User defined functions

[ThomasNieto]

Summary of the new feature / enhancement

As a user I want to be able to define my own functions to perform logic within the config document.

Proposed technical implementation details (optional)

Use the existing extension framework used for secrets and imports to to be able to define namespaced user functions.

ARM and BICEP examples. However , they do not meet my needs of being able to call an external program to perform logic.

https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/user-defined-functions
https://learn.microsoft.com/en-us/azure/azure-resource-manager/templates/user-defined-functions

[Gijsreyn]

Looking at the docs, I guess the initial work on ARM language v2 is a requirement and can perhaps be added to this list?

[michaeltlombardi]

This is certainly something I've wanted for myself. Using an extension-provided function requires resolving a few different issues (non-exhaustive, just the first ones that come to mind):

• How should DSC indicate that a function doesn't exist? Ideally, I would like to parse the configuration document for all function invocations before further actions and report whether/which functions will fail because they don't exist. We can't necessarily pre-validate the function values, because they could point to the output of other functions, but we can probably determine whether you're using an unrecognized function.
• How does DSC indicate to higher order tools whether the document depends on any extensions?
• How do we verify that the configuration functions behave as-expected for the existing design principles?
• How do we inform editor integrations about extensions?

[ThomasNieto]

@SteveL-MSFT is this something we can squeeze into 3.2? This feature is one of the blockers for adoption in my org.

[SteveL-MSFT]

@SteveL-MSFT is this something we can squeeze into 3.2? This feature is one of the blockers for adoption in my org.

I'll have to look into how complicated it will be to implement

[ThomasNieto]

@SteveL-MSFT after reviewing the PR I have the following questions.

Does the PR support creating a user function to call an external program with arguments similar to a secret extension?

Some use cases I have are:

• System info discovery like getting a customer identifier [myOrg.Customer()] and environment context [myOrg.EnvContext()]
• Integrating with key/value REST service [myOrg.ConfigService().prop.childProp]

[SteveL-MSFT]

@ThomasNieto no, the PR supports my understanding of what ARM user functions supports (which is basically a macro). If you need to call an external program, you could use the PSScript resource to do that and wrap the output as JSON so then you can reference() it elsewhere.

If you need to call a REST endpoint, I think it would be better if we had a general REST resource to do that.

[ThomasNieto]

@SteveL-MSFT like with the secret discussion within the parameter block #1079 both of those solutions do not work for that scenario. The goal would be able to use the key/value pair endpoint to be the default values within the parameter block so that if no parameter is supplied it uses the endpoint config but if a user wanted to override for testing they would be able to pass a parameter.

[SteveL-MSFT]

@SteveL-MSFT like with the secret discussion within the parameter block #1079 both of those solutions do not work for that scenario. The goal would be able to use the key/value pair endpoint to be the default values within the parameter block so that if no parameter is supplied it uses the endpoint config but if a user wanted to override for testing they would be able to pass a parameter.

Ok, so if I understand correctly, you need the default parameter values to be dynamic whereas currently they have to be static, right? I think you should open a new issue for that. Perhaps we could have an extension point just for that purpose?