API Gateway integration response mapping: parse statusCode and body from Step Function output

Question

I want to parse 'statusCode' and 'body' values from API Gateway integration response using VTL and return those as a method response like this:

Request status: 201
Response body: {"firstName":"He","lastName":"Man","email":"he.man@eternia.com"}

My API Gateway Step Function integration is returning the following integration response body (this is before transformation, non-relevant attributes are removed from output):

{
  "output": "{\"statusCode\":201,\"body\":{\"firstName\":\"He\",\"lastName\":\"Man\",\"email\":\"he.man@eternia.com\"}}"
}

I would assume this to work:

#set ($output = $util.parseJson($input.json('$.output')))
#set ($statusCode = $output.statusCode)
#set ($context.responseOverride.status = $statusCode)
$output.body

But status is not updated and body is empty

Request status: 200
Response body: <empty>

With this approach I can parse the body:

#set ($bodyObj = $util.parseJson($input.body))
#set ($output = $util.parseJson($bodyObj.output))
#set ($context.responseOverride.status = $output.statusCode)
$output.body

statusCode is updated but body is returned as object representation i.e. not JSON.

Request status: 201
Response body: {firstName=He, lastName=Man, email=he.man@eternia.com}

How to serialize $output.body correctly to JSON in above case? API Gateway doesn't seem to have $util.toJson function like AppSync does (https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-programming-guide.html)

I've confirmed parsing output-variable works correctly:

#set ($output = $util.parseJson($input.json('$.output')))
$output

Request status: 200
Response body: {"statusCode":201,"body":{"firstName":"He","lastName":"Man","email":"he.man@eternia.com"}}

Relevant reference documentation:

score 1 · Accepted Answer · answered Aug 30 '22 at 01:28

I ran into this exact issue. There indeed does not seem to be a way to convert to JSON directly, so I devised a way to write JSON to the API Gateway response using Apache VTL:

#set ($outputObj = $util.parseJson($input.path('$.output')))
#foreach ( $key in $outputObj.body.keySet() )
  "$key": $outputObj.body[$key] #if ($foreach.hasNext),#end
#end

Hope this helps!

(This works for a simple use case like the one you described, but doesn't generalize to nested objects, etc.)

score 1 · Answer 2 · answered Dec 13 '22 at 10:42

I ran into this same issue. This is how I solved it in the end. The solution provided by @noahtk7 was good for simple object, I have large nested object I needed.

For this to work you need to have your body returned as a base64 encoded string. This is easy to do in a step function. E.G:

"Pass": {
  "Type": "Pass",
  "Next": "Success",
  "Parameters": {
    "Base64.$": "States.Base64Encode(States.JsonToString($.Response.Body))"
  },
  "ResultPath": "$.Response.Body"
}

Then in the api response mapper:

#set($output = $util.parseJson($input.path('$.output')))
#set($context.responseOverride.status = $output.Response.Status)

$util.base64Decode($output.Response.Body.Base64)

Would give me a result of:

{
    "foo": "bar",
    "Id": "c2b2220c-00e3-4499-87c0-d06ea5e5f1f4"
}

API Gateway integration response mapping: parse statusCode and body from Step Function output

2 Answers2