Software Engineering for Students A Programming Approach

38 Chapter 4 ■ Requirements engineering
On the other hand, some people argue that it is impossible to divorce specification
and implementation. Indeed, in several major approaches to specification they are
intermixed. In such a method, the first task is to understand and to document the
workings of an existing system. (This might be a manual or a computer-based system,
or some combination.) This investigation serves as the prelude to the development of
a new computer system. Thus the implementation of one system (the old) acts as a
major ingredient in the specification for the new system. For example, suppose we
wished to develop a computer system for a library that currently does not use computers.
One approach would be to investigate and document all the current manual
procedures for buying books, cataloging, shelving, loaning, etc. Having accomplished
this task, the next step is to decide which areas of activity are to be computerized (for
example, the loans system). Finally, the specification of the new system is derived from
the design (implementation) of the old system. This approach to development seems
very appealing and logical. However, it does mean that implementation and specification
are intertwined.
There are several, additional and powerful reasons why the analyst must think about
implementation during specification. First, they must check that a requirement is technically
possible. For example, is it possible to achieve a response time of 0.1 second?
Second, it is vital to consider implementation in order to estimate the cost and delivery
date of the software. In order to estimate figures for performance and cost it will
almost certainly be necessary to carry out some outline development at least as far as
architectural design.
So, an ideal specification stipulates what, not how. But this is not always practical.
4.3 ● The qualities of a specification
We have seen that, ideally, a specification should confine itself to what is needed. We
now present a list of desirable qualities for a specification. A good specification should
exhibit the following characteristics:
■ implementation free – what is needed, not how this is achieved
■ complete – there is nothing missing
■ consistent – no individual requirement contradicts any other
■ unambiguous – each requirement has a single interpretation
■ concise – each requirement is stated once only, without duplication
■ minimal – there are no unnecessary ingredients
■ understandable – by both the clients and the developers
■ achievable – the requirement is technically feasible
■ testable – it can be demonstrated that the requirements have been met.
This list of desirable features can be used as a checklist when a specification is
drawn up. Additionally it can be used as a checklist to examine and improve an existing
specification.