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Rebecca Wirfs-Brock’s Blog and Informal Essays

Agile Architecture Myths #2 Architecture Decisions Should Be Made At the Last Responsible Moment

In Lean Software Development: An Agile Toolkit, Mary and Tom Poppendieck describe “the last responsible moment” for making decisions:

Concurrent development makes it possible to delay commitment until the last responsible moment, that is, the moment at which failing to make a decision eliminates an important alternative.

And Jeff Atwood, in a thought-provoking blog argues that “we should resist our natural tendency to prepare too far in advance” especially in software development. Rather than carry along too many unused tools and excess baggage, Jeff admonishes,

Deciding too late is dangerous, but deciding too early in the rapidly changing world of software development is arguably even more dangerous. Let the principle of Last Responsible Moment be your guide.

And yet, something about the principle of the last responsible moment has always made me feel slightly uneasy. I’ve blogged about a related topic (just barely enough design) before. And be aware that in both my personal and professional life that I am not known as someone who plans things far out in advance. As a consequence I rarely use frequent flyer miles because I don’t anticipate vacation plans far enough in advance. I am not known to get to the airport hours ahead of my flight either. My just-in-time decision-making and actions have been known to make my traveling companions a bit uneasy. They prefer a not so tight timeline.

But what about software development? Well, if I find an approach that seems worth pursuing, I’ll typically go for it. I like to knock off decisions that have architecturally relevant impacts early so I can get on to the grunt work. A lot of code follows after certain architectural choices are made and common approaches are agreed upon. Let’s make a rational decision and then vet it, not constantly revisit it, is my ideal.

Yet I know too-early architecture decisions are particularly troublesome as they may have to be undone (or if not, result in less-than-optimal architecture).

So what is it about forcing decision-making to be just-in-time at the last responsible moment that bugs me, the notorious non-planner? Well, one thing I’ve observed on complex projects is that it takes time to disseminate decisions. And decisions that initially appear to be localized (and not to impact others who are working in other areas) can and frequently do have ripple affects outside their initially perceived sphere of influence. And, sometimes, in the thick of development, it can be hard to consciously make any decisions whatsoever. How I’ve coded up something for one story may inadvertently dictate the preferred style for implementing future stories, even though it turns out to be wrongheaded. The last responsible moment mindset can at times lull me (erroneously) into thinking that I’ll always have time to change my mind if I need to. I’m ever the optimist. Yet in order to work well with others and to produce habitable software I sometimes need a little more forethought.

And so, I think I operate more effectively if I make decisions at the “most responsible moment” instead of the “last responsible moment”.

I’m not a good enough of a designer (or maybe I am too much of an optimist) to know when the last responsible moment is. Just having a last-responsible moment mindset leaves me open to making late decisions. I’m sure this is not what Mary and Tom intended at all.

So I prefer to make decisions when they have positive impacts. Making decisions early that are going to have huge implications isn’t bad or always wasteful. Just be sure they are vetted and revisited if need be. Deferring decisions until you know more is OK, too. Just don’t dawdle or keep changing your mind. And don’t just make decisions only to eliminate alternatives, but make them to keep others from being delayed or bogged down waiting for you to get your act together. Remember you are collaborating with others. Delaying decisions may put others in a bind.

In short: make decisions when the time is right. Which can be hard to figure out sometimes. That’s what makes development challenging. Decisions shouldn’t be forced or delayed, but taken up when the time is right. And to help me find the right times, I prefer the mindset of “the most responsible moment” not the “last responsible one.”