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172 Model Driven Systems Development with Rational Products
we can do the math simply enough and see when the power/size relationship
violates a constraint; but in a more complex set of constraints we might want to
set up the constraint network and allow a constraint solver to warn us when one
of the constraints is violated.
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We can see then, that parametrics in SysML provide us powerful capabilities for
reasoning about non-functional requirements and systems concerns that are not
available in UML, and provide us with semantics for modeling systems
engineering concerns.
Summary of SysML basics
In this chapter we have discussed some of the different capabilities that SysML
offers to system engineers and product designers. SysML is aimed at supporting
the conceptual stage of the life cycle of the product. This stage is preceded by
the decomposition of the customer needs into product features. We have seen
that SysML allows the representation of these features as requirements in the
model. In turn, these requirements can be allocated to the use cases, to the
sub-systems and components (whether functional or physical) identified for the
product.
The conceptual stage requires the specification of the various sub-systems
and the need for details depends on their level of integration. SysML provides
a set of constructs to support the description of the structure of the product.
Blocks are used to model sub-systems and components, and ports support
the description of their interfaces. Dependencies (for example, analytical)
between structural properties are expressed using constraints and
represented using the parametric diagram.
In addition to structure, the conceptual stage should clarify how the product
behavior is expressed through the interaction of its components. For example,
behavior modeling gives a detailed description of the product use cases.
SysML provides three means for explicating the product behavior, namely
interactions, state machine and activities. These three mechanisms are built
as a unified behavior concept and can consequently be orchestrated in a
single, uniform and complex behavior model for the whole product.
A complex product model is form by several sub-models of different nature
(for example, requirements, blocks, constraints, activities). SysML provides a
mechanism to relate different aspects of the model and to enforce traceability
across it.
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A simple example of how this can be done is provided by S.V. Hovater in Implementing a
domain-specific constraint in IBM Rational Systems Developer, IBM developerWorks,
http://www.ibm.com/developerworks/rational/education/dw-rt-rsdconstraint/