Is TDD Enough?

11 Jun 2012

One thing that the Rails community prides itself on is that test-driven development (TDD) is widely employed. But is this sufficient to produce good quality code that stands the test of changing requirements over a long period of time?

In my recent roundup of Ruby resources, Sandi Metz is one author I recommended. In her excellent talk at the 2009 Gotham Ruby Conference, Sandi contested that TDD is not enough. Sandi’s talk, which concentrates on applying SOLID design principles in Ruby, is well worth listening to in its entirety.

2009 - Sandi Metz - SOLID Object-Oriented Design from Gotham Ruby Conference on Vimeo

As Sandi explains, central to SOLID design is avoiding dependencies by striving for code that is:

• loosely coupled
• highly cohesive
• easily composable
• context independent

As she demonstrates applying these principles through refactoring a small Ruby application, Sandi shows that, if we want code to endure in a state that makes it fun to work with, we need to do more than use TDD. As Sandi refactors she emphasises that when we reach a point where the tests pass, we should not be satisfied. Nor should ensuring that the code is DRY be enough. Other questions that need to be asked about the class under test are:

• Does it have one responsibility?
• Does everything in it change at the same rate?
• Does it depend on things that change less often than it does?

Only when the answer to all of these questions is “yes” should we move on.

I know these are questions I should ask myself more often. As Sandi stresses, “test driven development is good, but it’s not enough.”

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