03 May 2005

Over the past few weeks I’ve given a number of presentations on service-oriented design and how it relates to both OO and n-tier models. One constant theme has been my recommendation that service methods use a request/response or just request method signature:<?xml:namespace prefix = o ns = “urn:schemas-microsoft-com:office:office” />

response = f(request)

or

f(request)

“request” and “response” are both messages, defined by a type or schema (with a little “s”, not necessarily XSD).

The actual procedure, f, is then implemented as a Message Router, routing each call to an appropriate handler method depending on the specific type of the request message.

In concept this is an ideal model, as it helps address numerous issues around decoupling the client and service, and around versioning of a service over time. I discussed many of these issues in my article on SOA Covenants.

Of course the devil is in the details, or in this case the implementation. Today’s web service technologies don’t make it particularly easy to attach multiple schemas to a given parameter - which we must do to the “request” parameter in order to allow our single endpoint to accept multiple request messages.

One answer is to manually create the XSD and/or WSDL. Personally I find that answer impractical. Angle-brackets weren’t meant for human creation or consumption, and programmers shouldn’t have to see (much less type) things like XSD or WSDL.

Fortunately there’s another answer. In this blog entry, Microsoft technology expert Bill Wagner shows how to implement “method overloading” in asmx using C# to do the heavy lifting.

 

While the outcome isn’t totally seamless or perfect, it is pretty darn good. In my mind Bill’s answer is a decent compromise between getting the functionality we need, and avoiding the manual creation of cryptic WSDL artifacts.

 

The end result is that it is totally possible to construct flexible, maintainable and broadly useful web services using VB.NET or C#. Web services that follow a purely message-based approach, implement a message router on the server and thus provide a good story for both versioning and multiple message format stories.

 

Indigo, btw, offers a similar solution to what Bill talks about. In the Indigo situation however, it appears that we’ll have more fine-grained control over the generation of the public schema through the use of the DataContract idea, combined with inheritance in our VB or C# code.