2 Why We Need Model-Based Testing

Systems with Finite Models 61
topics. When they do, the topic list appears the same as when last seen; any previous
selections regarding message excerpts and sort order are still in effect.
5.3 Preliminary analysis
Writing a model program always begins with a preliminary analysis where we
decide on the purpose of the model, choose the features to include in the model,
select the level of abstraction, identify the actions, write some sample traces, and
identify the state variables.
During preliminary analysis we study the implementation. Depending on what is
available, we might study the implementation’s requirements and design documents,
or inspect its API or its code. In this example, we just observe its behavior.
5.3.1 Decide on the purpose
Modeling is a goal-directed activity; every model program has a purpose. We write
a model program in order to answer some questions about the implementation (by
testing or analysis). We have to formulate those questions before we can see how to
write the model program.
In this example, our questions arise from the observation that the newsreader
presents several kinds of pages that can be formatted in various styles, and several
options for selecting pages and styles. Our questions are: What options are available
when a particular page and style is displayed? What page and style appears when
the user selects a particular option?
Our questions guide testing and analysis. To create a test suite, we express the
questions this way: Are the intended options made available for each particular page
and style? Does the intended page and style appear when each particular option is
chosen? To perform liveness analysis, we ask: Is every option available (on some
page)? Can every page and style be made to appear (by choosing some sequence of
options)? Are there are any deadlocks (pages with no options) or livelocks (pages
from which other pages cannot be reached)? To perform safety analysis, we ask:
Are the options that are available on each page the ones that should be there? Is it
possible to request some combination of page and style that doesn’t make sense?
5.3.2 Select the features
We usually do not model a large system with a single model program. Instead, we
select a subset of related features that comprise a self-contained package of functionality.
A large implementation can be covered by several small model programs
that represent different subsets of features. Choose a feature subset that makes a
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