construct CfnReferences for CfnInclude'ed resources

[anguslees]

A way to construct a CfnReference for a resource (with known logicalId) that was included via CfnInclude.

Use Case

I have some non-trivial existing cfn. I want to start using cdk. I have a NestedStack that is basically just a CfnInclude of my old cfn. Great.

Now I want to refer to resources within that NestedStack from my parent/sibling stacks. I can manually create CfnOutputs using getAttr in my NestedStack, and reference them appropriately using getAttr again on the NestedStack, but that's boring. Ideally I could create a CfnReference and the usual token/resolve magic would do all that for me.

Except CfnReference is (currently) private, so I can't. I think the only place it is currently exposed is via CfnResource.getAttr, and I don't want the rest of the CfnResource implementation (because I'm not actually creating new cfn output for my resource).

Proposed Solution

Either:

1. Move CfnResource.getAtt up to CfnRefElement.getAtt. Basically anything that is read-only should maybe go to CfnRefElement/CfnElement.

2. Create a new dummy "IncludedResource" class intended specifically for use with CfnInclude. I think it should extend CfnRefElement, and output nothing to the synthesized cfn . Give it a fixed logicalId, and (at least) a getAtt method that uses CfnReference.

In addition, the naive Fn.getAtt("S3Bucket", "Arn") example on the end of https://docs.aws.amazon.com/cdk/latest/guide/use_cfn_template.html should be updated to use our new improved solution, and thus be cross-stack safe.

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

---

This is a 🚀 Feature Request

[anguslees]

This isn't the way I'd implement it, but the following looks like it works as a PoC:

// About the only useful thing you can do with this is ref and getAtt()
export class DummyResource extends cdk.CfnResource {
    constructor(scope: cdk.Construct, logicalId: string) {
        super(scope, logicalId, {type: "Dummy"});
        this.overrideLogicalId(logicalId);
    }

    public _toCloudFormation(): object {
        return {};
    }
}

[eladb]

@skinny85 is currently working on a solution for this.

[skinny85]

@skinny85 is currently working on a solution for this.

#7304

[skinny85]

This is now possible with our cfn-include module.

Resolving, let us know if need anything else from our side for this issue.