12.1 Introduction

12.1.1 Overview of Critical Systems

Let's start by explaining what we mean by "critical" systems. A simple definition: systems in which failure(s) may have significant and far-reaching consequences. This is, of course, a very broad view of things, so something more precise is needed. To put things into perspective, consider the following representative real-world examples (Figure 12.1). The first four systems can be categorized as "safety-critical"; the others are "mission-critical." The essential difference is self-evident:

Figure 12.1: Typical systems – failures and their consequences

Assume that your computer forms part of a critical system. What should (or can) you do to put operational quality into your product? There is no simple answer; it all depends on how systems must operate when faults occur. Very broadly, we can categorize system requirements as shown in Figure 12.2. First, we have...