Support ShouldProcess in Invoke-DscResource when calling the Set method

[michaeltlombardi]

As an operator using DSC Resources to conform my environment, I want to be able call to Invoke-DscResource with the Set method in a no-op mode to see what it would do to change my system state/step through the state-modifying statements so I can better understand what it is doing/debug behavior on my system without having to understand PowerShell debugging intimately.

With the current implementation of DSC, a user can:

• retrieve the current state with the Get method
• learn whether the resource is in or out of state - and only in/out, not how (see Add support for reporting how a DSC Resource is out-of-state via the Test method #77, which addresses the need for more detailed information from Test)
• enforce the desired state with the Set method

But there's no way for a user to see how the set method would behave without actually running it. This functionality is particularly useful when inspecting the behavior (or misbehavior) of a resource interactively and when exploring implementations as a resource author.

It's also functionality that exists in many higher-order configuration management tools. With Puppet, users can specify the --noop flag. With Chef, users can specify the --why-run flag. With CFEngine, users can specify the --dry-run flag. With Ansible, users can specify the --check and --diff flags. With Salt, users can specify test=True.

In PowerShell, there is already a functional abstraction for this in the form of ShouldProcess and the WhatIf and Confirm switch parameters. Support for this on the Invoke-DscResource cmdlet would both enable interactive users to check the behavior of DSC Resources and make supporting this functionality much easier for integrating vendors / higher order configuration management tools.

Proposal

I propose that the Invoke-DscResource cmdlet be updated to support ShouldProcess, that ShouldProcess support should be extended to methods defined on PowerShell classes, and that guidance be added to help DSC Resource authors understand how they can write DSC Resources that use this behavior effectively.

I haven't dug into the underlying code at all, but I suspect this is somewhat bigger than just adding ShouldProcess to the Invoke-DscResource cmdlet as I don't believe there is any current way to annotate that a PowerShell class method supports ShouldProcess behavior. I'm not sure on the implementation and whether it can be accomplished without the need for a $PSClass variable similar to $PSCmdlet.