December 12, 2006
Re: Domain-Driven Design - Implications to SOA?
Jon recently wrote up a nice review of a talk he attended on Domain-Driven Design (DDD). In his review, he raised the following questions:
"Now something that really got me buzzing was something Eric said in passing about reuse. He was talking about context, and how domain-driven design and model development almost always apply to a particular context. A model is used in solving a problem in a particular context. What Eric said is that sometimes these models are wildly successful-the model really works in solving a set of problems. And what happens then? People start using the model out of context, to solve other problems. And what happens then? A lot of ugliness and failures. The model was created for a purpose, and is now being abused because it's being used out of context.
At this point some synapses fired and I thought to myself: isn't this a problem in SOA? You see, even though SOA is service oriented, we also expose a model. Sometimes implicitly and sometimes explicitly. And the model we expose was probably created to solve some particular problem in some domain.
The problem I see is this: SOA encourages reuse, and so the chance of a good model being used out of context is increased dramatically. What are people doing about this? Is anyone working on domain-driven design in an SOA context? Do we need strict governance systems around the reuse of services to prevent models from being abused? Doesn't SOA make domain-driven design more difficult?"
First let me respond by saying that I am working with DDD in an SOA context. Second, although there is some buzz around the reuse possibilities of SOA, I'm firmly in the "you use a service, you don't reuse it" camp. In my experience, a service that encapsulates a DDD context works well, while services that cross context boundaries lead to tight coupling. This has to do with the business level cohesiveness requirements found in a context.
To the question of whether SOA makes DDD more difficult I'd have to say that the answer is yes and no. If you are trying to take an entire enterprise model and divide it up between services, you'll have a hard time. If you take a given set of services and try to do DDD within each of them, you may find that a mistake in assigning responsibility to services will become apparent in the domain model. I see that each one gives feedback to the other, keeping you from veering to far off the golden path.
Posted by Udi Dahan at 04:11 PM Permalink
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