How do I resolve the "Attribute 'Key' does not exist" error when I use the Fn::GetAtt function on my resource provider resource in CloudFormation?

Short description

CloudFormation returns the "Attribute 'Key' does not exist" error because it isn't receiving the required property. The ReadHandler of your resource must return a property that's specified from the readOnlyProperties list in the organization-service-resource.json resource provider schema file.

For other errors that are related to using a resource provider, see the following articles:

• How do I resolve the "Resource specification is invalid" error when I run the cfn generate command using the CloudFormation CLI in CloudFormation?
• How do I resolve the "Model validation failed (#: extraneous key [Key] is not permitted)" error in CloudFormation?
• How do I resolve the "Resource timed out waiting for creation of physical resource" error when I create a resource using my resource provider type in CloudFormation?
• How do I resolve the "java.lang.ClassNotFoundException: com.example.package.resource.HandlerWrapper" error in CloudFormation?

Resolution

1.    In your organization-service-resource.json file, confirm that the readOnlyProperties definition uses the following format, where Output is a property that's defined in the properties section. For example:

"readOnlyProperties": [
    "/properties/Output"
],

Note: The organization-service-resource.json format is located in the root directory of your project.

2.    In your ReadHandler, set the property in the model object. For example:

final ResourceModel model = request.getDesiredResourceState();
model.setOutput("abcdxyz");
return ProgressEvent.<ResourceModel, CallbackContext>builder()
    .resourceModel(model)
    .status(OperationStatus.SUCCESS)
    .build();

Related information

AWS CloudFormation CLI (from the GitHub website)