rclcpp: Cannot load yaml config to composable node in dashing

@mjcarroll any update on this? The last of the discussion looks to be before the dashing release. We’re coming up on Foxy and this feels like something that should be included in the next LTS.

Hey @christianrauch, @wjwwood and I spent some time discussing this today, and here’s the conclusion for the dashing release:

The way that you are currently passing the parameters to the load command ros2 component load /ComponentManager test Test -r __params:=ros2 pkg prefix test/share/test/cfg/demo.yaml is essentially a remapping argument. The parameter file that is read in that case has to have the parameters nested under node_name/ros__parameters in the YAML file, which is consistent with how a standalone node will work. In this case though, the container does not receive the parameters via the parameters field in the service call, but instead receives a remapping argument for __params.

The current implementation of the ComposableNode launch description does not understand the nested structure in the YAML file, which causes the parameters to be (incorrectly) expanded to:

CompositionContainer received parameters:
name: test_node_otto.ros__parameters.some_int, type: integer
name: test_node_otto.ros__parameters.a_string, type: string

I think that there are two potential short-term workarounds (without requiring major changes in the way that the ComposableNode description works)

1. Pass your existing yaml file in as a remapping argument in the launch description:

composable_node = ComposableNode(package='test', node_plugin='Test',
                                 remappings=[('__params', get_package_share_directory('test')+"/cfg/demo.yaml")])

This will then pass in parameters similar to the load call above.

1. Use a new yaml file that doesn’t have any of the nesting:

some_int: 42
a_string: "Hello World"

And then pass that in as parameters to the ComposableNode description:

composable_node = ComposableNode(package='test', node_plugin='Test',
                                     parameters=[get_package_share_directory('test')+"/cfg/demo_no_nesting.yaml"])

The caveat on approach 1 is that any additional loose parameters passed via the parameters kwarg will always supersede what’s in the remappings argument, where in approach 2, the ordering in the list passed to parameters is important (later definitions will supersede earlier definitions).

So in the case of:

composable_node = ComposableNode(
        package='test',
        node_plugin='Test',
        parameters=[
            'some_file.yaml',  # {something: 'foo'}
            {'something': 'bar'},
            'some_other_file.yaml',  # {something: 'baz'}
        ])

The parameter something will be baz, but in the case of:

composable_node = ComposableNode(
        package='test',
        node_plugin='Test',
        parameters=[{'something': 'blah'}],
        remappings=[('__params', get_package_share_directory('test')+"/cfg/demo.yaml")])

The parameter something will be blah.