[dasil003]

To me the crux of the issue is the interface. If you can define a very clean interface without having to do a lot of contortions to get the data you want where you need it, then extracting a service can be relatively low overhead. But what often happens is at the 30,000-foot view it looks like a service makes sense, but then when you get into the details you realize the separation can not be as clean as you first envisioned.

[jaegerpicker]

I'd disagree with this. In my experience everytime it has seemed like the interface was too complex too support services correctly, it's been because the break for the services was at the wrong level of abstraction. SOA tends to work best when you define discreet chunks of functionality and each service is only responsible for that chunk. Just like developing testable code, you want to make those chunks as small as possible, without losing your mind at the shear number of services. For example having an order service and shipping service as opposed to a just an order service that handles everything is more likely to make sense IMO.

[dasil003]

Your argument doesn't seem to address my point. You're saying if the interface isn't good you're at the wrong level of abstraction. Okay. How does that contradict the idea that the interface is everything for creating a successful SOA?

[jaegerpicker]

I'm saying that it's rare that SOA isn't a proper fit for a web application and if it seems like a poor fit you likely haven't abstracted your interfaces to the right level. Your monolithic app is going to suffer from poor design just as much as an SOA based one. Your point seemed to be that certain applications could be well designed and still not a proper fit for a SOA. I think if an app is well designed it will by default be a proper fit for SOA. If you were not implying that I apologize. I guess my position is that nearly any complex web application would benefit from SOA.

[dasil003]

My point was really nothing to do with whether an application is a fit for SOA. It's more about how to design an SOA. Your point about a poorly designed SOA being an equally poorly designed app is well taken, but I think it's more work to design a good SOA than a good monolithic app. And this is where the effort of designing the interface comes in.

In a monolithic app interfaces can be more fluid because you can have automated tests and static analysis and compile time checks verifying that a given change works. That means you can prototype and iterate faster while the business requirements may still be churning considerably. To realize the benefits of an SOA you need a much more stable interface and some way to handle validation and correctness of the wire protocol. If you do it right and come up with a stable interface, you gain the benefits of decoupling system administration, scalability, and even development to a great extent. But if you do it wrong you end up a lot more work for an equivalent business logic architecture, and if you don't have any scaling issues than the cost-benefit is likely not to be there.

My rule of thumb about whether an SOA is a good idea to pursue at a given point in time is much more related to the team size than the nature of the app.