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

Handling Events
without contention.
This pushes us to consider a different
design.
Replacing the 
Delivery History's
collection of 
Handling Events 
with a query would allow
Handling Events
to be added without raising any integrity issues outside its own 
AGGREGATE
. This
change would enable such transactions to complete without interference. If there are a lot of
Handling Events
being entered and relatively few queries, this design is more efficient. In fact, if
a relational database is the underlying technology, a query was probably being used under the
covers anyway to emulate the collection. Using a query rather than a collection would also reduce
the difficulty of maintaining consistency in the cyclical reference between 
Cargo
and 
Handling
Event
.
To take responsibility for the queries, we'll add a 
REPOSITORY
for 
Handling Events
. The 
Handling
Event Repository
will support a query for the 
Events
related to a certain 
Cargo
. In addition, the
REPOSITORY
can provide queries optimized to answer specific questions efficiently. For example, if a
frequent access path is the 
Delivery History
finding the last reported load or unload, in order to
infer the current status of the 
Cargo
, a query could be devised to return just that relevant
Handling Event
. And if we wanted a query to find all 
Cargoes 
loaded on a particular 
Carrier
Movement
, we could easily add it.
Figure 7.6. Implementing Delivery History's collection of Handling
Events as a query makes insertion of Handling Events simple and free