AWS Step function parameters are not moving to the next step - amazon-web-services

Question

I am running a step function with many different step yet I am still stuck on the 2nd step.
The first step is a Java Lambda that gets all the input parameters and does what it needs to do.
The lambda returns null as it doesn't need to return anything.
The next step is a call for API gateway which needs to use one of the parameters in the URL.
However, I see that neither the URL has the needed parameter nor do I actually get the parameters into the step. ("input": null under TaskStateEntered)
The API gateway step looks as follows: (I also tried "Payload.$": "$" instead of the "Input.$": "$")
"API Gateway start": {
"Type": "Task",
"Resource": "arn:aws:states:::apigateway:invoke",
"Parameters": {
"Input.$": "$",
"ApiEndpoint": "aaaaaa.execute-api.aa-aaaa-1.amazonaws.com",
"Method": "GET",
"Headers": {
"Header1": [
"HeaderValue1"
]
},
"Stage": "start",
"Path": "/aaa/aaaa/aaaaa/aaaa/$.scenario",
"QueryParameters": {
"QueryParameter1": [
"QueryParameterValue1"
]
},
"AuthType": "IAM_ROLE"
},
"Next": "aaaaaa"
},
But when my step function gets to this stage it fails and I see the following in the logs:
{
"name": "API Gateway start",
"input": null,
"inputDetails": {
"truncated": false
}
}
And eventually:
{
"error": "States.Runtime",
"cause": "An error occurred while executing the state 'API Gateway start' (entered at the event id #9). Unable to apply Path transformation to null or empty input."
}
What am I missing here? Note that part of the path is a value that I enter at the step function execution. ("Path": "/aaa/aaaa/aaaaa/aaaa/$.scenario")
EDIT:
As requested by #lynkfox, I am adding the lambda definition that comes before the API gateway step:
And to answer the question, yes its standard and I see no input.
"Run tasks": {
"Type": "Task",
"Resource": "arn:aws:states:::lambda:invoke",
"OutputPath": "$.Payload",
"Parameters": {
"Payload.$": "$",
"FunctionName": "arn:aws:lambda:aaaaaa-1:12345678910:function:aaaaaaa-aaa:$LATEST"
},
"Retry": [
{
"ErrorEquals": [
"Lambda.ServiceException",
"Lambda.AWSLambdaException",
"Lambda.SdkClientException"
],
"IntervalSeconds": 2,
"MaxAttempts": 6,
"BackoffRate": 2
}
],
"Next": "API Gateway start"
},

Answer 1

So yes, as I commented, I believe the problem is the OutputPath of your lambda task definition. What this is saying is Take whatever comes out of this lambda (which is nothing!) and cut off everything other than the key Payload.
Well you are returning nothing, so this causes nothing to be sent to the next task.
I am assuming your incoming vent already has a key in the Json that is named Payload, so what you want to do is remove the OutputPath from your lambda. It doesn't need to return anything so it doesn't need an Output or Result path.
Next, on your API task, assuming again that your initializing event has a key of Payload, you would have "InputPath": "$.Payload" - if you have your headers or parameters in the initializing json Event then, you can reference those keys in the Parameters section of the definition.
Every AWS Service begins with an Event and ends with an Event. Each Event is a JSON object. (Which I'm sure you know). With State Machines, this continues - the State Machine/Step Function is just the controller for passing Events from one Task to the next.
So any given task can have an InputPath, OutputPath, or Result Path - These three definition parameters can decide what values go into the Task and what are sent onto the Next Task. State machines are, by definition, for maintaining State between Tasks, and these help control that 'State' (and there is pretty much only one 'state' at any given time, the event heading to the next Task(s)
The ResultPath is where, in that overall Event, the task puts the data. If you put ResultPath: "$.MyResult" by itself it appends this key to the incoming event
If you add OutputPath, it ONLY passes that key from the output event of the Task onto the next step in the Step Functions.
These three give you a lot of control.
Want to Take an Event into a Lambda and respond with something completely different - you don't need the incoming data - you combine OutputPath and ResultPath with the same value (and your Lambda needs to respond with a Json Object) then you can replace the event wholesale.
If you have ResultPath of some value and OutputPath: "$." you create a new json object with a single Key that contains the result of your task (the key being the definition set in ResultPath
InputPath allows you to set what goes into the Task. I am not 100% certain but I'm pretty sure it does not remove anything from the next Task in the chain.
More information can be found here but it can get pretty confusing.
My quick guide:
ResultPath by itself if you want to append the data to the event
ResultPath + OutputPath of the same value if you want to cut off the Input and only have the output of the task continue (and it returns a JSON style object)