Dynamically create Step Function state machines locally from CFN template - aws-cloudformation

Question

Goal
I am trying to dynamically create state machines locally from generated Cloud Formation (CFN) templates. I need to be able to do so without deploying to an AWS account or creating the definition strings manually.
Question
How do I "build" a CFN template into a definition string that can be used locally?
Is it possible to achieve my original goal? If not, how are others successfully testing SFN locally?
Setup
I am using Cloud Development Kit (CDK) to write my state machine definitions and generating CFN json templates using cdk synth. I have followed the instructions from AWS here to create a local Docker container to host Step Functions (SFN). I am able to use the AWS CLI to create, run, etc. state machines successfully on my local SFN Docker instance. I am also hosting a DynamoDB Docker instance and using sam local start-lambda to host my lambdas. This all works as expected.
To make local testing easier, I have written a series of bash scripts to dynamically parse the CFN templates and create json input files by calling the AWS CLI. This works sucessfully when writing simple state machines with no references (no lambdas, resources from other stacks, etc.). The issue arises when I want to create and test a more complicated state machine. A state machine DefinitionString in my generated CFN templates looks something like:
{'Fn::Join': ['', ['{
"StartAt": "Step1",
"States": {
{
"StartAt": "Step1",
"States": {
"Step1": {
"Next": "Step2",
"Retry": [
{
"ErrorEquals": [
"Lambda.ServiceException",
"Lambda.AWSLambdaException",
"Lambda.SdkClientException"
],
"IntervalSeconds": 2,
"MaxAttempts": 6,
"BackoffRate": 2
}
],
"Type": "Task",
"Resource": "arn:', {'Ref': 'AWS::Partition'}, ':states:::lambda:invoke",
"Parameters": {
"FunctionName": "', {'Fn::ImportValue': 'OtherStackE9E150CFArn77689D69'}, '",
"Payload.$": "$"
}
},
"Step2": {
"Next": "Step3",
"Retry": [
{
"ErrorEquals": [
"Lambda.ServiceException",
"Lambda.AWSLambdaException",
"Lambda.SdkClientException"
],
"IntervalSeconds": 2,
"MaxAttempts": 6,
"BackoffRate": 2
}
],
"Type": "Task",
"Resource": "arn:', {'Ref': 'AWS::Partition'}, ':states:::lambda:invoke",
"Parameters": {
"FunctionName": "', {'Fn::ImportValue': 'OtherStackE9E150CFArn77689D69'}, '",
"Payload.$": "$"
}
}
}
}
]
},
"TimeoutSeconds": 10800
}']]}
Problem
The AWS CLI does not support json objects, the CFN functions like 'Fn::Join' are not supported, and there are no references allowed ({'Ref': 'AWS::Partition'}) in the definition string.

Answer 1

There is not going to be any magic here to get this done. The CDK renders CloudFormation and that CloudFormation is not truly ASL, as it contains references to other resources, as you pointed out.
One direction you could go would to be to deploy the SFN to a sandbox stack, and allow CFN to dereference all the values and produce the SFN ASL in the service, then re-extract that ASL for local testing.
It's hacky, but I don't know any other way to do it, unless you want to start writing parses that turn all those JSON intrinsics (like Fn:Join) into static strings.