ros.ComposableNode, launch_ros.actions.LoadComposableNodes, launch_ros.actions.ComposableNodeContainer plus examples from Add composable node launch actions #9

Implementation considerations

launch_ros

See slide 8 of https://roscon.ros.org/2018/presentations/ROSCon2018_launch.pdf

        ExecuteComposableNodeProcess(
            package='rclcpp_components', container_executable='rclcpp_node_container', output='screen',
            composable_node_descriptions=[
                ComposableNodeDescription(
                    package_name='demo_nodes_cpp', node_plugin_name='talker',
                    name='my_talker'),
                ComposableNodeDescription(
                    package_name='demo_nodes_cpp', node_plugin_name='listener'
                    name='my_listener')
            ]
        )

ExecuteComposableNodeProcess should start a container executable with a defined API. This allows a user to create a custom container process (e.g. create a container with a custom executor). The API may be a config file and/or a ROS API. The acceptance criteria for this ticket is limited to a ROS service API that can load and unload composable nodes (see also ros2/rcl_interfaces#60).

rclcpp

There must be a new ROS package (rclcpp_components?) with a container process for rclcpp nodes. The acceptance criteria for this task is to be able to use either a single threaded executor or multithreaded executor in a container process. For example, this could be two container processes, or one with a CLI argument to choose the executor.

Launching a composable node requires a common API for constructing a node.
One option is to mimic ROS 1 nodelets which required a default constructor on the node and had an init() member function to initialize the nodes with parameters.
Another option is having a constructor with one argument and and a factory class to call it.
Regardless this task should require ros2/rclcpp#492 so there is one object NodeArguments/NodeOptions so new arguments to a node don't require changing the signature of a function.

• https://github.com/ros2/rclcpp/tree/wjwwood/components_wip
• https://github.com/ros2/rclcpp/blob/wjwwood/components_wip/rclcpp_components/doc/source/architecture.rst
• https://github.com/ros2/rcl_interfaces/tree/wjwwood/components_wip

CMake API

This ticket includes creating a CMake API for creating a composable node. This API should

• call add_library(... SHARED ..)
• optionally add_executable() with a generated main()
• register the composable node with pluginlib

[hidmic]

@sloretz @mjcarroll can we close this one?

[mjcarroll]

Yup, I believe so.