Why Model?

Why do engineers build models? Why do aerospace engineers build models of aircraft? Why do structural engineers build models of bridges? What purposes do these models serve?

These engineers build models to find out whether their designs will work. Aerospace engineers build models of aircraft and then put them into wind tunnels to see whether they will fly. Structural engineers build models of bridges to see whether they will stand. Architects build models of buildings to see whether their clients will like the way they look. Models are built to find out whether something will work.

This implies that models must be testable. It does no good to build a model if you cannot apply criteria to that model in order to test it. If you can't evaluate the model, the model has no value.

Why don't aerospace engineers simply build the plane and try to fly it? Why don't structural engineers simply build the bridge and then see whether it stands? Very simply, airplanes and bridges are a lot more expensive than the models. We investigate designs with models when the models are much cheaper than the real thing we are building.

Why Build Models of Software?

Can a UML diagram be tested? Is it much cheaper to create and test than the software it represents? In both cases, the answer is nowhere near as clear as it is for aerospace engineers and structural engineers. There are no firm criteria for testing a UML diagram. We can look at it, evaluate it, and apply principles and patterns to it, but in the end, the evaluation is still subjective. UML diagrams are less expensive to draw than software is to write but not by a huge factor. Indeed, there are times when it's easier to change source code than it is to change a diagram. So when does it make sense to use UML?

I wouldn't be writing some of these chapters if UML didn't make sense to use. However, UML is also easy to misuse. We make use of UML when we have something definitive we need to test and when using UML to test it is cheaper than using code to test it. For example, let's say that I have an idea for a certain design. I need to test whether the other developers on my team think that it is a good idea. So I write a UML diagram on the whiteboard and ask my teammates for their feedback.

Should We Build Comprehensive Designs Before Coding?

Why do architects, aerospace engineers, and structural engineers all draw blueprints. The reason is that one person can draw the blueprints for a home that will require five or more people to build. A few dozen aerospace engineers can draw blueprints for an airplane that will require thousands of people to build. Blueprints can be drawn without digging foundations, pouring concrete, or hanging windows. In short, it is much cheaper to plan a building up front than to try to build it without a plan. It doesn't cost much to throw away a faulty blueprint, but it costs a lot to tear down a faulty building.

Once again, things are not so clear-cut in software. It is not at all clear that drawing UML diagrams is much cheaper than writing code. Indeed, many project teams have spent more on their diagrams than they have on the code itself. It is also not clear that throwing away a diagram is much cheaper than throwing away code. Therefore, it is not at all clear that creating a comprehensive UML design before writing code is a cost-effective option.