2 Why We Need Model-Based Testing

96 Exploring and Analyzing Finite Model Programs
Topics
WithText
ByMostRecent
ShowTitles()
ShowText()
Topics
TitlesOnly
ByMostRecent
Figure 6.1. Newsreader scenario FSM: state transition diagram.
SortByFirst()
SortByMostRecent()
Topics
TitlesOnly
ByFirst
Figure 6.1 shows a state transition diagram for the same run. This is another way
to represent the same FSM that is shown in Table 6.1. The diagram is a picture of a
data structure called a graph, where bubbles called nodes (or vertices) are connected
by arrows called links (or arcs or edges). In a state transition diagram, the nodes
represent states. The links represent state transitions. Each is labeled by its action (a
method invocation). The diagram shows a directed graph because each link points
in one direction, from the current state to the next state. We can reconstruct the run
from the diagram by following the links. In our diagrams the initial state is usually
distinguished by shading.
It is important to understand that the effects of an action only depend on the
current state. To compute (or predict) the effects (the next state), it is not necessary
to keep track of the history leading up to the current state.
An FSM represents behavior compactly because each transition only occurs once
in the FSM, no matter how many times it occurs in the run. In a state transition
diagram, each state also appears only once. If the run reaches the same state more
than once, the diagram contains loops called cycles. In this example there are cycles
through all of the states.
6.1.1 Runs and scenarios
We generated our FSM (Table 6.1, Figure 6.1) from a run of our model program.
It is also possible to generate runs from an FSM. This is how we do offline test
generation.
Each FSM describes every run that can be obtained by traversing a path around
the nodes and links in its graph. Here is an informal description of the algorithm for
generating one run: Begin in the initial state. In a state that has transitions to next
states, choose any one. For example, the middle node in Figure 6.1 has two outgoing
transitions, so two choices are available. If we reach a state with no outgoing transitions,
the run finishes in that state. Or, if the state has been designated an accepting
state, we may choose to finish the run in that state. In this example, we have have
not designated any accepting states, so every state is considered an accepting state;
the run may finish in any state.
We often describe this process in terms of actions, rather than transitions. An
action is enabled in a state if it is allowed to begin in that state; otherwise, it is
more free ebooks download links at:
http://www.ebook-x.com