Information Technology Reference
In-Depth Information
action that changes the user identity, this will imply use of a multi-user binder.
And there will have to be such an action, because the detail of the sign-on
operation will involve it."
\We are asking a lot of our tools, aren't we?" said Claire, tilting her head.
The same sun picked out the highlights in her blond hair. \But we should!"
Alex replied. \The tools are much better than the programmers are at spotting
simple logical implications from different parts of the design, and for doing
boring but crucial consistency checks as the design develops." \And if the
logic isn't simple, or the side eects not obvious?" She was teasing him, and
he knew it. \Then heaven preserve us from clever programmers. If the system
depends on a line of code so obscure you want to put it on a T-shirt, I'm
betting the system will be a pig to maintain."
7.1
The Need for Correspondences
So far, we have concentrated on the set of five viewpoint languages. It is
all too easy to leave it at that, but we need to relate the different viewpoints
so that they are woven together to give a unified system specification. This
chapter aims to redress the balance by concentrating on the correspondences
between elements in different viewpoints.
If the viewpoints are seen as representing the areas of concern of the differ-
ent stakeholders, then the correspondences represent the agreements between
these stakeholders about how their views of the world overlap and what enti-
ties are to be found in multiple views.
Each correspondence in ODP links a pair of viewpoints, and so has two
endpoints, one in each viewpoint. All the correspondences between a par-
ticular pair of viewpoints together form what is called a correspondence
specification . It would, in principle, be possible to define multi-way corre-
spondences, but these are more dicult to understand and manage, so a mesh
of binary relationships is used for simplicity. As a result, with five viewpoints,
there could be at most ten correspondence specifications, but there is not nec-
essarily something significant to say in every possible case, so in practice there
are less than ten. However, there cannot be less than four sets of correspon-
dences in total, or the overall specification will not be completely connected
and will fall apart.
The idea of a correspondence in ODP is deliberately kept quite general
because the various viewpoints may be specified using different languages,
satisfying their individual needs. A correspondence is therefore a general link
between specification elements with an associated rule or constraint indicating
the way the elements in the two different viewpoints are related. The linkage
of the two specification elements can be made independent of the languages
involved, although the expression of the additional rules to be applied generally
