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

Chapter Fifteen. Distillation
These four equations, along with the definitions of their terms and the body of mathematics
they rest on, express the entirety of classical nineteenth-century electromagnetism.
—
James Clerk Maxwell,
A Treatise on Electricity and Magnetism
, 1873
How do you focus on your central problem and keep from drowning in a sea of side issues? A
LAYERED ARCHITECTURE
separates domain concepts from the technical logic that makes a computer
system run, but in a large system, even the isolated domain may be unmanageably complex.
Distillation
is the process of separating the components of a mixture to extract the essence in a
form that makes it more valuable and useful. A model is a distillation of knowledge. With every
refactoring to deeper insight, we abstract some crucial aspect of domain knowledge and priorities.
Now, stepping back for a strategic view, this chapter looks at ways to distinguish broad swaths of
the model and distill the domain model as a whole.
As with many chemical distillations, the separated by-products are themselves made more
valuable by the distillation process (as 
GENERIC SUBDOMAINS
and 
COHERENT MECHANISMS
), but the
effort is motivated by the desire to extract that one particularly valuable part, the part that
distinguishes our software and makes it worth building: the "
CORE DOMAIN
."
Strategic distillation of a domain model does all of the following:
Aids all team members in grasping the overall design of the system and how it fits together
1.
Facilitates communication by identifying a core model of manageable size to enter the
UBIQUITOUS LANGUAGE
2.
Guides refactoring
3.
Focuses work on areas of the model with the most value
4.
Guides outsourcing, use of off-the-shelf components, and decisions about assignments
5.
This chapter lays out a systematic approach to strategic distillation of the 
CORE DOMAIN
, and it
explains how to effectively share a view of it within the team and provides the language to talk
about what we are doing.

---