How do you design complex systems with TDD?

Question

Similar to Does TDD mean not thinking about class design?, I am having trouble thinking about where the traditional 'design' stage fits into TDD.

According to the Bowling Game Kata (the 'conversation' version, whose link escapes me at the moment) TDD appears to ignore design decisions made early on (discard the frame object, roll object, etc). I can see in that example it being a good idea to follow the tests and ignore your initial design thoughts, but in bigger projects or ones where you want to leave an opening for expansion / customisation, wouldn't it be better to put things in that you don't have a test for or don't have a need for immediately in order to avoid time-consuming rewrites later?

In short - how much design is too much when doing TDD, and how much should I be following that design as I write tests and the code to pass them (ignoring my design to only worry about passing tests)?

Or am I worrying about nothing, and code written simply to follow tests is not (in practice) difficult to rewrite or refactor if you're painted into a corner? Alternatively, am I missing the point and that I should be expecting to rewrite portions of the code when I come to test a new section of functionality?

Answer 1

I would base your tests on your initial design. In many ways TDD is a discovery process. You can expect to either confirm your early design choices or find that there are better choices you can make. Do as much upfront design as you are comfortable with. Some like to fly by the seat of the chairs doing high level design and using TDD to flesh the design out. While others like to have everything on paper first.

Part of TDD is refactoring.