AWS CloudFormation - when to use !Ref vs !GetAtt?

When using AWS CloudFormation and the intrinsic function!GetAtt, the behaviour you expect might not always work.

A common thing you might to do is to refer to a S3 Bucket and gets its BucketName as an input for another resource.
In my case, I’m trying to get the BucketName to set it as an environment variable to be used by a Lambda:

Environment:
  Variables:
    S3_BUCKET: !GetAtt S3Bucket.BucketName
    or
    S3_BUCKET: !GetAtt [S3Bucket, BucketName]

However, running this will produce the following error:

message: 'Template error: resource S3Bucket does not support attribute type BucketName in Fn::GetAtt'

Solution

Its not obvious at first but if you read the documentation on how !Ref works, its worth noting its behaviour is not consistent across all resources:

if you use !Ref to refer to a AWS::SNS::Topic, it returns the ARN of the resource
if you use !Ref to refer to a AWS::S3::Bucket, it returns the BucketName

So in the end my solution looks like this:

Environment:
  Variables:
    S3_BUCKET_NAME: !Ref S3Bucket

Ralph Willgoss

Solution