In Defense of Design Before Coding

Software design as a separate activity from implementation — “up front” design — got a bad press with agile methods.

Agile advocates say the design should be emergent. They say, design without coding is waterfall. It’s a waste of time.

I understand that you don’t want to design the whole system up front. But at the feature level, a bit of thinking before coding does miracles, I say.

My first argument is visible design. Looking at the code doesn’t reveal the whole design because code only shows the static structure. The design is more than that. To understand how the system works you must run it, but even then the sequencing of events is still invisible. If you want effective feedback on the design, you must make it visible. People that jump directly to code still end up sketching or drawing things for their colleagues to explain their design and get feedback. Designing up front makes the design visible up front.

My second argument is speed of iteration. Even with higher-level programming languages, there is a gap between the concepts and the implementation. There is some work needed to implement the thing for real and take care of all the details. Running the system in your your head, or on paper, to challenge the design enables faster iterations on the design. CRC sessions are for instance a nice way to explore the design space effectively, without coding yet.

My third argument is better reasoning. The code level is just one of the many abstraction levels you can use to reason about the system. When you’re trying to identify the main abstractions, what their responsibilities are, and how they play together, this abstraction level is often too low. One abstraction with a clear responsibility might map to several classes. There might be for instance a “scheduler” that will be implemented using several concurrency primitives. These are implementation details (althrough interesting ones!) irrelevant for now. Working at the code level forces you to think at one specific abstraction level. Working on paper enables you to choose the optimal abstraction level to work out the design.

And finally, my fourth argument is tracking rationale. The code defines how the system works but gives in itself little clues as to why it was designed so. Design is all about trade offs: what were the they? If you never learn to design up front on paper you will never learn to document software understandably, too. And without documentation later, the rationale will be lost.

You should design up front so far you can. Then switch to a computer.

How far you can design up front depends on your intellect and your knowledge of the problem domain. Learn to assess the confidence in your up front design correctly, and identify when to stop, since there lies the danger: too much time spent designing on paper something that doesn’t work. But some design up front has its place.


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