Map step function response to the response body of an HTTP API Gateway V2 integration

Question

Ive got an `HTTP API Gateway V2` invoking a synchronous Express Step Function thru AWS Service Integration via `StepFunctions-StartSyncExecution`. My problem is that the response from the Step Function seems to always be in the form of:

```
{
   "billingDetails": { 
      "billedDurationInMilliseconds": number,
      "billedMemoryUsedInMB": number
   },
   "cause": "string",
   "error": "string",
   "executionArn": "string",
   "input": "string",
   "inputDetails": { 
      "included": boolean
   },
   "name": "string",
   "output": "string",
   "outputDetails": { 
      "included": boolean
   },
   "startDate": number,
   "stateMachineArn": "string",
   "status": "string",
   "stopDate": number,
   "traceHeader": "string"
}
```
And that gets passed as the response to the client calling the API Gateway. I just want the value of the `output` field   (as JSON) from that big JSON blob.
As far as I can tell there doesn’t seem to be a way with `HTTP API Gateway V2` to transform the integration response into the clean final response. As far as I can tell, `parameter mapping` will only work for `headers`, I can’t see how to rewrite the `body`. I can get the value in Integrations->parameter mapping but there doesn’t seem to be a way to overwrite the  body. Only header.*
The old v1 REST API gateway had mapping templates, but I can’t find anything like that  for V2

Is there a way to do this that I'm missing?

Accepted Answer

Hi,

Unfortunately it is not possible to receive just the value of the output field from the StepFunctions execution, since it is the expected response format of the [StartSyncExecution API operation](https://docs.aws.amazon.com/step-functions/latest/apireference/API_StartSyncExecution.html). As you say, you could filter it using the integration response, but this is just supported on the [REST API](https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-vs-rest.html#http-api-vs-rest.differences.development).

Finally, note that both REST APIs and HTTP APIs are valid RESTful API products, and you should chose depending on your use case needs.

Answer

Should it be possible to do this with the OpenAPI `x-amazon-apigateway-integration` and the v2 API HTTP Gateway as per the article [How to Build Lightning Fast APIs with AWS Step Functions](https://www.readysetcloud.io/blog/allen.helton/how-to-build-lightning-fast-apis-with-aws-step-functions/)? It's not clear to me if they are using the v2 API Gateway or not. I might try this when I get a chance.

Answer

There is no way to do payload transformation in HTTP API (v2), only in REST API (v1).

Map step function response to the response body of an HTTP API Gateway V2 integration

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