Pat Myron

@vladtsir does https://github.com/aws-cloudformation/aws-cloudformation-resource-schema/pull/81 fix this?

`.cfn.yaml` would probably be better since it'd still be unique, but integrations without specific support for the new extension would still at least recognize it as YAML

seen [`AWS::StackId` psuedoparameter](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/pseudo-parameter-reference.html#cfn-pseudo-param-stackid) used for some similar use cases

I think these [cross-resource references](https://github.com/aws-cloudformation/aws-cloudformation-resource-schema#relationships) are all we have planned for the near future

would be nice, been maintaining [minimal examples here](https://github.com/PatMyron/cloud/blob/master/cloudformation/minimal.yaml) in the meantime

more general issue: aws-cloudformation/cfn-language-discussion#44

catching some of these in `cfn validate` because **[`readOnlyProperties` should never overlap with `writeOnlyProperties` or `createOnlyProperties` or `required`](https://github.com/aws-cloudformation/aws-cloudformation-resource-schema/#resource-semantics):** https://github.com/aws-cloudformation/cloudformation-cli/pull/663 https://github.com/aws-cloudformation/cloudformation-cli/pull/668 still not sure why CloudFormation diverged from [`readOnly` and `writeOnly`...

@rjlohan trying to assemble a list of likely stateful resource types here to help prevent data loss: https://github.com/aws-cloudformation/cfn-python-lint/blob/f476b226753a6df494091f392d78990ef93517ef/src/cfnlint/rules/resources/UpdateReplacePolicyDeletionPolicyOnStatefulResourceTypes.py#L22-L44

In addition to this problem with `createOnlyProperties` being more nuanced than being expressable in just a boolean, the same problem applies to `readOnlyProperties` and `writeOnlyProperties` as well https://twitter.com/InesParnisari/status/1327040431138299904

added some of these to `cfn validate`: https://github.com/aws-cloudformation/cloudformation-cli/pull/663, https://github.com/aws-cloudformation/cloudformation-cli/pull/668, https://github.com/aws-cloudformation/cloudformation-cli/pull/675, https://github.com/aws-cloudformation/cloudformation-cli/pull/729