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

S
PECIFICATION
 Stack Content for "Cheap Container"
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AndSpecificationOperator (F
LY
W
EIGHT
)
NotSpecificationOperator (F
LY
W
EIGHT
)
Armored
NotSpecificationOperator
Ventilated
When you want to test a candidate, you have to interpret this structure, which can be done by
popping off each element, then evaluating it or popping off the next as required by an operator.
You would end up with this:
and(not(armored), not(ventilated))
This design has pros (
+
) and cons (
–
):
+
Low object count
+
Efficient use of memory
–
Requires more sophisticated developers
You have to find an implementation with trade-offs that work for your circumstances. The same
pattern and model can underlie very different implementations.
Subsumption
This final feature is not usually needed and can be difficult to implement, but every now and then
it solves a really hard problem. It also elucidates the meaning of a 
SPECIFICATION
.
Consider again the chemical warehouse packer from the example on page 235. Recall that each
Chemical
had a 
Container Specification,
and the 
Packer
SERVICE
guaranteed that all these
would be satisfied when 
Drums
are assigned to 
Containers
. All is well... until someone changes
the regulations.
Every few months a new set of rules is issued, and our users would like to be able to produce a list
of the chemical types that now have more stringent requirements.
Of course, we could give a partial answer (and one the users probably also want) by running a
validation of each 
Drum
in the inventory, with the new 
SPECIFICATIONS
in place, and finding all
those that no longer meet the 
SPEC
. This would tell the users which 
Drums 
in the existing
inventory they need to move.
But what they 
asked for
was a list of chemicals whose handling has become more stringent.
Perhaps there are none in-house right now, or perhaps they just happened to be packed into a
more stringent container. In either case, the report just described would not list them.
Let's introduce a new operation for directly comparing two 
SPECIFICATIONS
.
boolean subsumes(Specification other);
A more stringent 
SPEC
subsumes a less stringent one. It could take its place without any previous

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requirement being neglected.