Why does TDD work?
- by CesarGon
Test-driven development (TDD) is big these days. I often see it recommended as a solution for a wide range of problems here in Programmers SE and other venues. I wonder why it works.
From an engineering point of view, it puzzles me for two reasons:
The "write test + refactor till pass" approach looks incredibly anti-engineering. If civil engineers used that approach for bridge construction, or car designers for their cars, for example, they would be reshaping their bridges or cars at very high cost, and the result would be a patched-up mess with no well thought-out architecture. The "refactor till pass" guideline is often taken as a mandate to forget architectural design and do whatever is necessary to comply with the test; in other words, the test, rather than the user, sets the requirement. In this situation, how can we guarantee good "ilities" in the outcomes, i.e. a final result that is not only correct but also extensible, robust, easy to use, reliable, safe, secure, etc.? This is what architecture usually does.
Testing cannot guarantee that a system works; it can only show that it doesn't. In other words, testing may show you that a system contains defects if it fails a test, but a system that passes all tests is not safer than a system that fails them. Test coverage, test quality and other factors are crucial here. The false safe feelings that an "all green" outcomes produces to many people has been reported in civil and aerospace industries as extremely dangerous, because it may be interepreted as "the system is fine", when it really means "the system is as good as our testing strategy". Often, the testing strategy is not checked. Or, who tests the tests?
I would like to see answers containing reasons why TDD in software engineering is a good practice, and why the issues that I have explained above are not relevant (or not relevant enough) in the case of software. Thank you.
