Behavior-driven development catches on

BDD gets users involved earlier for improved results in agile application development

Behavior-driven development (BDD), which helps users get more involved in describing an application's intended behavior, is becoming popular with developers, who are now latching on to tools that fit into the BDD vein. It's particularly relevant to agile application development approaches.

In BDD, application stakeholders as well, analysts, and testers are brought together to determine requirements. BDD features tests written in plain text. "Test-driven development, BDD, executable specifications -- all of that is becoming more popular and is the future," says agile development coach Warren Elliott.

How behavior-driven development works
BDD is an evolution of test-driven development that brings the customers into the picture, says Aslak Hellesoy, creator of the Cucumber BDD tool and a consultant at Norway's Bekk Consulting. BDD lets the customer write tests that everyone understands with assistance from developers and testers.

The BDD approach holds that an idea for a requirement can be turned into an implemented, tested, production-ready code as long as the requirement is specific enough that everyone knows what is going on, says developer Dan North of ThoughtWorks, who is credited with coining the term "BDD" around 2003. To do this, a way to describe the requirement is needed so that everyone from businesspeople to analysts, developers, and testers have a common understanding of the scope of the work. Parties agree on a common definition of "done."

In the BDD approach, a BDD "story" is developed, featuring a description of a requirement, a business benefit, and criteria for completeness. A story could be developed, for example, for an application for withdrawing cash from an ATM.

"BDD is about helping the customers, the stakeholders, have this slightly structured conversation with the delivery team," North says. "[BDD is] allowing people to write better software," he says, adding that it enables a focus on just delivering features.

Hellesoy advises against using BDD in a traditional, waterfall method of application development. "I would only use it on an agile project," he says, because BDD treats the specification as a living, changing document. "You can't possibly come up with it up front," he says, which methods such as waterfall require.

Tools leveraging BDD concepts include Cucumber, RSpec, JBehave, and FitNesse. They focus on different aspects of application development and testing, so can be complementary.

Cucumber is a current BDD darling
Cucumber drew crowds at the RailsConf 2009 conference in Las Vegas in early May. As of May, Cucumber had been downloaded about 40,000 times since its first release eight months ago. "I love Cucumber," says Martin Emde, a lead developer at Naughty America, an adult entertainment venture. "It gives me something to show my bosses, so my boss knows what it is that the application really does," he says, "There's a clarity in communication."

"The reason Cucumber interests me is it's a little more expressive than FitNesse," with developers able to write business scenarios and execute them against code, says app dev coach Elliott. Developers can work with an executable rather than just a Word or HTML document, he notes. "Cucumber has a long lineage" dating to 2003, Hellesoy says, and is written in the Ruby language. "The Ruby community didn't have a tool to do what the Java people have," he notes.