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Figure 1: A representation of the architecture
can lie. The Base Layer describes those elements that are perceived as independent from
any particular situation; to some extent, this layer represents those base elements that we
would see permanently in each frame, provided that we were able to see the whole fi lm at
one glance. To clarify the meaning of these aspects, we now review a fragment of the sample
dynamic model shown in Rumbaugh, Blaha, Premerlani, Eddy and Lorensen (1991), which
represents a programmable thermostat. In this example, the comparison between the actual
room temperature and the programmed (desired) temperature is identifi ed permanently
and independently of which of these temperatures is higher. Therefore, the
comparison
is
a base (status-independent) feature. At any given moment, one of the temperatures will be
higher, but this situation will vary as the system develops: the
status of the comparison
is
a purely dynamic feature.
The differences between the two layers that we have shown with respect to the model
of a system can also be identifi ed at higher levels of abstraction. A detailed discussion
about modeling and metamodeling levels goes beyond the scope of this chapter; for such
a discussion, see for instance Smolander, Lyytinen, Tahvanainen and Marttiin (1991) or
Hofstede and Verhoef (1997). Henceforth, we will use the terminology of a particular ap-
proach to metamodeling, that of the Four Layer Metamodeling Architecture (OMG, 2003,
pp. 2-5). In this Architecture, the Metamodel Level “defi nes the language for specifying a
model.” At this level, the differences between the base (status-independent) features and the
snapshot (status-dependent) features are revealed in the modeling artifacts that each specifi c
language, technique or method provides to represent one or other feature. For example, in
Rumbaugh et al. (1991), the Statecharts formalism is used to create a model of the ther-
mostat. Under our perspective, Statecharts concepts such as
state, transition, condition
and
variable
belong to the Base Layer, and so the 'standard' statechart (1) of Figure 2 would
become an instance belonging to this aspect (in particular, a condition is used to model the
comparison between the temperatures). On the other hand, concepts such as
active state,
compound transition, enabled compound transition, true condition,
etc., are related to the
Snapshot Layer. Therefore, diagrams (2), (3) and (4) of Figure 2, which represent several
consecutive situations of the thermostat by means of a widened notion of statechart, are
related to the Snapshot Layer (and thus, the value of the condition models the status of the
comparison). It must be noticed that the standard statechart notations do not offer graphical
representations for the concepts we have gathered in the Snapshot Layer, such as
active state
or
enabled transition.
In spite of this, several authors and tool developers have represented
in a visual manner some of these purely dynamic aspects of the behavior of a statechart, and
they have chosen graphical representations for such dynamic aspects. For instance, several
