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

Cargo
with an empty
Delivery History
, and a null 
Delivery Specification
.
The two-way association between 
Cargo
and 
Delivery History means that neither Cargo
nor
Delivery History
is complete without pointing to its counterpart, so they must be created
together. Remember that 
Cargo
is the root of the 
AGGREGATE
that includes 
Delivery History
.
Therefore, we can allow 
Cargo's
constructor or 
FACTORY
to create a 
Delivery History
. The
Delivery History
constructor will take a 
Cargo
as an argument. The result would be something
like this:
public Cargo(String id) {
trackingID = id;
deliveryHistory = new DeliveryHistory(this);
customerRoles = new HashMap();
}
The result is a new 
Cargo
with a new 
Delivery History
that points back to the 
Cargo
. The
Delivery History
constructor is used exclusively by its 
AGGREGATE
root, namely 
Cargo
, so that
the composition of 
Cargo
is encapsulated.
Adding a Handling Event

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Each time the cargo is handled in the real world, some user will enter a 
Handling Event
using the
Incident Logging Application
.
Every class must have primitive constructors. Because the 
Handling Event
is an 
ENTITY
, all
attributes that define its identity must be passed to the constructor. As discussed previously, the
Handling Event
is uniquely identified by the combination of the ID of its 
Cargo
, the completion
time, and the event type. The only other attribute of