Specification of Reactive Hardware/Software Systems - Electronic ...

5.3 Design Philosophy 121
this. The idea is that architecture design is kept rather informal. This is necessary because
of the multiformity of architecture itself. We do not prescribe graphical notations, nor
do we offer special symbols (yet). Designers are free to choose what is appropriate for
the goal. In practical cases there may be several forms of architecture that play a role
simultaneously. There will also be several forms on different levels of abstraction. All
this information must gradually be mapped as constraints into the specification.
A formalisation process is performed via subsequent steps, called transformations (defined
in Section 5.9 and in Chapter 10). Objects that must be joined into modules
according to an architecture are placed in boundaries that correspond to architecture
modules. This is done by transforming Object Instance Models. Wherever possible,
clusters will be used to implement boundaries.
5.3 Design Philosophy
5.3.1 Conceptual Solutions
Design is often connected to creativity. The phenomenon creativity is not well understood
yet. Most useful ideas in design are based on existing ideas, or are variations or
combinations of them. Design ideas are usually formed by recognition of structure or
requirements seen before, or by discovering analogies. There are, for instance, many
technical systems that are designed in analogy to nature. To be able to do this, humans
use mental models. Good designers are able to create abstractions in their mind of
complex systems or problems and solutions. Solving a design problem on system level
is in most cases finding a conceptual solution and gradually verifying whether this can
really solve the problem. This cannot be done immediately because the problem to be
solved is only just understood after playing around with a model (or a prototype) for
some time.
Besides technical considerations there are also rather non-technical considerations that
play a role during design. Designers can give preference to architectures just because
they are more appealing in the sense of elegance or beauty. Aesthetic properties have
to do with taste rather that technical arguments. They are related to acceptance, recognisability
and comprehensibility, which is very important for a specification. These
psychological phenomena may lead to a choice for conceptual solutions on rather arbitrary
grounds. Most projects start with the selection of a conceptual solution. This is
also related to a so-called feasibility study. Contractors or principles often require or
firmly suggest a conceptual solution before a project is approved. Such decisions must
be taken very often on quite intuitive grounds. In general design is not a pure process,
where the most economical solution is selected from the universe of possible solutions.
A conceptual solution has usually an implicit specific structure that determines an
architecture of a (sub)system. So architecture often comes naturally with a chosen
concept. Without perseverance of the implicit structure the chosen conceptual solution
will not be recognised anymore in a specification model. This is already reason enough