Can unit testing be successfully added into an existing production project? If so, how and is it worth it?

Question

I\'m strongly considering adding unit testing to an existing project that is in production. It was started 18 months ago before I could really see any benefit of TDD

Answer 1

I suggest reading a brilliant article by a TopTal Engineer, that explains where to start adding tests: it contains a lot of maths, but the basic idea is:

1) Measure your code's Afferent Coupling (CA) (how much a class is used by other classes, meaning breaking it would cause widespread damage)

2) Measure your code's Cyclomatic Complexity (CC) (higher complexity = higher change of breaking)

You need to identify classes with high CA and CC, i.e. have a function f(CA,CC) and the classes with the smallest differences between the two metrics should be given the highest priority for test coverage.

Why? Because a high CA but very low CC classes are very important but unlikely to break. On the other hand, low CA but high CC are likely to break, but will cause less damage. So you want to balance.