20.1. Characteristics of Software Quality

Software has both external and internal quality characteristics. External characteristics are characteristics that a user of the software product is aware of, including the following:

• Correctness The degree to which a system is free from faults in its specification, design, and implementation.

• Usability The ease with which users can learn and use a system.

• Efficiency Minimal use of system resources, including memory and execution time.

• Reliability The ability of a system to perform its required functions under stated conditions whenever required having a long mean time between failures.

• Integrity The degree to which a system prevents unauthorized or improper access to its programs and its data. The idea of integrity includes restricting unauthorized user accesses as well as ensuring that data is accessed properly that is, that tables with parallel data are modified in parallel, that date fields contain only valid dates, and so on.

• Adaptability The extent to which a system can be used, without modification, in applications or environments other than those for which it was specifically designed.

• Accuracy The degree to which a system, as built, is free from error, especially with respect to quantitative outputs. Accuracy differs from correctness; it is a determination of how well a system does the job it's built for rather than whether it was built correctly.

• Robustness The degree to which a system continues to function in the presence of invalid inputs or stressful environmental conditions.

Some of these characteristics overlap, but all have different shades of meaning that are applicable more in some cases, less in others.

External characteristics of quality are the only kind of software characteristics that users care about. Users care about whether the software is easy to use, not about whether it's easy for you to modify. They care about whether the software works correctly, not about whether the code is readable or well structured.

Programmers care about the internal characteristics of the software as well as the external ones. This book is code-centered, so it focuses on the internal quality characteristics, including

• Maintainability The ease with which you can modify a software system to change or add capabilities, improve performance, or correct defects.

• Flexibility The extent to which you can modify a system for uses or environments other than those for which it was specifically designed.

• Portability The ease with which you can modify a system to operate in an environment different from that for which it was specifically designed.

• Reusability The extent to which and the ease with which you can use parts of a system in other systems.

• Readability The ease with which you can read and understand the source code of a system, especially at the detailed-statement level.

• Testability The degree to which you can unit-test and system-test a system; the degree to which you can verify that the system meets its requirements.

• Understandability The ease with which you can comprehend a system at both the system-organizational and detailed-statement levels. Understandability has to do with the coherence of the system at a more general level than readability does.

As in the list of external quality characteristics, some of these internal characteristics overlap, but they too each have different shades of meaning that are valuable.

The internal aspects of system quality are the main subject of this book and aren't discussed further in this chapter.

The difference between internal and external characteristics isn't completely clear-cut because at some level internal characteristics affect external ones. Software that isn't internally understandable or maintainable impairs your ability to correct defects, which in turn affects the external characteristics of correctness and reliability. Software that isn't flexible can't be enhanced in response to user requests, which in turn affects the external characteristic of usability. The point is that some quality characteristics are emphasized to make life easier for the user and some are emphasized to make life easier for the programmer. Try to know which is which and when and how these characteristics interact.

The attempt to maximize certain characteristics inevitably conflicts with the attempt to maximize others. Finding an optimal solution from a set of competing objectives is one activity that makes software development a true engineering discipline. Figure 20-1 shows the way in which focusing on some external quality characteristics affects others. The same kinds of relationships can be found among the internal characteristics of software quality.

Figure 20-1. Focusing on one external characteristic of software quality can affect other characteristics positively, adversely, or not at all

The most interesting aspect of this chart is that focusing on a specific characteristic doesn't always mean a tradeoff with another characteristic. Sometimes one hurts another, sometimes one helps another, and sometimes one neither hurts nor helps another. For example, correctness is the characteristic of functioning exactly to specification. Robustness is the ability to continue functioning even under unanticipated conditions. Focusing on correctness hurts robustness and vice versa. In contrast, focusing on adaptability helps robustness and vice versa.

The chart shows only typical relationships among the quality characteristics. On any given project, two characteristics might have a relationship that's different from their typical relationship. It's useful to think about your specific quality goals and whether each pair of goals is mutually beneficial or antagonistic.