Software Design Methodology

48 Chapter 3. Design Principles

3.1 BASIC RULES OF SOFTWARE DESIGN

3.1.1 Causes of difficulties

Software development is complicated and difficult. Design is one of the most

difficult tasks in software development. Before we start to search for methods that

allow us to make good designs of software systems, we need to understand why

software development is difficult.

As discussed in Chapter 1, all designers face difficulties. The question is

whether or not there are specific reasons that make software design even more

difficult in additional to the general reasons. In the paper No Silver Bullet: Essence

and Accidents of Software Engineering [

1], Fred Brooks pointed out some of the

principal causes of the difficulties of software development. These causes are

classified into two types: the essence and the accidents. Essences are the

difficulties inherent in the nature of software. They are irreducible, will not

disappear and cannot be solved through technical solutions. Accidents are

difficulties that today attend its production but are not inherent. Brooks’ four

essences of difficulties of software development are the following.

(a) Complexity

Complexity is an essential property of software. Software entities are complex in

terms of the sizes of their state spaces and the way these states are interrelated one

to another. This makes conceiving, describing, and testing software hard. Digital

computers are more complex than most things that people build. They have large

numbers of states. Software systems have orders of magnitude more states than

computers do. Software entities are also complex in terms of the numbers of their

elements and the way they interact with each other. Elements in a software entity

are not repetitions of the same because if two parts are similar, we make them a

subroutine or procedure. Therefore, elements in a software entity are all essentially

different from each other. A scale up of a software entity, therefore, is not merely

increasing the repeated occurrences of same elements or enlarging the elements in

sizes. It is necessarily the increase of the number of different elements. These

elements in a software entity interact with each other nonlinearly in most cases, e.g.

the state space of the whole system is often a nonlinear function of the state spaces

of the elements.

The complexity of software and its nonlinear increase with size is the main

cause of many difficulties in software development. The complexity of the problem