Guardian.co.uk, Software design

An ABC of R2: D is for domain driven design

…which Mat Wall and I have written about extensively before, However, for this piece let me say this…

When you have a huge number of people for whom you are building software (1500 staff, 20 million unique users, and an entire wired economy influencing which way you should go next) then simply following instructions is insufficient, because your users’ demands will change and evolve over time — even if not during the current project then certainly before Version Two. So to minimise the cost of those changes you need to understand the way your users are thinking.

That’s where domain driven design (DDD) comes from. It’s about taking the concepts in your users’ heads and embedding them straight into the software you’re writing. And then when those concepts evolve and change the cost of changing your software is directly proportional to the mental shift that your users are making. When your users say “I want to make a small change” then usually it’s a small cost; if they propose a big change then they should understand when you walk them through all the implications.

For the R2 project we used DDD from the start, and it was key to many of our successes: when we discovered new opportunites which arose only from direct use of what we had implemented, then DDD allowed us to realise them.

Take, for example, the idea of “tone” — the principle that we should be able to categorise content by its “voice” or “style” (well, its tone). Its tone might be obituary, blog post, match report, and so on. The vague notion of this had been around since the start of the project, but we hadn’t settled on many details, let alone how to implement them. But after a few releases, and when the software started getting real use, it became apparent that applying a tone should be very like applying a keyword. Suddenly things fell into place. The functional requirements were clear, as were the implementation details. Tone has become a very powerful feature (here are all our obituraries, all our blog posts, all our match reports,…) yet it was a relatively straightforward piece of work because it was, in the end, a relatively intuitive idea.

Of course, domain driven design has a lot more to it than I’ve described here, and our use of it has been much deeper than feature implementation. If you’re lucky enough to be going to QCon San Francisco then today you can see our software architect Phil Wills presenting a lot more detail there.

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