In the latest event in the discussion about SOA and objects with Steve, he responds to me the following: “I’d argue that you are passing objects, they’re just wrapped up in messages.”
Steve, yes, my point is to support your previous assessments that what matters is not how are your objects on the inside done or how much logic they may have, but the WSDL and the XSD, and the policy, applied to your service (in a WS based service). But to understand my point, I’ll go back to what I said here: the ASMX is evil in the sense that it convinces people there’s no difference between expressing an object in XML (serialization) and passing XML messages around. This assumption is only valid in simple cases.
Let’s make a simple comparison: Is a word document better off being a dump of the object layout in word, or as an xml document? Which one is tightly couple and a maintenance hell? Which one can support versioning and the open content model, and things like inherit by extension? Response: The first was the word file format, a mess to keep backward compatible according to office devs, and difficult to envolve. And it was exactly that, an OLE Stream file…
Objects limit you to a CLR view. The problem with starting from saying you map objects in XML is that you:
1. Restrict your view of how other systems are going to consume your web service, that won’t use the same objects as you, or the same language, or the same platform, or even the same paradigm. There ARE going to be a lot of people using batch files to query web services. I’m already doing it, and it’s far more productive for certain tasks than code.
your view on how extensible your data will be. An object can only be modified through
inheritance or interfaces in the .net world. In the xsd world, validation is there,
inheritance by restriction and inheritance by extension. These intrinsic values of
XML are lost when you try to stick with objects.
Let me expand a bit. When you map on objects, you either map to an XmlNode for the open content model, or you die upon deserialization. You are sensitive to changes in the data being sent. If you deal with an xml message, you don’t really care what is happening in the extension points you provide. You’ll always be able to ignore the data not interesting to you, and access what you need. You’re more open.
3. End up providing a bad message format, as you don’t design your xsd first. I agree that not everybody agrees on contract design first, but I strongly do, and believe that the architects around the world should unite to speak as one voice for schema design first. Why design first? Because you can then talk about the schema as being the contract of your service, and very early know what your service is doing, why, and how. And you let other people expand it.
I’m not saying asmx is the source of all things evil (although I’m tempted). I’m saying that to think about the object mapping feature of asmx as the way to go works only in simple cases no gateway no proxy no router no header processing no non-.net or no non-oolanguage cases. As soon as you try and move forward… It becomes so much more complex.