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

Sales Management
System
mustn't use dimensions that the 
Allocation Checker
can't turn into 
Enterprise
Segments
. (Without applying the 
ENTERPRISE SEGMENT
pattern, the same constraint would force
the sales system to use only dimensions that can be used in a query to the 
Cargo Repository
.
This approach is feasible, but the sales system spills into other parts of the domain. In this design,
the 
Cargo Repository
need only be designed to handle 
Enterprise Segment
, and changes in
the sales system ripple only as far as the 
Allocation Checker
, which was conceived as a 
FACADE
in the first place.)
Performance Tuning
Although the 
Allocation Checker's
interface is the only part that concerns the rest of the domain
design, its internal implementation can present opportunities to solve performance problems, if
they arise. For example, if the 
Sales Management System
is running on another server,
perhaps at another location, the communications overhead could be significant, and there are two
message exchanges for each allocation check. There is no alternative to the second message,
which invokes the 
Sales Management System
to answer the basic question of whether a certain
cargo should be accepted. But the first message, which derives the 
Enterprise Segment
for a
cargo, is based on relatively static data and behavior compared to the allocation decisions
themselves. One design option would be to cache this information so that it could be relocated on
the server with the 
Allocation Checker
, reducing messaging overhead by half. There is a price
for this flexibility. The design is more complicated and the duplicated data must now be kept up to
date somehow. But when performance is critical in a distributed system, flexible deployment can
be an important design goal.
[ Team LiB ]

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[ Team LiB ]
A Final Look
That's it. This integration could have turned our simple, conceptually consistent design into a
tangled mess, but now, using an 
ANTICORRUPTION LAYER
, a 
SERVICE
, and some 
ENTERPRISE
SEGMENTS
, we have integrated the functionality of the