You may be asking 'what is testing?' and we'll look more closely at the defi-
nition of testing in Section 1.2. For the moment, let's adopt a simple everyday-
life usage: 'when we are testing something we are checking whether it is OK'.
We'll need to refine that when we define software testing later on. Let's start by
considering why testing is needed. Testing is necessary because we all make mis-
takes. Some of those mistakes are unimportant, but some of them are expensive
or dangerous. We need to check everything and anything we produce because
things can always go wrong - humans make mistakes all the time - it is what we
do best!
Because we should assume our work contains mistakes, we all need to check
our own work. However, some mistakes come from bad assumptions and blind
spots, so we might make the same mistakes when we check our own work as we
made when we did it. So we may not notice the flaws in what we have done.
Ideally, we should get someone else to check our work - another person is more
likely to spot the flaws.
In this book, we'll explore the implications of these two simple paragraphs
again and again. Does it matter if there are mistakes in what we do? Does it
matter if we don't find some of those flaws? We know that in ordinary life, some
of our mistakes do not matter, and some are very important. It is the same with
software systems. We need to know whether a particular error is likely to cause
problems. To help us think about this, we need to consider the context within
which we use different software systems.
1.1.2 Software systems context
Testing Principle - Testing is context dependent
Testing is done differently in different contexts. For example, safety-critical software is
tested differently from an e-commerce site.
These days, almost everyone is aware of software systems. We encounter them
in our homes, at work, while shopping, and because of mass-communication
systems. More and more, they are part of our lives. We use software in day-to-
day business applications such as banking and in consumer products such as
cars and washing machines. However, most people have had an experience with
software that did not work as expected: an error on a bill, a delay when waiting
for a credit card to process and a website that did not load correctly are
common examples of problems that may happen because of software problems.
Not all software systems carry the same level of risk and not all problems
have the same impact when they occur. A risk is something that has not hap-
pened yet and it may never happen; it is a potential problem. We are concerned
about these potential problems because, if one of them did happen, we'd feel a
negative impact. When we discuss risks, we need to consider how likely it is that
the problem would occur and the impact if it happens. For example, whenever
we cross the road, there is some risk that we'll be injured by a car. The likeli-
hood depends on factors such as how much traffic is on the road, whether there
is a safe crossing place, how well we can see, and how fast we can cross. The
impact depends on how fast the car is going, whether we are wearing protective
gear, our age and our health. The risk for a particular person can be worked out
and therefore the best road-crossing strategy.
