How do I use JSON Lambda output in Step Functions

Question

I have a (Python) Lambda function which returns a JSON object. I'd like to join that object on to my Step Functions state, and access its sub-fields in the next state... Something like:

```python
# Lambda Function
def handler(event, context):
    return json.dumps({ "ThingARN": "...", "AnotherField": "..." })
```

```json
{
  "StartAt": "LambdaStuff",
  "States": {
    "LambdaStuff": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "Parameters": {
        "FunctionName": "MyCoolLambdaFunction",
        "Payload.$": "$"
      },
      "ResultPath": "$.LambdaResult",
      "Next": "NextActivity"
    },
    "NextActivity": {
      "Type": "Task",
      "Resource": "...",
      "Parameters": {
        "ARN.$": "$.LambdaResult.ThingARN"
      },
      "ResultPath": "$.AnotherResult",
      "End": true
    }
  }
}
```

...But my state machine is failing (cancelling the `NextActivity` task) because what gets populated to `$.LambdaResult` is a big object with Lambda metadata and the ***stringified*** result payload - like this:

```json
{
  "ExecutedVersion": "$LATEST",
  "Payload": "{\"ThingARN\": \"...\", \"AnotherField\": \"...\"}",
  "SdkHttpMetadata": {
    "HttpHeaders": { ... }, // All kinds'a stuff
    "HttpStatusCode": 200
  },
  "SdkResponseMetadata": {
    "RequestId": "..."
  },
  "StatusCode": 200
}
```

[This docs page][1] seems to imply that what I'm trying to do is possible, but the example is clearly very simplified/stripped-down.

All the other worked examples I've found (e.g. ["InputPath, OutputPath and ResultPath Example"][2]) just seem to have Lambdas returning simple strings, rather than structured objects.

I guess I'm missing something obvious?

[1]: https://docs.aws.amazon.com/step-functions/latest/dg/concepts-state-machine-data.html#concepts-state-machine-data-state-input-output
  [2]: https://docs.amazonaws.cn/en_us/step-functions/latest/dg/input-output-example.html

Accepted Answer

If you invoke your lambda function from SFN using the full function ARN in the Resource attribute, you will get back only what was returned from the Lambda function. Also, you do not want to json.dump the object that you return. Just return a map and that will be included in your output.

For instance, the following function:

def lambda_handler(event, context):
        # TODO implement
        body = {
            'Test': 'abc',
            'Value': 123
        }
        return body

When invoked like this:

"Hello": {
      "Type": "Task",
      "Resource": "arn:aws:lambda:eu-west-1:xxxxxxxxx:function:HelloWorld:$LATEST",
      "ResultPath": "$.LambdaResult",
      "Next": "World"
    },

With the following input:

{
      "Input": "My input"
    }

returned the following:

{
      "Input": "My input",
      "LambdaResult": {
        "Test": "abc",
        "Value": 123
      }
    }

How do I use JSON Lambda output in Step Functions

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