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Figure 4: Petri Nets diagrams for thermostat furnace relay
corresponds to the Base Layer of the architecture. Therefore, a statecharts drawing tool can
concentrate its efforts on this layer. However, a CASE shell developer interested in provid-
ing the tool with some simulation or animation features must be aware that the simulated
execution of a statechart model involves the combined application of the concepts related to
the
T
0
T
0
T
map, the Snapshot Layer and the
T
map, applied to the particular case of the statechart
T
formalism. Some up-to-date, commercial CASE tools supporting UML State Machines are
based on similar approaches since they have been provided with some animation features
(for example, the above-mentioned Rhapsody tool provides animated views of the modeled
application, and in particular, allows the observation of the behavior of a state machine using
a color scheme to differentiate between, for instance, active or inactive states). However,
it must be stressed that this kind of approach is one part of particular tools, and does not
belong to the standard State Machines defi nition. In any case, a system analyst or designer
would benefi t from the use of a CASE tool that follows the architecture, since such a tool
would provide, as a sub-product, a pattern to the modeling of behavior. In particular, a
software engineer should not be burdened with a detailed specifi cation of the maps of the
architecture, but he/she must be aware of their availability in the tool.
Furthermore, it is necessary to stress that the issues we are analyzing are not exclusive
to State Machines, although this language is our main example. For instance, with regard to
Petri Nets (Peterson, 1981), within our architecture, a non-marked Petri Net belongs to the
base view, whereas a Petri Net together with a particular marking belongs to the snapshot
view. The process of fi ring transitions and re-marking of places is equivalent to the change
of status in State Machines. Figure 4 illustrates this situation by means of a petri net model
of the thermostat, representing the base layer (1), two consecutive statuses of the snapshot
layer (2 and 4) and an intermediate situation (3). It is worth noting that we are not stating
here that the statechart model in Figure 2 and the petri net model in Figure 4 are (or are
not) equivalent (see Palanque et al., 1993, for a related discussion). What this example
shows is that the differences between the base and the snapshot aspects can be analyzed in
a language-independent way, and it is therefore of great value to be provided with a frame-
work that allows the analysis of the behavior of models of techniques in a general way. In
particular, as we have explained in the 'Related Work' section, the proposed architecture can
be a valuable artifact for method engineers in order to design, construct or adapt a method
to be used within a particular project.
