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 Tuesday, May 24, 2005

It is clear to me that Service Oriented Architecture has gone about as far as it can go in a vacuum. And Service Oriented Programming continues to evolve somewhat randomly as vendors and/or standards bodies see fit (WSE, Indigo, etc).

 

So there needs to be more rigorous thought apply to Service Oriented Design. If we assume the SOA concepts of autonomous entities communicating via contract-defined messages over negotiated channels. And if we assume we’ll have programming tools that provide contractual synchronous and asynchronous communication vectors, then we can move ahead and work on thought models for designing services and systems composed of services.

 

Note that there are two very different aspects here. One is how to create a service, the other is now to build systems where services interact. The inside view and the outside view. And the rules and concepts are very different for each.

 

In that vein, here's a good/interesting article on SO design from Rich Turner at Microsoft. He’s taking the inside view and discussing some high-level design concepts for how you might build an autonomous service that still requires other services to function. He’s proposing a couple thought models that might be useful in design efforts – exactly the kind of dialog that is required to move SOD forward.

 

Personally I think that a number of the object-oriented design concepts behind CRC cards (class, responsibility, collaboration) apply equally well to services. In such a scheme, Rich’s relationships (constellation or fractal) would be listed as collaborators from a service.

 

I’m convinced that at least the first side of a CRC card, where the class (now service), responsibility and list of collaborators (other services) are a good design tool for services. Some of the other sides (like the list of behaviors) aren’t so useful, since a service is an atomic procedure and thus by definition has exactly one behavior. Still, applying those parts of CRC that do work is very helpful.

Tuesday, May 24, 2005 9:02:10 AM (Central Standard Time, UTC-06:00)  #    Disclaimer  |  Comments [0]  | 
 Sunday, May 08, 2005
I try not to do too much simple link-fowarding, but this is a very good, clear article on SOA.
Sunday, May 08, 2005 6:45:51 PM (Central Standard Time, UTC-06:00)  #    Disclaimer  |  Comments [0]  | 
 Tuesday, May 03, 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:

 

   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.

Tuesday, May 03, 2005 10:15:53 AM (Central Standard Time, UTC-06:00)  #    Disclaimer  |  Comments [0]  | 
There are many ways to tweak VPC to get better performance. Putting the vhd file on its own physical (fast) hard drive is key, and Brian Randell has other tips.
 
But the single biggest thing you can do is have so much RAM in the host computer that there's no paging. I just upgraded my desktop so it has 2.5 gigs of RAM. My VPC machine has 768 megs allocated and it is fast. And at the same time on the host I'm able to run Word, Outlook, RSSBandit, Windows Media Player, Trillian, Avant Browser and all the other random stuff in my system tray.
 
The key of course, is to avoid paging, and the only real way to do that is to have about 20% more RAM than you'll ever actually use. 2.5 gigs seems to be a sweet spot for me, since even with everything running at once I still show almost 1 gig of free memory.
 
Prior to this I was at 1.5 gig total RAM, and with a VPC loaded the whole system was super-slow. The VPC was almost unusable, and other apps on the host loaded very slowly and ran with sub-par performance.
 
Now, last night I fired up a game and was happily playing away - then I realized that I'd never shut down the VPC. Here I was playing a memory-hungry modern game and a VPC and I didn't even notice the difference.
 
More memory is happiness!
Tuesday, May 03, 2005 9:25:31 AM (Central Standard Time, UTC-06:00)  #    Disclaimer  |  Comments [0]  | 
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