As I mentioned yesterday Doug Purdy posted an insightful entry in response to Ted Neward's about the inappropriateness of returning ADO.NET DataSets from XML Web Services. Today Ted Neward has a post entitled  Why Purchase Orders are the root of all evil? which almost entirely misses the point of Doug's post.

Ted writes

Could you tell me what the schema should be? Doug, it's right there in front of you: the class definition itself is a schema definition for objects of its type. The question I think you mean to ask is, "What the XML schema should be for this Purchase Order?", but I can't do that, because you've already stepped way out into la-la land as far as XML/XSD goes by making use of generic types (like Dictionary) for which there is no XSD equivalent; sure, we can rpc-encode one up, but we're back to turning objects into XML and back again, and I thought we didn't like that....?

Could you tell me what each particle of the schema means? Well, the LineItemAddedEvent certainly isn't a schema construct, so I'm guessing that'll have to be the XML-based representation of a .NET delegate.... the IAddress has no implementation behind it that I can see so once again I'll have to punt....

Oh, I get it... Doug's using one of them anti-pattern thingies to show us what not to do when trying to define types in XML/XSD for use in Web services (or WebServices or web services or however we've decided to spell these silly things anyway).

You're absolutely right, Doug--the way that thing is written, Purchase Orders, while perhaps not the root of ALL evil, are certainly evil and therefore should be banned from the WS-* camp immediately.

Seriously, dude, DataSets as return values from Web services are evil. Get over it.

What I find interesting is that Ted Neward is looking at XML Web Services through the perspective of distributed objects. His entire arguments hinge around the fact that his applications convert XML into Java or CLR objects so the XML returned must be something that is condusive to converting to objects easily. Doug accurately points out that there is no one-to-one mapping between an XML schema and a CLR object. Arguing that your favorite platform has one-to-one mappings for some XML schemas and not others thus banning various XML formats from participating in XML Web Services is a very limiting viewpoint. I'd like to ask Ted whether he also would ban XBRL, wordProcessingML or UBL documents from being used in XML Web Services because there aren't easy ways to convert them to a handy, dandy Java object with strongly typed members and all that jazz.  

I don't dispute the practical reasons for discouraging developers from returning ADO.NET DataSets from XML Web Services since most developers trying to access the XML Web Services just use a toolkit that pretends you are building distributed object applications. Usually such toolkits either barf horribly when faced with XML they don't grok or force developers to have deal with scary angle brackets directly instead of the object facade they know & love (ASP.NET XML Web Services included). This is a practical reason to avoid exposing ADO.NET DataSets from XML Web Services that may be accessed from Java platforms especially since such platforms don't make it easy to deal with raw XML.

On the other hand, claiming that there is some philosophical reason not to expose data from an XML Web Service that may be be semi-structured and full of unknown data (i.e XML data) seems quite antithetical to the entire point of XML Web Services and the Service Oriented Architecture fad.