Unable to pass Lambda Funciton ARN to Swagger Definition

[GarimaBathla]

I have a standard setup:

SAM-template.yml file contains
    ---  AWS Rest API
    --- Lambda Function
Swagger.yml file contains
  --- REST Definition

In the above setup I am able to refer Lambda FunctionName in Swagger.yml

    ABCApi:
        Type: AWS::Serverless::Api
        DependsOn:
          - FunctionA
          - FunctionB
        Properties:
            StageName: dev
            DefinitionUri: ./swagger.yml
            Variables:
              RefA: !Ref FunctionA
              RefB: !Ref FunctionB

IN Swagger URI I can use RefA and RefB variables:

uri: !Sub arn:aws:apigateway:us-west-2:lambda:path/2015-03-31/functions/arn:aws:lambda:us-west-2:xxxxxxxxxx:function:${RefA.FunctionName}/invocations

This Works.

Now I am trying to pass Lambda Functions Arn instead of just the name - but that won't work

Like this

ABCApi:
    Type: AWS::Serverless::Api
    DependsOn:
      - FunctionA
      - FunctionB
    Properties:
        StageName: dev
        DefinitionUri: ./swagger.yml
        Variables:
          ArnA: !GetAtt FunctionA.Arn

IN Swagger for uri If I use the ArnA variable it error's out:

uri: !Sub arn:aws:apigateway:us-west-2:lambda:path/2015-03-31/functions/arn:aws:lambda:us-west-2:xxxxxxxxxx:function:${ArnA}/invocations

Why Name can be passed but Arn cannot be?

Please note I have already read the alternative of using AWS Transform by inlining the Swagger in cft directly.

I am very curious to understand why approach A is feasible and not B. If Name is known, why ARN won't be known by this time?

[jlhood]

@GarimaBathla We're sorry you're experiencing issues passing stage variables to your swagger definition. AWS::Serverless::Api's Variables property does support CloudFormation intrinsic functions like !Ref, !Sub, and !GetAtt. There are some parts of your problem description that are making it difficult for us to determine what's happening in your specific case.

In your first example sets stage variables RefA and RefB, and then you say you use those variables in your swagger document like this: uri: !Sub arn:aws:apigateway:us-west-2:lambda:path/2015-03-31/functions/arn:aws:lambda:us-west-2:xxxxxxxxxx:function:${RefA.FunctionName}/invocations. I'm assuming that's a line from your external swagger definition file (./swagger.yml), which should be inside a x-amazon-apigateway-integration block. However, there are two issues with that line. First, you can't use CloudFormation intrinsic functions in external swagger documents, so using !Sub in the uri definition shouldn't work. Second, when referencing API stage variables in your swagger document, the correct format to use is ${stageVariables.<your stage variable name>}. So for your first example, the correct line to put into your x-amazon-apigateway-integration block is uri: arn:aws:apigateway:us-west-2:lambda:path/2015-03-31/functions/arn:aws:lambda:us-west-2:xxxxxxxxxx:function:${stageVariables.RefA}/invocations

In your second example (the one using !GetAtt in the stage variable), you should be able to use !GetAtt to set the value of a stage variable. However, the way you're trying to use it in the x-amazon-apigateway-integration has some issues: uri: !Sub arn:aws:apigateway:us-west-2:lambda:path/2015-03-31/functions/arn:aws:lambda:us-west-2:xxxxxxxxxx:function:${ArnA}/invocations

Again, you can't use !Sub in an external swagger file and you're not referencing the stage variable correctly. You could change it to this: uri: arn:aws:apigateway:us-west-2:lambda:path/2015-03-31/functions/arn:aws:lambda:us-west-2:xxxxxxxxxx:function:${stageVariables.ArnA}/invocations. However, there's an additional issue that you are already providing the beginning of the function ARN (arn:aws:lambda:us-west-2:xxxxxxxxxx:function:) before adding your stage variable ARN so even with the stage variable value substituted in correctly, the resulting function ARN won't be valid.

Also note, even if you correct the above issues, API Gateway does not allow you to use an account id in a stage variable (see this Stack Overflow). I haven't personally tried it, but I don't think API Gateway will let you substitute the entire function ARN like you're trying to do. I believe the only supported swagger syntax is to manually construct the ARN and then substitute the function name as you are doing in the first example.

[GarimaBathla]

thanks @jhood. I will test tonight and report back on the status. For me when using ${RefA.FunctionA} works fine for now (not sure why missing stageVariables prefix is not erring.

[johnc44]

It would be a lot simpler if the swagger spec could take in a function name instead of the full uri. I presume it wouldn't be simple to add this in as an alternative? I don't really know how swagger and cloudformation fit together but it does seem a bit unnecessary to put the full uri in there.

[jlhood]

@johnc44 Thanks for your feedback. The x-amazon-apigateway-integration swagger extension is an API Gateway feature. The full URI is necessary, because API Gateway supports more types of integrations than just Lambda functions. For example, it can call other AWS services directly, e.g., Amazon DynamoDB, or it can call an http endpoint (more information here).

[jlhood]

@GarimaBathla Closing this issue. Feel free to reopen with updated details from your testing if you are still experiencing issues.