Tuesday, February 26, 2008

Behavior Driven Development is not Domain Driven Design?

I had been aware of behavior driven development for a while, but researching my previous post on Test Driven Development I came across something that really had me scratching my head. The general view of BDD is an evolution or refinement of TDD with the goal of making TDD more natural and easier to understand. A big part of BDD is a change in mindset from thinking about testing to thinking about behavior and class design. To this end doing BDD has behavior classes, not test cases. BDD also has methods that specify behavior with names that clearly describe the expected behavior and begin with "should" instead of "test". All this is well and good and I expect will make things easier to understand, especially for developers new to agile development.

What has me more than a little bothered is the linking of BDD with "domain driven design" and the "ubiquitous language" as described by Eric Evans in his excellent book. The Behaviour-Driven Development web site and Wikipedia entry read as if BDD is somehow a logical extension of domain driven design. I read everything I could find and saw nothing to backup the claim. Maybe the site is incomplete, or perhaps it just shows the weakness of implementing the Behaviour-Driven Development site as a wiki. So far I am not convinced that BDD has the depth to accomplish domain driven design. From Domain-Driven Design -
"Domain-driven design is not a technology or a methodology. It is a way of thinking and a set of priorities, aimed at accelerating software projects that have to deal with complicated domains."
Stories will not produce the rich domain model that is needed. Here is the template for a story -

Title (one line describing the story)

As a [role]
I want [feature]
So that [benefit]

Acceptance Criteria: (presented as Scenarios)
This is just an variation on the agile user story and is still too simple, low level, to narrow in focus to result in deep domain knowledge. BDD may give you a well crafted set of classes that implement a feature or story. But how will those classes look in the context of the domain? Will the result superficial or insightful? Will the object connections and interactions be deep and rich, full of meaning, or shallow and empty?

Will the developers following BDD understand the vision of the domain model? Will they understand that changing the code is changes the model and the changing the model requires changing the code? It's an issue of the old saying "not seeing the forest for the trees". How do you see the big picture when the focus is so tight on a story and feature and the classes that implement them.

"Getting the Words Right" and method names that start with "should" and read like sentences won't guide anyone toward a ubiquitous language for the domain. Getting the language right requires a thorough understanding the the problem domain and that does not come from coding user stories.

Don't get me wrong I'm a big believer in agile development and unit testing. I understand that TDD and BDD are as much about design as they are about testing. I been using JUnit since something like 1999. I know how much better unit tests make my code and how much they can improve the design of the classes being tested. But I don't use tests to for domain modeling.


DanNorth.net » Introducing BDD
Beyond Test Driven Development: Behaviour Driven Development (Google Video)
In pursuit of code quality: Adventures in behavior-driven development
Test Early » Using BDD to drive development


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