http://yourdon.com/strucanalysis/wiki/index.php?title=Chapter_17  

   

   

 

THE ESSENTIAL MODEL

 

 

The essential system model is a model of what the system must do in order to satisfy the user’s requirements, with as little as possible (and ideally nothing) said about how the system will be implemented. As mentioned earlier, this means that our system model assumes that we have perfect technology available and that it can be readily obtained at zero cost.

Specifically, this means that when the systems analyst talks with the user about the requirements of the system, the analyst should avoid describing specific implementations of processes (bubbles in the dataflow diagram) in the system; that is, he or she should not show the system functions being carried out by humans or an existing computer system. As illustrated by Figure 17.4(a) and (b), these are arbitrary choices of how the system might be implemented; but this is a decision that should be delayed until the systems design activity has begun.[5] Figure 17.4(c) shows a more appropriate essential model of what the system function must carry out regardless of its eventual implementation.

 

PHYSICAL MODEL I 

Figure 17.4(a): A model of how a system function will perform its job; source: Image:Figure174a.graffle

 

 

  PHYSICAL MODEL II

 

Figure 17.4(b): Another model of how the system function will be performed; source: Image:Figure174b.graffle

 

 

 

 

 

 

 

LOGICAL MODEL

 

Figure 17.4(c): A model of what the system function is; source: Image:Figure174c.graffle

 

 

The same is true of dataflows and data stores: the essential model should describe the content of dataflows and data stores, without describing the medium (e.g., disk or tape) or physical organization of the data.