Invoke State Machine from API Gateway Call

Question

Currently, I am having difficulty successfully deploying a CloudFormation Stack that contains 3 lambdas, an API Gateway and a State Machine. I am at my wits end for trying to debug this issue and would love a second set of eyes. The deployment fails when it attempts to construct the AWS::ApiGateway::Method and kicks back with:

apiGatewayRootMethod: entered status CREATE_FAILED, reason: Invalid ARN specified in the request (Service: AmazonApiGateway; Status Code: 400; Error Code: BadRequestException; Request ID: a2ba00f1-179b-400f-a42a-f3a4e90231af; Proxy: null)

Below is a snippet of the template used for the deployment,

apiGatewayRootMethod:
    Type: "AWS::ApiGateway::Method"
    Properties:
      AuthorizationType: "NONE"
      HttpMethod: "POST"
      Integration:
        CacheKeyParameters:
          - 'method.request.path.proxy'      
        Credentials: !Ref 'apiGatewayRole'
        IntegrationHttpMethod: "POST"
        PassthroughBehavior: "NEVER"
        RequestParameters:
          integration.request.path.proxy: 'method.request.path.proxy'   
        RequestTemplates:
          application/json: !Sub
            - "{\"input\": \"$util.escapeJavaScript($input.json('$'))\",\"stateMachineArn\": \"${SPIStateMachine.Arn}\"}"
            - StateMachineArn: !GetAtt [ SPIStateMachine, Arn ]
        Type: "AWS"
        Uri: !Sub
          "arn:aws:apigateway:${AWS::Region}:states:action/StartExecution"         
      ResourceId: !GetAtt "apiGateway.RootResourceId"
      RequestParameters:
        method.request.path.proxy: true    
      RestApiId: !Ref "apiGateway"



SPIStateMachine:
    Type: "AWS::StepFunctions::StateMachine"
    Properties:
      StateMachineName: !Sub ${AWS::StackName}-state-machine
      RoleArn: !GetAtt [ StatesExecutionRole, Arn ]
      DefinitionString: 
        Fn::Sub: 
          |-
            {
                "Comment": "Calling Step Functions from Lambda SQS",
                "StartAt": "APIGatewayLambda",
                "States": {
                    "APIGatewayLambda": {            
                        "Type": "Task",
                        "Resource": "${SPIApiFunction.Arn}",
                        "Catch": [
                            {
                                "ErrorEquals": ["CustomError"],
                                "Next": "PythonLambda"
                            }
                        ],
                        "End": false
                    },
                    "PythonLambda": {
                        "Type": "Task",
                        "Comment": "This is the mandatory lambda which will process the information and possible call the PowerShell script",
                        "Resource": "${SPILambdaFunctionOne.Arn}",
                        "Next": "Needs Additional Language?"  
                    },
                    "Needs Additional Language?": {
                        "Type": "Choice",
                        "Choices": [
                            {
                                "Variable": "$.platform",
                                "StringEquals": "powerbi",
                                "Next": "PowerShellLambda"
                            },
                            {
                                "Not": {
                                    "Variable": "$.platform",
                                    "StringEquals": "powerbi"
                                },
                                "Next": "PassThrough"
                            }
                        ]
                    },
                    "PowerShellLambda": {
                        "Type": "Task",
                        "Comment": "This is the PowerShell Lambda, to be run for PowerBI updates",
                        "Resource": "${SPILambdaFunctionTwo.Arn}",
                        "End": true
                    },
                    "PassThrough": {
                        "Type": "Pass",
                        "End": true
                    }    
                }
            }

EDIT:

application/json: !Sub

- "{\"input\": \"$util.escapeJavaScript($input.json('$'))\",\"stateMachineArn\": \"${StateMachineArn}\"}"

- StateMachineArn: !Sub arn:aws:states:${AWS::Region}:${AWS::AccountId}:stateMachine:SPIStateMachine

This results in the same apiGatewayRootMethod: entered status CREATE_FAILED, reason: Invalid ARN specified in the request (Service: AmazonApiGateway; Status Code: 400; Error Code: BadRequestException; Request ID: 9585e311-69e1-4809-bc3c-f59a67a4546e; Proxy: null)

Answer 1

Seems like this doesnt work

   - StateMachineArn: !GetAtt [ SPIStateMachine, Arn ]

quoting from the documentation

Fn::GetAtt Fn::GetAtt returns a value for a specified attribute of this type. The following are the available attributes and sample return values.

Arn Not currently supported by AWS CloudFormation.

You can construct the ARN using Fn::Sub

 Fn::Sub: arn:aws:states:${AWS::Region}:${AWS::AccountId}:stateMachine:mystatemachine

EDIT

so the uri should be in this form

arn:aws:apigateway:{region}:{subdomain.service|service}:path|action/{service_api}

which translates in case of step function:

Fn::Sub: arn:aws:apigateway:${AWS::Region}:states:action/StartExecution

and this should be in there in the cloudformation template for the arn

    #set($input = $input.json('$'))
{
  "input": "$util.escapeJavaScript($input)",
  "stateMachineArn": "<STATE_MACHINE_ARN>"
}

This will pass the json payload posted to API Gateway to Step Function.

You can create another URI for describing the execution

Fn::Sub: arn:aws:apigateway:${AWS::Region}:states:action/DescribeExecution

For this the request template would be like below

    requestTemplates:
      application/json:
        Fn::Sub: |-
          {
            "executionArn": "arn:aws:states:${AWS::Region}:${AWS::AccountId}:execution:${Workflow.Name}:$input.params().path.get('executionId')"
          }

As I described same stuff works here you can find the working example.