Customise emitted SNS message in state machine

Question

I have the following state machine step:
```
    "Step Name": {
      "Type": "Task",
      "Resource": "arn:aws:states:::sns:publish",
      "Parameters": {
        "TopicArn": "${TopicARN}",
        "Message.$": "States.JsonToString($)",
        "MessageAttributes": {
          "type": {
            "DataType": "String",
            "StringValue": "$.type"
          },
          "id": {
            "DataType": "String",
            "StringValue": "$.id"
          }
        }
      },
      "End": true
    }
```
which works fine but I'd like to customise the body of the SNS message, somehow like this:
```
    "Step Name": {
      "Type": "Task",
      "Resource": "arn:aws:states:::sns:publish",
      "Parameters": {
        "TopicArn": "${TopicARN}",
        "Message.$": {
          "id": "$.id",
          "param1": "$.param1",
          "param2": "$.param2",
          "metadata": "$"
        },
        "MessageAttributes": {
          "type": {
            "DataType": "String",
            "StringValue": "$.type"
          },
          "id": {
            "DataType": "String",
            "StringValue": "$.id"
          }
        }
      },
      "End": true
    }
```
However it does not work because `Message` has to be a `String`. I was hoping to wrap my object in `States.JsonToString` but that also doesn't seem to work.

Is there a way to achieve this or will I have to resort to using a lambda instead of the SNS integration?

Accepted Answer

To specify that a parameter use a path to reference a JSON node in the input, end the parameter name with .$ (from [doc](https://docs.aws.amazon.com/step-functions/latest/dg/connect-parameters.html)).

In your case:
```
    "Step Name": {
      "Type": "Task",
      "Resource": "arn:aws:states:::sns:publish",
      "Parameters": {
        "TopicArn": "${TopicARN}",
        "Message": {
          "id.$": "$.id",
          "param1.$": "$.param1",
          "param2.$": "$.param2",
          "metadata.$": "$"
        },
        "MessageAttributes": {
          "type": {
            "DataType": "String",
            "StringValue.$": "$.type"
          },
          "id": {
            "DataType": "String",
            "StringValue.$": "$.id"
          }
        }
      },
      "End": true
    }
```

Customise emitted SNS message in state machine

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