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

Cargo
is undetermined, but that it must accomplish the goal set out
in the 
Delivery Specification
.
3.
A 
role
distinguishes the different parts played by 
Customers
in a shipment. One is the "shipper,"
one the "receiver," one the "payer," and so on. Because only one 
Customer
can play a given role
for a particular 
Cargo
, the association becomes a qualified many-to-one instead of many-to-
many. 
Role
might be implemented as simply a string, or it could be a class if other behavior is
needed.
Carrier Movement
represents one particular trip by a particular 
Carrier
(such as a truck or a
ship) from one 
Location
to another. 
Cargoes
can ride from place to place by being loaded onto
Carriers
for the duration of one or more 
Carrier Movements
.
Delivery History
reflects what has actually happened to a 
Cargo
, as opposed to the 
Delivery
Specification
, which describes goals. A 
Delivery History
object can compute the current
Location
of the 
Cargo
by analyzing the last load or unload and the destination of the
corresponding 
Carrier Movement
. A successful delivery would end with a 
Delivery History
that
satisfied the goals of the 
Delivery Specification
.
All the concepts needed to work through the requirements just described are present in this
model, assuming appropriate mechanisms to persist the objects, find the relevant objects, and so
on. Such implementation issues are not dealt with in the model, but they must be in the design.
In order to frame up a solid implementation, this model still needs some clarification and
tightening.
Remember, ordinarily, model refinement, design, and implementation should go hand-in-hand in
an iterative development process. But in this chapter, for clarity of explanation, we are starting
with a relatively mature model, and changes will be motivated strictly by the need to connect that

---

model with a practical implementation, employing the building block patterns.
Ordinarily, as the model is being refined to support the design better, is should also be refined to
reflect new insight into the domain. But in this chapter, for clarity of explanation, changes will be
strictly motivated by the need to connect with a practical implementation, employing the building
block patterns.
[ Team LiB ]

---

[ Team LiB ]