Calidad Del Software

1 Softw are quality
E ngineering seeks quality; software engineering is the production of quality software.
This book introduces a set of techniques which hold the potential for remarkable improvements in the quality of software products. Before studying these techniques, we must clarify their goals. Software quality is best described as a combination of several factors. This chapter analyzessome of these factors, shows where improvements are most sorely needed, and points to the directions where we shall be looking for solutions in the rest of our journey.

1.1 EXTERNAL AND INTERNAL FACTORS
We all want our software systems to be fast, reliable, easy to use, readable, modular, structured and so on. But these adjectives describe two different sorts of qualities. On one side, we areconsidering such qualities as speed or ease of use, whose presence or absence in a software product may be detected by its users. These properties may be called external quality factors.
Under “users” we should include not only the people who actually interact with the final products, like an airline agent using a flight reservation system, but also those who purchase the software or contract outits development, like an airline executive in charge of acquiring or commissioning flight reservation systems. So a property such as the ease with which the software may be adapted to changes of specifications — defined later in this discussion as extendibility — falls into the category of external factors even though it may not be of immediate interest to such “end users” as the reservationsagent.

Other qualities applicable to a software product, such as being modular, or readable, are internal factors, perceptible only to computer professionals who have access to the actual software text. In the end, only external factors matter. If I use a Web browser or live near a computer-controlled nuclear plant, little do I care whether the source program is readable or modular if graphics takeages to load, or if a wrong input blows up the plant. But the key to achieving these external factors is in the internal ones: for the users to enjoy the visible qualities, the designers and implementers must have applied internal techniques that will ensure the hidden qualities.

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SOFTWARE QUALITY §1.2

The following chapters present of a set of modern techniques for obtaining internalquality. We should not, however, lose track of the global picture; the internal techniques are not an end in themselves, but a means to reach external software qualities. So we must start by looking at external factors. The rest of this chapter examines them.

1.2 A REVIEW OF EXTERNAL FACTORS
Here are the most important external quality factors, whose pursuit is the central task ofobject-oriented software construction.

Correctness Definition: correctness
Correctness is the ability of software products to perform their exact tasks, as defined by their specification. Correctness is the prime quality. If a system does not do what it is supposed to do, everything else about it — whether it is fast, has a nice user interface… — matters little. But this is easier said than done. Even thefirst step to correctness is already difficult: we must be able to specify the system requirements in a precise form, by itself quite a challenging task. Methods for ensuring correctness will usually be conditional. A serious software system, even a small one by today’s standards, touches on so many areas that it would be impossible to guarantee its correctness by dealing with all components andproperties on a single level. Instead, a layered approach is necessary, each layer relying on lower ones:

Application system Compiler Operating System Hardware
In the conditional approach to correctness, we only worry about guaranteeing that each layer is correct on the assumption that the lower levels are correct. This is the only realistic technique, as it achieves separation of concerns...