Step Function: Dynamic input to ItemReader

Question

I am working on a use case where I need to process a JSON file stored on S3 through a map state. Here is how I am passing the bucket name and Key to ItemReader:

```
"ItemReader": {
              "Resource": "arn:aws:states:::s3:getObject",
              "ReaderConfig": {
                "InputType": "JSON"
              },
              "Parameters": {
                "Bucket": "bucket-name",
                "Key.$": "$.StudyID"
              }
 }
```

The bucket name is static (can be sourced from state's input if necessary) and Key varies (sourced from state's input).

Question - Can we add a static prefix to the key? Like `"Key.$": "static-prefix/$.StudyID"`. I have tested this but doesn't work. Is there any alternate solution to this?

Answer

Hi.

Try using the States.Format Intrinsic function, it give you the possibility to manipulate the string.

```
"Key.$": "States.Format('static-prefix/{}.', $.StudyID)"
```

See [the documentation for more information](https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-intrinsic-functions.html#asl-intrsc-func-generic)

Step Function: Dynamic input to ItemReader

Relevant content