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

Explanatory Models
The thrust of this book is that one model should underlie implementation, design, and team
communication. Having separate models for these separate purposes poses a hazard.
Models can also be valuable as education aids to teach about the domain. The model that drives
the design is one view of the domain, but it may aid learning to have other views, used only as
educational tools, to communicate general knowledge of the domain. For this purpose, people can
use pictures or words that convey other kinds of models unrelated to software design.
One particular reason that other models are needed is scope. The technical model that drives the
software development process must be strictly pared down to the necessary minimum to fulfill its
functions. An explanatory model can include aspects of the domain that provide context that
clarifies the more narrowly scoped model.
Explanatory models offer the freedom to create much more communicative styles tailored to a
particular topic. Visual metaphors used by the domain experts in a field often present clearer
explanations, educating developers and harmonizing experts. Explanatory models also present the
domain in a way that is simply different, and multiple, diverse explanations help people learn.
There is no need for explanatory models to be object models, and it is generally best if they are
not. It is actually helpful to avoid UML in these models, to avoid any false impression of
correspondence with the software design. Even though the explanatory model and the model that
drives design do often correspond, the similarities will seldom be exact. To avoid confusion,
everyone must be conscious of the distinction.
Example
Shipping Operations and Routes
Consider an application that tracks cargos for a shipping company. The model includes a detailed
view of how port operations and vessel voyages are assembled into an operational plan for a cargo
(a "route"). But to the uninitiated, a class diagram may not be very illuminating.