The Working Model
Like the cavemen who modeled their hunting system, an architect draws up final building plans, which are referred to as “working drawings.” Using these drawings and a written specification, the builder constructs the building. Why are the plans called working drawings? Because they represent what will be a working system.
If the construction engineer erects the building exactly as the working drawings show, the building will function correctly. Indeed, before construction can begin, the architect uses the working drawings to obtain approval and to demonstrate to the proper authorities that the building will be safe and meets code. The architect also uses the working drawings to demonstrate to the owner of the building that it will function as desired.
Because it represents a working system, the analysis model is also called a working model of the system. How do you know from studying the model that the system will work? You can tell from looking at the data that enter and leave each of the processes. Ask, “Is there enough data entering a process for it to produce its output data?”
For example, in Figure 2.1.1, the bubble HUNT WOOLLY MAMMOTHS receives the incoming data CLUBS and MOST LIKELY TRAILS. These data are sufficient for the cavemen to hunt for food. When the process of hunting is finished, the cavemen return with MAMMOTH SIGHTINGS and, if they were sufficiently agile, a CARCASS or two. In this case, we can say that the process has enough input to produce its output. In other words, it works.
When you can say the same thing for the other processes in the system, then you know you have a working model of a correctly functioning system and therefore the correct specification of the system. How else can this kind of model help in the task of systems development? To answer, let’s first look at how you understand systems.