Add support for Fn::GetAtt in the Step Function Resource Attributes

[alexanderluiscampino]

Currently, step functions can be written using the new functionality in AWS toolkit, which is quite helpful. One thing I do, is handle all deployment using serverless. For that, I parameterize my resources using the Fn::GetAtt instead of hardcoding the name of the lambda function in the JSON definition.

For example, the following state will be okay with serverless framework, but the rendering does not accept it:

"Downloader":{
      "Type": "Task",
      "Resource": {"Fn::GetAtt": ["download-handler", "Arn"]},
      "Catch": [
        {
          "ErrorEquals": [
            "Runtime.ExitError"
          ],
          "Next": "RedefineParameters"
        },
        {
          "ErrorEquals": [
            "Task.Failed"
          ],
          "Next": "HandleFailure"
        }
      ],
      "Retry": [ {
              "ErrorEquals": ["Lambda.ServiceException", "Lambda.AWSLambdaException", "Lambda.SdkClientException", "Lambda.TooManyRequestsException"],
              "IntervalSeconds": 5,
              "MaxAttempts": 10,
              "BackoffRate": 2
            } ],
      "ResultPath": "$.results",
      "Next": "IsCommpleted"
    }

Describe the solution you'd like
I would like this to not be flagged as a faulty JSON definition and the rendering to work

Describe alternatives you've considered
I have been manually removing all my parameterized resource names with empty "". But this can be tedious and error prone as your state machine grows in size

Additional context
Serverless will accept the JSON as write above, which is great. But the rendering of the state machine is not working which causes problems with dev speed

[wong-a]

Related to #1016

The plugin only validates ASL, which doesn't include intrinsic functions and placeholders used in CloudFormation templates.

[alexanderluiscampino]

Related to #1016

The plugin only validates ASL, which doesn't include intrinsic functions and placeholders used in CloudFormation templates.

Hi @wong-a, thank you for your reply.
#1016 is indeed related to ignore a invalid ARN, so if I remove the {"Fn::GetAtt": ["download-handler", "Arn"]} and replace with empty "" it will render it, with an invalid ARN.

But if I leave the {"Fn::GetAtt": ["download-handler", "Arn"]} , which is still invalid in ASL, it does not render it. Am I wrong, or should it render either way? Both empty string and the function are invalid ARN, they should just be both ignored by the renderer.

I don't necessarily need it to recognize the placeholder, I wish for it just to render the overall state-machine.

[alexanderluiscampino]

Just a quick update on a workaround that Im using. If you define the state as follows, the renderer seems to not throw an error:

                "AddPartition": {
                        "Type": "Task",
                        "Resource": "arn:aws:states:::lambda:invoke",
                        "Parameters":{
                            "FunctionName": {"Ref": "add-partition"},
                            "Payload": "$"
                        },
                        "TimeoutSeconds": 60,
                        "Catch": [
                            {
                            "ErrorEquals": [
                                "Runtime.ExitError",
                                "Lambda.Unknown",
                                "States.Timeout",
                                "States.TaskFailed"
                            ],
                            "ResultPath": "$.errorInfo",
                            "Next": "FluxFailure"
                            }
                        ],
                        "Retry": [ {
                                "ErrorEquals": [
                                    "Lambda.ServiceException",
                                    "Lambda.AWSLambdaException",
                                    "Lambda.SdkClientException",
                                    "Lambda.TooManyRequestsException"
                                ],
                                "IntervalSeconds": 5,
                                "MaxAttempts": 10,
                                "BackoffRate": 2
                                } ],
                        "End": true
                    

[wong-a]

Thanks for the follow-up @alexanderluiscampino.

The validator only checks that Parameters is an object and does not validate the fields for each service integration. This is why the CloudFormation syntax isn't marked as invalid there.

[alanbo]

@alexanderluiscampino , you major pain point is not being able to render the state machine when it is invalid but in fact could be rendered, right?

[alexanderluiscampino]

@alanbo, that's exactly my pain point.
I'd like to be able to use this sort of nomenclature:

"Resource": {"Fn::GetAtt": ["download-handler", "Arn"]},

And still be able to render it and visualize it on the right hand side.

[ffxsam]

Just wanted to throw my hat into the ring here, in support of this change/fix. I understand "Resource": {"Fn::GetAtt": ["download-handler", "Arn"]} is not valid as far as a raw state definition. But if the purpose is to help us visualize the step function before deploying it, and many people are using SAM or Serverless Framework, it would make sense to allow Resource to be an object.

It's a bit tedious to go through and temporarily replace the resources with strings.

[alanbo]

@ffxsam , I recommend you looking into definition substitutions. This way you can use placeholders in your state machine json and add a list of substitutions with "DefinitionSubstitutions" property. Example of a placeholder: ${dollar_sign_brace} .
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-stepfunctions-statemachine.html#cfn-stepfunctions-statemachine-definitionsubstitutions

[bryceitoc9]

Closing at the request of @alanbo (SFN team).

[ffxsam]

@bryceitoc9 I noticed this was closed with no details given. Is this a wontfix? What's the status of this?