Domain-Driven Design: Tackling Complexity in the Heart of Software

Figure 16.7. Refactoring the model to conform to the new layering
structure

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This factoring makes the 
Route Bias Policy
more explicit while making 
Transport Leg
more
focused on the fundamental concept of transportation capability. A large-scale structure based on
a deep understanding of the domain will often push the model in directions that clarify its meaning.
This new model now smoothly fits into the large-scale structure.
Figure 16.8. The restructured and refactored model
A developer accustomed to the chosen layers can more readily discern the roles and dependencies
of the parts. The value of the large-scale structure increases as the complexity grows.
Note that although I'm illustrating this example with a modified UML diagram, the drawing is just a
way of 
communicating
the layering. UML doesn't include this notation, so this is additional
information imposed for the sake of the reader. If code is the ultimate design document for your
project, it would be helpful to have a tool for browsing classes by layer or at least for reporting
them by layer.
How Does This Structure Affect Ongoing Design?

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Once a large-scale structure has been adopted, subsequent modeling and design decisions must
take it into account. To illustrate, suppose that we must add a new feature to this already layered
design. The domain experts have just told us that routing restrictions apply for certain categories
of hazardous materials. Certain materials may not be allowed on some transports or in some
ports. We have to make the 
Router
obey these regulations.
There are many possible approaches. In the absence of a large-scale structure, one appealing
design would be to give the responsibility of incorporating these routing rules to the object that
owns the