serverless-application-model: Unsupported intrinsic functions

The problem

Some properties have limited support for intrinsic functions (e.g. Ref, Fn::GetAtt, Fn::If, etc.).

Using unsupported intrinsic functions in such properties can cause issues with deployment.

Why it happens

AWS SAM is a AWS CloudFormation macro; it receives a SAM template as input (as-is, along with the intrinsic functions), and returns a CloudFormation template which is then deployed by CloudFormation.

This means that SAM is unable to resolve some intrinsic functions. For example, an !If condition with a Ref to a stack parameter is resolvable (SAM has access to the stack parameters), but a Ref to a stack resource is not (as the transform happens before deployment). Furthermore, SAM supports limited intrinsic function resolution for only some properties.

It’s not an issue for properties that are passed as-is to properties of the underlying CloudFormation resources, but it becomes an issue when SAM must know the value for its transform logic.

Workarounds

Add the `AWS::LanguageExtensions` transform

The AWS::LanguageExtensions transform will resolve intrinsic functions if the value is known when Transforms are run.

Replace:

Transform: AWS::Serverless-2016-10-31

With:

Transform:
  - AWS::LanguageExtensions
  - AWS::Serverless-2016-10-31

The AWS SAM CLI currently doesn’t process AWS::LanguageExtensions locally, so it won’t work where local transforms are needed.

[!NOTE] There is a known issue where AWS::Serverless-2016-10-31 and AWS::LanguageExtensions can conflict; see https://github.com/aws-cloudformation/cfn-language-discussion/issues/109.

Use a pass-through property, if available

Pass-through properties are passed directly to the underlying CloudFormation resources, hence intrinsic functions work. Check the “AWS CloudFormation compatibility” notice under properties to see whether the property is passed as-is to an underlying CloudFormation resource.

For example for the Schedule event type, use the State property instead of Enabled.

Use raw CloudFormation

If nothing else works, you can always switch to using the underlying CloudFormation resources directly. Since they are not processed by SAM, CloudFormation will be able to resolve the intrinsic functions.

You can get the transformed CloudFormation template of a stack <my-stack> using:

aws cloudformation get-template --query TemplateBody --change-set-name "$(aws cloudformation describe-stacks --query 'Stacks[0].ChangeSetId' --output text --stack-name <my-stack>)"

Which you can then use to replace the affected resources.

See also https://github.com/aws/serverless-application-model/issues/3007#issuecomment-1464194429 for other ways of transforming a SAM template into a CloudFormation template.

About this issue

Original URL
State: open
Created 2 years ago
Reactions: 13
Comments: 21 (5 by maintainers)

Most upvoted comments

I am coming from https://github.com/aws/serverless-application-model/issues/1616. Is there any way to set string to whole Step Functions TimeoutSeconds? Just like TimeoutSecondsPath for Task. Now the whole Step Functions TimeoutSeconds is only for int, and can’t get string from path.

ozcsdog on Oct 31, 2023

This breaks CodeUri when deploying AWS lambda from a prebuilt JAR file using !Sub (see aws/serverless-application-model#271). Let’s say I’m deploying some lambda function I built with Maven, and I don’t want to hard-code the JAR version in my SAM template.

Resources:
  FooFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: !Sub "target/foo-bar-functions-${Ver}.jar"
      Handler: com.example.FooBar::foo

SAM gives me an error:

'CodeUri' requires Bucket and Key properties to be specified.

If I try the workaround noted here of adding AWS::LanguageExtensions, that SAM will at least upload my JAR to the S3 bucket. But then when it fails with:

'CodeUri' is not a valid S3 Uri of the form 's3://bucket/key' with optional versionId query parameter.

The problem is that SAM is not updating the CodeUri value before it uploads the template to S3, as it would if the value were a literal string. Shouldn’t SAM know the dereferenced value at the time of uploading the template? Hasn’t the template already been processed? There must be a bug where SAM forgets it needs to update the CodeUri value before uploading the template if !Ref was used. I filed bug aws/aws-sam-cli#5249.

garretwilson on Jun 2, 2023

@tariromukute If I’m understanding correctly the problem you’re encountering is !Ref to local paths in AWS::Serverless::LayerVersion not working. I was able to reproduce it, and confirmed it works with CodeUri in AWS::Lambda::Function. I’ve created https://github.com/aws/aws-sam-cli/issues/4767 for the issue. There’s not much else that can be done on the SAM transform side as the intrinsic resolution to the local path must happen before the SAM transform receives it. While not ideal, as workaround in the meantime you could use sed or similar to replace the text.

hoffa on Feb 25, 2023