Predictive versus Adaptive

　　Separation of Design and Construction

　　The usual inspiration for methodologies is engineering disciplines such as civil or mechanical engineering. Such disciplines put a lot of emphasis on planning before you build. Such engineers will work on a series of drawings that precisely indicate what needs to be built and how these things need to be put together. Many design decisions, such as how to deal with the load on a bridge, are made as the drawings are produced. The drawings are then handed over to a different group, often a different company, to be built. It's assumed that the construction process will follow the drawings. In practice the constructors will run into some problems, but these are usually small.

　　Since the drawings specify the pieces and how they need to be put together, they act as the foundation for a detailed construction plan. Such a plan can figure out the tasks that need to be done and what dependencies exist between these tasks. This allows for a reasonably predictable schedule and budget for construction. It also says in detail how the people doing the construction work should do their work. This allows the construction to be less skilled intellectually, although they are often very skilled manually.

　　So what we see here are two fundamentally different activities. Design which is difficult to predict and requires expensive and creative people, and construction which is easier to predict. Once we have the design, we can plan the construction. Once we have the plan for the construction, we can then deal with construction in a much more predictable way. In civil engineering construction is much bigger in both cost and time than design and planning.

　　So the approach for software engineering methodologies looks like this: we want a predictable schedule that can use people with lower skills. To do this we must separate design from construction. Therefore we need to figure out how to do the design for software so that the construction can be straightforward once the planning is done.

　　So what form does this plan take? For many, this is the role of design notations such as the UML. If we can make all the significant decisions using the UML, we can build a construction plan and then hand these designs off to coders as a construction activity.

　　But here lies the crucial question. Can you get a design that is capable of turning the coding into a predictable construction activity? And if so, is cost of doing this sufficiently small to make this approach worthwhile?

　　All of this brings a few questions to mind. The first is the matter of how difficult it is to get a UML-like design into a state that it can be handed over to programmers. The problem with a UML-like design is that it can look very good on paper, yet be seriously flawed when you actually have to program the thing. The models that civil engineers use are based on many years of practice that are enshrined in engineering codes. Furthermore the key issues, such as the way forces play in the design, are amenable to mathematical analysis. The only checking we can do of UML-like diagrams is peer review. While this is helpful it leads to errors in the design that are often only uncovered during coding and testing. Even skilled designers, such as I consider myself to be, are often surprised when we turn such a design into software.

　　Another issue is that of comparative cost. When you build a bridge, the cost of the design effort is about 10% of the job, with the rest being construction. In software the amount of time spent in coding is much, much less McConnell suggests that for a large project, only 15% of the project is code and unit test, an almost perfect reversal of the bridge building ratios. Even if you lump in all testing as part of construction, then design is still 50% of the work. This raises an important question about the nature of design in software compared to its role in other branches of engineering.

　　These kinds of questions led Jack Reeves to suggest that in fact the source code is a design document and that the construction phase is actually the use of the compiler and linker. Indeed anything that you can treat as construction can and should be automated.

　　This thinking leads to some important conclusions:

　　In software: construction is so cheap as to be free
In software all the effort is design, and thus requires creative and talented people Creative processes are not easily planned, and so predictability may well be an impossible target.
We should be very wary of the traditional engineering metaphor for building software. It's a different kind of activity and requires a different process.

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