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Why software engineering is NOT like structural engineering

121_2While cycling home from work today I was listening to the Deep Fried Bytes episode with Rico Mariani. I should have been warned by his introduction; “He has an analogy for everything”. But since he seems to be a smart person the good old “Software engineering is just like structural engineering” caught me by surprise. I thought we ditched that one together with the waterfall process.

Usually this analogy is used to get people to do big up front design. “You wouldn’t start pouring concrete for the foundations when you don’t know how many stories your building should have” In structural engineering you need to finish a design before you start building.

But in software engineering all we do is design. Sure, in waterfall you have a design and implementation phase. But if you look more closely implementation has nothing to do with building anything. It’s design on a more detailed level, code is design too just like UML. The only thing we do that looks the slightest bit like building is deploying our software. Luckilly I’ve never seen anyone deploy software that they havent designed yet.

The forces in software design are also completely different from structural engineering. Any good software design is highly decoupled. This means that you don’t have a foundation that should change whenever another part of the “building” changes. In structural engineering you don’t have that luxury. We don’t have the technology to decouple gravity yet so every time you add to your building you’re going to need a bigger foundation.

Another reason we’re so big on decoupling is that software engineering is hard for completely different reasons than structural engineering. Complexity is a much bigger factor in software. Requirements change a lot more too. I’m not trying to say software engineering is harder in any way or that structural engineers don’t have problems with complexity or customers who change their minds. But in software engineering these things play a much bigger role. Luckilly we can handle these problems by decoupling the different parts of our design so we only have to handle the complexity of the individual components and can change around pieces if the customer needs something new.

So if software engineering is all about complex and flexible designs how should we go about creating them? Waterfall tells us to finish our high level design first and then fill in the details. The problem with this is changing requirements and no feedback on the quality of the design. Should we do bottom up design then? Start small and add more pieces later like agile tells us to do? I think we should do both. Work bottom up but be shure you have a rough sketch of what you’re trying to design. Don’t spend too much time on it because you’re probably going to throw it a way at least a couple of times before your building is finished.


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