Monday, February 25, 2013

We shouldn't build software like we build houses

On January 25th, 2013, Wired published an article by Leslie Lamport describing the need to build software like we build houses. This wasn’t the first time I heard this argument, but I was surprised that a publication as forward-thinking as Wired would publish such a backwards-looking article. It absolutely doesn’t make sense to build software like we build houses.
In his article, Lamport describes the need for specifications to help software developers “think clearly about what we are building.” Strangely, he seems to think that programmers don’t like thinking. In my experience, programmers are proud to do work that is so cerebral, and they enjoy opportunities to think deeply about their work. Another surprise is Lamport’s conflation of thinking and specifying; the two are hardly one and the same, and there are more ways to think than by writing specs. In general, though, Lamport’s point is that software should have specs so that programmers won’t make mistakes and software won’t be so hard to change.
Lamport’s overall argument is weak; he tries to make points through metaphors more often than he does through logic. I won’t dwell on this, though, as it’s been covered at length in the article’s comments. Instead I will point out what is implied by and missing from Lamport’s apparent worldview.

Lamport's Worldview

Lamport seems to believe that making mistakes is a big problem in software development and that writing specs is the best way to avoid it. This thinking rests on the assumptions that the writer of the spec knows exactly what the final state of the software should be – what it should do and how it should do it. It assumes that requirements are fully understood, technology is fully understood, and business conditions do not change. Over the past few decades, the software industry has come to realize that this is not often the case. I’m surprised that Lamport hasn’t  been made privy to this discovery, and I wonder whether the nature of his own programming work at Microsoft Research has somehow shielded him from it.
The software industry now understands fairly well that processes with up-front investment in things that aren’t working software – things like specs – work great for situations like accounting, cooking, and building houses. Some people call these “defined process control models.” These processes, while great in other industries, fall down on their faces when it comes to software development, where we don’t often have a lot of certainty about what we are building in, or how it will be built.

The Modern Worldview

In software we look instead to the empirical process control model, which emphasizes inspecting and adapting (i.e., feedback loops) instead of planning and then executing. This is what’s called for when we don’t know exactly what our work product will ultimately look like or how it will work.
In empirical processes, instead of being asked to clearly and unambiguously describe the exact output desired, a customer describes what needs to be accomplished. Developers work in short iterations to incrementally build what is needed, involving the customer all along the way, until the necessary solution is discovered. The result of these processes is typically better software.

Programming is Nothing Like Housebuilding

Building a house is arguably uncreative work. We can categorize houses fairly simply based on their attributes and current state of degradation. Houses, for all their variations, are largely the same and generally serve the same purposes for anyone that lives in them.
Software development, on the other hand, is incredibly creative work. The process of developing any two pieces of software will be quite different, as will its ultimate outputs. Thinking one can accurately define the exact output of a software development project is at best ignorant and at worst arrogant.
We can’t know exactly what will be created at the beginning of a software development project and that is okay. Instead we involve the customer in the process so they can see exactly what is happening as we move along. Lamport might call the evolution of the software in concert with customer feedback a series of mistakes, but anyone who has developed software this way knows there is actually incredible value in these iterations. Users don’t need to visualize how the software will work because they can actually use it after each iteration of development. And neither they nor developers have to waste time investing in specs that will rapidly become obsolete.
And despite what Lamport said, well-built software is incredibly easy to change. Using techniques like TDD allow us to leverage the strengths of software creation, and avoid the limitations inherent in the production of physical objects.


  1. It's not even true for building houses.
    Marry Poppendieck a lean coach, went to a training given to PM from large building projects, to learn about how do they do it. And they lauched when she asked them. They don't build large building with BDUF

  2. Thanks for the comment, @Yves! I'll have to follow up with Mary because I am very curious what the response was! I'm being careful with the generalizations as there are often exceptions, but I believe BDUF is fairly common in single-resident house construction where there is plenty of opportunity for repetition. I wouldn't suspect the same opportunities for repetition in large construction projects, which is why I am guessing Mary received the response she did.

  3. By the completion of a tract of 100 essentially identical suburban homes, the design's pretty well debugged, and construction #100 looks pretty BDUF. And when the developer takes the plan to another site and builds 100 more, those all look pretty BDUF, too. But way back at houses #1-5, there's a BD, all right, but there's also a lot of learning and tweaking.

    1. @Jack, exactly! The buyer knows just about exactly what he will get with house #100 before it is built. Buyer #100 will still likely want to look at house #99 to see if there are any tweaks he would like to make, but in general the end product is known.

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