the information was available inside the model, it was
difficult and time-consuming to comprehend, verify,
and maintain the resulting system. As a result, even
subject matter experts had major problems in under-
standing the communication relationships they should
be already familiar with. Furthermore it was an es-
sential requirement, that the resulting model diagrams
could also be well understood by those who are not fa-
miliar with MBSE and system architectures. Unfortu-
nately, for reasons of confidentiality, the correspond-
ing system model and its contents cannot be made
available to the readership within this paper.
4 PROBLEM STATEMENT
It can be observed that the technological barriers are
not posing the greatest problem in here, but rather
those of human cognitive perception. Mapping the
structure and interrelationships of complex systems
as a digital model is a major challenge. To be able
to comprehend, understand and maintain what is rep-
resented beyond that, is an even greater task. Of
course, UML and SysML offer a variety of methods
to decompose the system to reach lower complexity.
On the other hand, even using the best decomposi-
tion, you eventually reach the point where you need
to represent the relations between the system com-
ponents in the model. Normally this is done using
elements and connectors, but even with just a few el-
ements, this may mean a large number of information
flows on a single diagram. Yet, it is the understand-
ing of this connector-representation between system
elements, that is a major cognitive challenge for the
human brain (Koning, 2002).
In the study User preference of graph layout aes-
thetics: a UML study (Purchase, 2000) on the bet-
ter comprehensibility of UML diagrams, 93% of the
participants stated that crossing connectors in the di-
agram are an enormous obstacle to comprehensibil-
ity. 91% testified that bends in the connectors were
a major barrier. These were the two most frequently
mentioned comprehension barriers in the study.
In order to solve the problems mentioned above
and to counteract the challenge of increasing com-
plexity in system models, we will elaborate our ap-
proach named InTra (Interaction-based Transforma-
tion), which significantly reduces the number of con-
nectors in the model, but at the same time preserves
their information content in a comprehensible way.
5 InTra APPROACH
The approach introduced in this article is designed to
significantly reduce the complexity of a system model
with the help of individual interaction rules that work
on the basic system structure. In this way it is pos-
sible to reduce the number of connectors even during
the initial modelling phase, thus keeping the resulting
model complexity small, which in turn improves the
readability and maintainability. The term ’rule-based’
describes the idea of recognizing repeating interaction
patterns in the basic structure of the model, and the
definition of rules for those patterns, which describe
the interaction concept behind those relationships. By
including these rules, it is no longer necessary to store
the said relations by connectors in the model. The
creation of a rule is useful, if an interaction between
certain element types or constellations always runs in
the same way. Although in this rule-based form it is
a lot harder to evaluate the model with simulations or
programs, it is fully completed and understandable in
its rule-based abstracted form. This means, that all
information of the relations to be captured is already
mapped in the model, partly abstracted as interaction
rules to be interpreted. By expressing this information
as rules, the representation of the model is changed,
but its information content remains the same. Avoid-
ing the redundancy of information in this way also
reduces the risk of errors in the further processing of
the model, since changes only need to be made to one
rule instead of all the connectors that a single rule rep-
resents. Furthermore, the usage of interaction rules
makes the model easier to understand for domain ex-
perts. This is the case because domain-specific inter-
actions can be represented very efficiently and com-
prehensibly as interaction rules. Nevertheless, an ex-
perienced system architect should be responsible for
identifying the basic structure of the system, recog-
nizing repetitive patterns in the relationships between
model elements, defining and configuring interaction
rules for those patterns, and finally linking them to the
relevant parts of the system’s basic structure.
In an optional second phase of model transforma-
tion, the defined interaction rules are able to trans-
form the model into a rule-independent version, suit-
able for data usage through e.g. simulations, by us-
ing the automatic model transformation algorithm of
InTra. Therefore linked interaction rules are parsed
by the algorithm, to define active filters for the pattern
matching of the target model. Pattern matching will
then find valid interaction paths inside the model and
apply the defined rewriting effects between all start
and endpoints of those found paths. Even though the
rule-based version of the model is fully expressive, we
InTra: A Pragmatic Approach of Using Rule-Based Model Transformation to Reduce Complexity of UML and SysML Models
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