Sommerville-Software-Engineering-10ed

Chapter 11 ■ Key points 335
Number of inputs
Figure 11.14
Distribution of inputs in
an operational profile
Input classes
...
different impressions of reliability because they use a system in different ways.
It is difficult to match all of these patterns of use in a single operational profile.
2. Users change the ways that they use a system over time. As users learn about
a new system and become more confident with it, they start to use it in more
sophisticated ways. Therefore, an operational profile that matches the initial
usage pattern of a system may not be valid after users become familiar with
the system.
For these reasons, it is often impossible to develop a trustworthy operational profile.
If you use an out-of-date or incorrect operational profile, you cannot be confident
about the accuracy of any reliability measurements that you make.
Key Points
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Software reliability can be achieved by avoiding the introduction of faults, by detecting and
removing faults before system deployment, and by including fault-tolerance facilities that allow
the system to remain operational after a fault has caused a system failure.
Reliability requirements can be defined quantitatively in the system requirements specification.
Reliability metrics include probability of failure on demand (POFOD), rate of occurrence of failure
(ROCOF), and availability (AVAIL).
Functional reliability requirements are requirements for system functionality, such as checking
and redundancy requirements, which help the system meet its non-functional reliability
requirements.