2 Why We Need Model-Based Testing

16 Reactive Systems
This chapter discusses testing of reactive systems. Reactive systems take inputs as
well as provide outputs in the form of spontaneous reactions. The behavior of a reactive
system, especially when distributed or multithreaded, can be nondeterministic.
Therefore, the main difference compared to testing of closed systems is that some
of the outputs of the implementation under test (IUT) may be outside the control
of the tester; that is, the tester cannot predict what the exact output will be during
testing. There may be multiple possible outputs and the subsequent tester inputs
may depend on those outputs.
Many software systems are reactive, for example many application-level network
protocols and multithreaded APIs are reactive. Such systems may produce spontaneous
outputs like asynchronous events. Factors such as thread scheduling are not
entirely under the control of the tester but may affect the observable behavior. In
these cases, a test suite generated offline may be infeasible, since all of the observable
behaviors would have to be encoded a priori as a decision tree, and the size of such a
decision tree can be very large.
The main topic of this chapter is on-the-fly testing of reactive systems. The
presentation of most of the material builds on on-the-fly testing of closed systems that
was the topic of Chapter 12. In reactive systems, the action vocabulary is divided into
controllable and observable action symbols. This terminology is tailored to match
the point of view of the tester. Actions whose symbol is controllable are under the
control of the tester and actions whose symbol is observable are under the control
of the IUT.
16.1 Observable actions
The action vocabulary of every model program can be split into two sets of action
symbols, controllable ones and observable ones. Whether an action symbol is controllable
or observable often depends on what part of the system is being tested and
more free ebooks download links at:
http://www.ebook-x.com
259