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Friday
Jan082010

The Big Bang Development Model

Last summer, on the first hot day we had (there weren’t that many hot days last year in New York), I turned on my air conditioner to find that although the outdoor compressor unit and the indoor air handler both appeared to be working (fans spinning), there was no cold air to be felt anywhere in the house. We bought the house about three years ago, and at that time the outdoor unit was relatively new (maybe five years old). In the non-summer seasons we've spent in the house, we've always made sure keep the compressor covered up so that rain, leaves and critters don't foul up the works, and I've even opened it up a couple of times to oil the fan and generally clean out whatever crud did accumulate in there, so I was surprised that the thing didn't last longer than eight years or so.

When the HVAC engineer came to check it out, he found that the local fuse that was installed inline with the compressor was not correctly rated (the original installer had chosen a 60A fuse; the compressor was rated at 40A), and that the compressor circuitry had burned out as a result. For the sake of the correct $5 part eight years ago, a new $4000 compressor was now required.

So this winter, when I turned up the thermostat on the first cold day and there was no heat, I readied the checkbook again. This furnace was the original equipment installed when the house was built in the ‘60s, so I felt sure that I’d need a replacement furnace. I was pleasantly surprised when this time the HVAC guy told me that a simple inexpensive part needed to be replaced (the flame sensor that shuts off the gas if the pilot light goes out). This was a great, modular design for a device that ensured that a full refit or replacement wasn’t required when a single component failed.

It occurred to me that I'd seen analogs of these two stories play out on software projects I've been involved with over the years. A single expedient choice (or a confluence of several such choices), each seemingly innocuous at the time, can turn into monstrous, expensive maintenance nightmares. Ward Cunningham originally equated this effect with that of (technical) debt [http://en.wikipedia.org/wiki/Technical_debt].

What makes the situation worse for software projects over the air conditioner analogy is that continuous change over the life of a software project offers more and more opportunity for such bad choices, and each change becomes more and more expensive as the system becomes more brittle, until all change becomes prohibitively expensive and the system is mothballed, its replacement is commissioned and a whole new expensive development project is begun. I think of this as a kind of Big Bang Development Model, and in my experience this has been the standard model for the finance industry.

For an in-house development shop, an argument can be made that this might not be so bad, although I wouldn't be one to make it – it’s success is highly dependent on your ability to retain the staff that “know where the bodies are buried”, which in turn is directly proportional to remuneration. If you're a vendor, Big Bang should not be an option - you need to hope that there's no other vendor waiting in the wings when your Big Bang happens.

Of course, today we try to mitigate the impact of bad choices with a combination of unit testing, iterative refactoring and abstraction, but all of this requires management vision, discipline, good governance and three or four of the right people (as opposed to several dozens or hundreds of the wrong ones). Those modern software engineering tools are also co-dependent: effective unit testing requires appropriate abstraction; fearless refactoring requires broad, automated testing; sensible abstractions can usually be refactored more easily than inappropriate ones when the need arises to improve them.

I'm not going to "refactor" my new $4000 compressor, but you can bet that I am going to use a $10 cover and a $5 can of oil.

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