cli: support `--import-existing-resources` flag in cdk diff

[tmokmss]

Describe the feature

Now that cdk deploy supports --import-existing-resources flag (#32676), cdk diff should support it as well.

Use Case

I want to know which resources are imported before deployment by just executing cdk diff.

Proposed Solution

Users should be able to just run cdk diff --import-existing-resources, and get a table of resources that will be imported.

CDK diff already supports displaying imported resources, so I guess all we need is to add a flag when calling createChangeset API here:

aws-cdk/packages/aws-cdk/lib/api/util/cloudformation.ts

Lines 469 to 482 in bbdd42c

	const changeSet = await options.cfn.createChangeSet({
	StackName: options.stack.stackName,
	ChangeSetName: options.changeSetName,
	ChangeSetType: options.resourcesToImport ? 'IMPORT' : options.exists ? 'UPDATE' : 'CREATE',
	Description: `CDK Changeset for diff ${options.uuid}`,
	ClientToken: `diff${options.uuid}`,
	TemplateURL: options.bodyParameter.TemplateURL,
	TemplateBody: options.bodyParameter.TemplateBody,
	Parameters: stackParams.apiParameters,
	ResourcesToImport: options.resourcesToImport,
	RoleARN: options.role,
	Capabilities: ['CAPABILITY_IAM', 'CAPABILITY_NAMED_IAM', 'CAPABILITY_AUTO_EXPAND'],
	});

Just like here:

aws-cdk/packages/aws-cdk/lib/api/deploy-stack.ts

Lines 541 to 550 in bbdd42c

	const changeSet = await this.cfn.createChangeSet({
	StackName: this.stackName,
	ChangeSetName: changeSetName,
	ChangeSetType: this.options.resourcesToImport ? 'IMPORT' : this.update ? 'UPDATE' : 'CREATE',
	ResourcesToImport: this.options.resourcesToImport,
	Description: `CDK Changeset for execution ${this.uuid}`,
	ClientToken: `create${this.uuid}`,
	ImportExistingResources: importExistingResources,
	...this.commonPrepareOptions(),
	});

Other Information

Currently it is possible to review the changeset before an actual deployment as described here, but it is not very handy.

aws-cdk/packages/aws-cdk/README.md

Lines 415 to 420 in 3a37256

	Use the `--method=prepare-change-set` flag to review which resources are imported or not before deploying a changeset.
	You can inspect the change set created by CDK from the management console or other external tools.
	```console
	$ cdk deploy --import-existing-resources --method=prepare-change-set
	```

Acknowledgements

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

CDK version used

2.174.1

Environment details (OS name and version, etc.)

macOS

[ashishdhingra]

CDK diff is one of the prior step for CDK deploy. Hence, for cdk diff only operation, it makes sense to support --import-existing-resources flag.

[tmokmss]

Hi @kaizencc, thank you for the last PR of cdk deploy!
I'm willing to add this feature, but it seems cdk cli is getting a huge change #32775. Should I wait for the diversion to complete, or is it okay to start to work on the issue immediately?

[rix0rrr]

Depends on how long you think it will take. But I don't think our reorganization is worth blocking any interesting ongoing work on.

Yes, it will be some work to transplant a PR if it happens to not land before the split happens... but that's not insurmountable, it's just a bit of drudgery (and probably not even too bad).

I would just start if I were you.

[tmokmss]

Thanks @rix0rrr for clarification! I'll start to work on this now then :)