RESTful web services - best way to return result of an operation?

I am designing a RESTful API and I would like to know what the most RESTful way is to return details about an operation.

E.g. an operation on a resource occurs when some data is POSTed to a URL. HTTP status codes will indicate either success or failure for the operation. But apart from succ开发者_如何学Goess/failure I need to indicate some other info to the client, such as an ID number.

So my question is, should the ID number be returned in an XML document in the response content, or should it be returned in some custom HTTP header fields? Which is more in line with the principles of REST? Or am I free to choose.

Returning an entity is a perfectly valid response to an HTTP POST.

You also do not need to return XML you could just use the content type text/plain and simply return a string value.

Using a header would require you to define a new custom header which is not ideal. I would expect clients would have an easier time parsing a response body than extracting the information from a header.

XML document makes the most sense.

If it is a just an ID number, it would save overhead to do it just as an HTTP header. Building a correct XML document just for a single number would add much more overhead to the request.