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82 Model Driven Systems Development with Rational Products
Localities and nodes
The UML documentation states that UML nodes are classifiers that have
processing ability and memory. Used in deployment diagrams, the UML node
semantics support reasoning about the hosting processors for the software
components. The implicit assumption is that the physical resources are outside
the software under consideration. For example, in software engineering, the
hardware is often seen as an enabling layer below the operating system. UML
does provide design and implementation-level artifacts for deployment diagrams:
Descriptor diagrams: For the design level
Instance diagrams: For the implementation level
In particular, instance deployment diagrams are meant to capture configurations
and actual choices of hardware and software, and to provide a basis for system
analysis and design, serving as an implementation level in the distribution
viewpoint.
The UML reference manual describes an instance version of a deployment
diagram as a diagram that shows the configuration of run-time processing nodes
and component instances and objects that live in them.
In MDSD, this intent is to model the places where services are performed, that is,
where the functionality described in the logical models happens. Modeling
localities allows for reasoning about the distribution of functionality. Localities
express a set of constraints on the realization of the functionality performed by
hardware, software and people. Using localities, engineers can model what
functionality can (and cannot) be grouped together.
Localities, services, and interfaces
A locality specifies places where logical services are provided. In practice, each
locality will provide a subset of the services of one or more of the logical
subsystems. The determination of those services is an outcome of the joint
realization.
The set of hosted subsystem services for a given locality should be captured with
UML or SysML interfaces. Subsystems are classifiers, and their services are
classifier operations. Both UML and SysML allow operations, and therefore
subsystem services, to be organized into interfaces. That is, an interface is a
subset of subsystem services. In this approach, we define the needed interfaces
for each of the subsystems and then assign them to the appropriate localities.
Generally, there will be more than one interface associated to a locality
2
.
2
See further discussion and illustration (Figure 5-1)