Passing params/references to groups/adapters

[SteveL-MSFT]

Summary of the new feature / enhancement

dsc only has visibility in the top most layer. This means that functions that get called such as parameter() and reference() can only access those within the same layer. A group can access those functions, but only within that group's layer.

Currently, there isn't a way to pass parameters to groups and a group cannot access a reference outside of its layer.

Proposed technical implementation details (optional)

I think we are ok to say that reference only works within a group, but I can see utility in passing parameters lower. In the case of groups, we could define a new to pass parameter values with the expectation that the either the group has a literal parameters section it resolves or we can have it inherit the parent's parameter definitions.

For adapters, we may just not have support for either.

[SteveL-MSFT]

With this PR https://github.com/PowerShell/DSC/pull/385, we can define a new Arg type to indicate passing parameters and another for parameter values which the group resource or adapter can use. In the case of the PS-Adapter, it could then define a global $DSCParameter hashtable that could be used with PS resource properties that get resolved before passing to the PS resource.

[michaeltlombardi]

I think for long-term functionality, we may need to think about a way to reference nested instances. Consider the following cases:

• I have a configuration that depends on platform information returned by Microsoft/OSInfo - I need to have slightly different handling depending on the output from the operating system family. For the configuration to be settable, the Microsoft/OSInfo instance needs to be nested in a Microsoft.DSC/Assertion group instance, but I then can't reference the value.

  In this case, my options seem to be

  1. To move the problem up a layer, pre-filtering my machines to select which parameters to pass to my configuration document, or
  2. To customize a resource to handle looking up the platform information internally, instead of reusing the existing resource.

• I have a configuration that uses a mix of PSDSC resources and command-based resources. I can't retrieve any outputs from my PSDSC resources to use with my command-based resources. If I can't reference adapted resources from an instance outside of that adapter, I'm locked into using only resources for that adapter or rewriting them as non-adapted resources myself.

[SteveL-MSFT]

Part of the challenge would be bi-directional support so that raw result content can be sent up to a parent so that a sibling can access it.