Correct answer is APP: API RESOURCE NOT FOUND
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1) A web client submits the request to the HTTP Listener.
2) The HTTP Request throws an "HTTP:NOT_FOUND" error, execution halts.
3) The On Error Propagate error Handler handles the error. In this case ,HTTP:NOT_FOUND error is mapped to custom error APP:API_RESOURCE_NOT_FOUND. This error processor sets payload to APP:API_RESOURCE_NOT_FOUND.
4) "APP:API_RESOURCE_NOT_FOUND. " is the error message returned to the requestor in the body of the HTTP request with HTTP Status Code: 500 Reference Diagram:

Transform Message Add write_date is coverting payload in JSON format and same JSON payload is avaialble to file write processor. However, if the payload is a different format (for example, not CSV) , you can place the transformation inside the Write operation to generate content that will be written without producing a side effect on the message in transit. This is not done in this case. By default, the connector writes whatever is in the message payload. Hence JSON payload will be written to file.