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

Evolving Team Decision
As with many strategic design decisions, an entire team must move to a 
SEGREGATED CORE
together. This step requires a team decision process and a team disciplined and coordinated
enough to carry out the decision. The challenge is to constrain everyone to use the same definition
of the 
CORE
while not freezing that decision. Because the 
CORE DOMAIN
evolves just like every other
aspect of a design, experience working with a 
SEGREGATED CORE
will lead to new insights into what
is essential and what is a supporting element. Those insights should feed back into a refined
definition of the 
CORE DOMAIN
and of the 
SEGREGATED CORE MODULES
.
This means that new insights must be shared with the team on an ongoing basis, but an individual
(or programming pair) cannot act on those insights unilaterally. Whatever the process is for joint
decisions, whether consensus or team leader directive, it must be agile enough to make repeated
course corrections. Communication must be effective enough to keep everyone together in one
view of the 
CORE
.
Example
Segregating the C
ORE
 of a Cargo Shipping Model
We start with the model shown in Figure 15.2 as the basis of software for cargo shipping
coordination.
Figure 15.2.

---

Note that this is highly simplified compared to what would likely be needed for a real application. A
realistic model would be too cumbersome for an example. Therefore, although this example may
not be complicated enough to drive us to a 
SEGREGATED CORE
, take a leap of imagination to treat
this model as being too complex to interpret easily and deal with as a whole.
Now, what is the essence of the shipping model? Usually a good place to start looking is the
"bottom line." This might lead us to focus on pricing and invoices. But we really need to look at the
DOMAIN VISION STATEMENT
. Here is an excerpt from this one.
. . . Increase visibility of operations and provide tools to fulfill customer requirements faster
and more reliably...
This application is not being designed for the sales department. It is going to be used by the front-
line operators of the company. So let's relegate all money-related issues to (admittedly important)
supporting roles. Someone has already placed some of these items into a separate package
(
Billing
). We can keep that, and further recognize that it plays a supporting role.
The focus needs to be on the cargo handling: delivery of the cargo according to customer
requirements. Extracting the classes most directly involved in these activities produces a
SEGREGATED CORE
in a new package called