AWS SAM custom domain fails when using !Ref

Question

If I'm defining a custom domain as a standalone resource that I'm referencing with !Ref my deployment fails. But if I use the same details for a nested object it works. Any idea why?

Custom Domains only works if both DomainName and CertificateArn are provided.

ApiDomain:
  Type: AWS::ApiGateway::DomainName
  Properties:
    DomainName: api.mydomain.com
    CertificateArn: "arn:aws:acm:blah-blah"
    Route53:
      HostedZoneName: "mydomain.com."

MyApi:
  Type: AWS::Serverless::Api
  Properties:
    StageName: Prod
    Cors: "'*'"
    Domain: !Ref ApiDomain

But if I nested the domain object - it magically works:

MyApi:
  Type: AWS::Serverless::Api
  Properties:
    StageName: Prod
    Cors: "'*'"
    Domain:
      DomainName: api.mydomain.com
      CertificateArn: "arn:aws:acm:blah-blah"
      Route53:
        HostedZoneName: "mydomain.com."

Answer 1

You can't use your ApiDomain this way. The Domain property of the AWS::Serverless::Api resource requires a DomainConfiguration object. ApiDomain is a DomainName, which is not the same as a DomainConfiguration object. Besides DomainName, a DomainConfiguration also creates base path mappings and Route 53 records.

If you want to define your custom domain as a standalone resource, you will have to define the base path mappings and the Route 53 record as standalone resources as well.