The Steering Premise | #98

PawCast with GeePaw Hill

Microtest TDD's Steering Premise is quite simple, which may be why it sometimes meets furious opposition. It says "Tests and testability are first-class citizens in design." Let's talk that over a little. There are, for any software problem, an infinite number of functionally correct designs. If implemented, they will work. But we don't *implement* an infinite number of designs. Why not? Because though they may be functionally correct, they still don't fit our context. We reject designs -- more often we reject individual choices in designs -- for a variety of reasons: reliability, cost of hardware, poor fit to our toolset, and so on. Those reasons are the "citizens" of the design process. And the steering premise says that one of the citizens -- not a secondary or minor or ex post facto consideration -- is whether that design can be tested. To probe at it: If you brought us a functionally correct design that required our app to run on a million AWS instances, we'd casually say "no, that's not valid." So obvious is this conclusion, that no one ever does it. Designs don't even get that far with such an issue.

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