Sommerville-Software-Engineering-10ed

310 Chapter 11 ■ Reliability engineering
Input set
I e
Inputs causing
erroneous outputs
Program
Figure 11.2 A system
as an input/output
mapping
Output set
O e
Erroneous
outputs
System reliability is not an absolute value—it depends on where and how that
system is used. For example, let’s say that you measure the reliability of an application
in an office environment where most users are uninterested in the operation of
the software. They follow the instructions for its use and do not try to experiment
with the system. If you then measure the reliability of the same system in a university
environment, then the reliability may be quite different. Here, students may explore
the boundaries of the system and use it in unexpected ways. This may result in system
failures that did not occur in the more constrained office environment. Therefore, the
perceptions of the system’s reliability in each of these environments are different.
The above definition of reliability is based on the idea of failure-free operation,
where failures are external events that affect the users of a system. But what constitutes
“failure”? A technical definition of failure is behavior that does not conform to
the system’s specification. However, there are two problems with this definition:
1. Software specifications are often incomplete or incorrect, and it is left to software
engineers to interpret how the system should behave. As they are not
domain experts, they may not implement the behavior that users expect. The
software may behave as specified, but, for users, it is still failing.
2. No one except system developers reads software specification documents. Users
may therefore anticipate that the software should behave in one way when the
specification says something completely different.
Failure is therefore not something that can be objectively defined. Rather, it is a
judgment made by users of a system. This is one reason why users do not all have the
same impression of a system’s reliability.
To understand why reliability is different in different environments, we need to think
about a system as an input/output mapping. Figure 11.2 shows a software system that