2 Why We Need Model-Based Testing

Systems with Finite Models 59
Figure 5.1. Newsreader Topics page in WithText mode.
sometimes, an infinite number. A scenario is a collection of runs that are pertinent to
some particular situation or purpose, such as a test suite. A scenario model program
defines a scenario; it can execute all of the runs of the scenario, but no others. A
scenario model program often has a small collection of runs; sometimes, just one.
Usually a model program only models a subset or slice of the implementation’s
features. Usually a model program represents the implementation at a level of
abstraction where many details are omitted or simplified.
5.2 Case study: user interface
We make our first model program very simple, so we can explain the process of
writing it in great detail, showing the reason for every step. We will pick up the pace
in the examples to come.
For our first model program we model the user interface for a Web-based newsreader,
where users select a “group” (news about a particular subject), and can then
read and post messages about topics in that group.
When the users first select a group, they see a list of message excerpts, each with
a title line indicating the topic (Figure 5.1). By clicking on the View titles only link,
they can select an alternate page that just shows a list of topics (so more topics fit
on the screen, Figure 5.2). On first viewing, the topics are sorted by the date of the
most recent message about that topic. Users can click on the link Sort by date of
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