Information Technology Reference
In-Depth Information
does require some knowledge of the languages being used. We shall return to
this point later.
The simplest kind of correspondence is to say that two elements refer to
the same thing. Thus, for example, we can state that the enterprise object
filling the role Branch staff corresponds to the computational object Repair-
CentreStaff, meaning that they are two representations in different viewpoints
for one and the same individual in the real world. Of course, in our simple
example, we could have aligned these names, but, in large-scale specifications
drawing on different existing libraries, this may be much more dicult.
7.2 Different Kinds of Correspondence
Most modern modelling languages have some concept of an object as a
representation of the entities that are being described, and an object has at-
tributes representing the properties of the real-world entities and behaviour
indicating how it interacts with its environment and evolves over time. While
the statement at the end of the previous section that a correspondence shows
that two descriptions are referring to the same entity may seem straightfor-
ward, it does not follow that the way the description is expressed is the same
in each case. The properties expressed may not be exactly the same; indeed,
they may be completely different.
So, even in the simple case of declaring that two objects represent the same
entity, we also need to be clear whether there is a correspondence between
their types as a whole or between individual attributes, and if so whether the
attribute types correspond. Even if two specifications are dealing with the
same entity, they may be describing quite different properties of it.
In fact, the types used in the different viewpoints will generally be different
because they will have been chosen to express different concerns, and so rep-
resent different abstractions of the same entity. For example, the enterprise
description of a phone may express its value, service bundle and ownership,
while an information description of the same phone may express its memory
capacity and SIM number.
Often, the two viewpoint specifications deal with similar properties but at
different levels of detail, so that an attribute in one description corresponds
loosely to a different attribute in the other, which may have a type that is a
subtype of the one used in the first description (because it needs to provide
more detailed information).
The correspondence can also add information to the specification by stat-
ing a correlation between attribute values. For example, we could make a
correspondence between phones with the premium service bundle and those
whose memory size is bigger than two megabytes. This now means that all
premium service users have phones with enough capacity, which is a fact that
