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150 Model Driven Systems Development with Rational Products
Traceability relationships can be deduced from the model using the structures
created in operations analysis. Requirements information can also be deduced
from joint realization tables.
Block semantics
As noted above, SysML provides a basic structural element called a Block,
whose aim is to provide a discipline-agnostic building block for systems. Blocks
can be used to represent any type of component of the system, for example,
functional, physical, and human. Blocks assemble to form architectures that
represent how different elements in the system co-exist.
Block definition diagram
The SysML Block Definition Diagram (BDD) is the simplest way to describe the
structure of the system. It is the equivalent to the class diagram in UML. It is used
to represent the system decomposition using for example associations and
composition relationships. The BDD is ideal to display the features of a block,
such as its properties, and operations. SysML allows blocks to own special types
of properties: Block properties and distributed properties.
Block properties impose additional constraints on classic UML properties, and
can for instance own a SysML
value type. Value types are designed to hold
units (for example, physical units) and dimensions.
Distributed properties let the user apply a probability distribution to the values
of the property. SysML proposes model libraries for possible values of units,
dimensions, and probability distributions.
In Figure 7-3 we show a BDD for the RSW. For the sake of readability of the
diagram, we do not render the associations between the sub-systems and the
Rain Sensing Wiper element, although these associations exist in the model.
Instead we use an illustrative box around each set of components (composite
and external) and a black diamond shape over the composite component as a
visual clue for composition. The main components of the RSW are an interface to
actuate the wiper, an electronic control unit, a sensor and the windshield
element. Both the interface and the windshield can exist in the car with or without
the RSW (In SysML they are so-called
reference properties).
The properties and the operations for each block are visible in Figure 7-3.
Properties (more precisely SysML block properties, shown using the
stereotype <<blockProperty>>) are used to model the physical characteristics
of the components. The operations (called sometimes services) represent the
functional aspects of the system.