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10.5. Model Extensions
10.5. Model Extensions
Model extensions are mainly needed in the case that an existing model 5 has to be extended or
adapted for a newer version of the ETCS SRS. Ideally, these extensions only have to be done
in the model but not in the meta model because any changes there might require additional
modifications for the code generators and the domain framework.
On the other hand, it seems doubtful that is possible to foresee any potential changes of the
SRS in the future for the design of the meta model or rather the openETCS formal specification
Language (in Figure 7.1).
Furthermore, even extensions on the model level should be traceable [78, p. 323][74] because
those are always related to the safety of the modelled system. One possibility is to trace
changes in the model by identifying the difference (of model elements) between two model /
SRS versions. Unfortunately, finding the difference in graphical models is not always trivial
because a modification of the graphical representation does not always mean a modification
of the abstract syntax model or the dynamic semantics. A loophole to this situation is a
transformation to a model representation that only holds the abstract syntax..
The GOPPRR meta model extension is such a model representation, which was introduced
in Chapter 4. Especially, the intermediate XML model in Section 4.3 can be directly used to
identify the syntactical differences between two models by using standard tools. Additionally,
a custom application could be developed that uses the GOPPRR C++ abstract syntax model
in Section 4.2.
In contrast to identifying model extensions between different model versions, it is also possible
to take model extensions directly into account for the design of the meta model. In other words,
the openETCS formal specification could offer a syntax to directly express extensions due to
new SRS versions in the graphical model. This would have the advantage of a more abstract
representation, compared to building the difference of XML files but also would increase the
complexity of the meta model and the corresponding model.
10.6. Conclusion
This chapter introduced the concrete model of the openETCS case study for a sub-subset of
the ETCS SRS. Representative examples of the model from all levels respectively graph types
were presented and described. The complete openETCS model can be found in Appendix C.
Furthermore, an important issue of model extensions for new ETCS SRS versions was
illuminated and a possible outlook on how the traceability of model extensions might be
increased in future work was given.
5 in terms of MDA the CIM
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