UML Weekend Crash Course™ - To Parent Directory

Session 2—UML and Development Methodologies 17
The progression from step to step follows a fairly strict set of rules to guide the translation
from each version of the diagram to the next. The process sets up a rhythm of build a
little and test it, build a little more and test a little more, which helps prevent surprise
problems from cropping up deep into the process.
The Shlaer-Mellor Method also places a great emphasis on iterative and incremental
development. But this methodology reduces and controls iteration in analysis by confining
it to a single domain at a time. Iteration in design is similarly controlled: Modifications to
the design are made entirely in the architectural domain and propagated to the entire
system through the standardized diagrams.
Reuse is yet another high priority. Because domains are kept completely separate from
one another until the final construction steps, they can be transported intact to other systems.
This applies particularly to the architectural domain: This domain is commonly reused
for other systems that have basically the same loading and performance characteristics.
Strengths of Shlaer-Mellor
By far the greatest strength of the Shlaer-Mellor Method is the ability to test your
diagrams through simulation. You actually execute your diagrams.
The process is extremely well defined in terms of rules that govern the construction
and testing of the diagrams.
The movement from one step in the process to the next (for example, from analysislevel
diagrams to design-level diagrams) is also defined with enough precision to
allow the generation of design diagrams directly from the analysis diagrams. This is
a huge time saver and prevents mistakes in the translation. It also speeds up the
process of applying changes because they can be propagated through the diagrams
automatically.
The method was developed for and maintains a strong emphasis on real-time systems
design. As such, it provides support that is largely lacking in other methodologies
that gloss over the unique demands of real time in favor of the more common
business functionality.
Weaknesses of Shlaer-Mellor
The strengths of the methodology can also be its weaknesses. Like the RUP, the tool
support is limited to vendors directly associated with the methodologists. This is
changing, but don’t expect it to be quick.
Learning the rules involves a definite learning curve and a serious investment of
time and effort. Steve Mellor is currently leading an enhancement to the UML,
called Action Semantics, to improve the definition of Statechart diagrams and build
much of this knowledge into the UML 1.5 standard. Tool support for this enhancement
should soon follow.
The methodology was developed for real-time systems, so it places heavy emphasis
on state modeling. Many business applications simply do not warrant a lot of work
on state transitions.