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

Figure 16.13. Conceptual dependencies and shearing points in a factory
automation system
Many businesses do not base their capability on plant and equipment. In financial services or
insurance, to name two, the potential is to a large extent determined by current operations. An
insurance company's ability to take on a new risk by underwriting a new policy agreement is based
on the diversification of its current business. The Potential layer would probably merge into
Operations, and a different layering would evolve.
One area that often comes to the fore in these situations is commitments made to customers.
Commitment
. What have we promised? This layer has the nature of Policy, in that it states
goals that direct future operations, but it has the nature of Operations in that commitments
emerge and change as a part of ongoing business activity.
Figure 16.14. Conceptual dependencies and shearing points in an
investment banking system

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The Potential and Commitment layers are not mutually exclusive. A domain in which both are
prominent, say a transportation company with a lot of custom shipping services, might use both.
Other layers more specific to those domains might be useful too. Change things. Experiment. But
it is best to keep the layering system simple; going beyond four or possibly five becomes unwieldy.
Having too many layers isn't as effective at telling the story, and the problems of complexity the
large-scale structure was meant to solve will come back in a new form. The large-scale structure
must be ferociously distilled.
Although these five layers are applicable to a range of enterprise systems, they do not capture the
salient responsibilities of all domains. In other cases, it would be counterproductive to try to force
the design into this shape, but there may be a natural set of 
RESPONSIBILITY LAYERS
that do work.
For a domain completely unrelated to those we've discussed, these layers might have to be
completely original. Ultimately, you have to use your intuition, start somewhere, and let the 
ORDER
EVOLVE
.
[ Team LiB ]

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[ Team LiB ]