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

Example
The Legacy Booking Application
In order to have a small, quick first release, we will write a minimal application that can set up a
shipment and then pass that to the legacy system through a translation layer for booking and
support operations. Because we built the translation layer specifically to protect our developing
model from the influence of the legacy design, this translation is an 
ANTICORRUPTION LAYER
.
Initially, the 
ANTICORRUPTION LAYER
will accept the objects representing a shipment, convert them,
pass them to the legacy system and request a booking, and then capture the confirmation and
translate it back into the confirmation object of the new design. This isolation will allow us to
develop our new application mostly independently of the old one, though we'll have to invest quite
a bit in translation.
With each successive release, the new system can either take over more functions of the legacy or
simply add new value without replacing existing capabilities, depending on later decisions. This
flexibility, and the ability to continually operate the combined system while making a gradual
transition, probably makes it worth the expense to build the 
ANTICORRUPTION LAYER
.

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A Cautionary Tale
To protect their frontiers from raids by neighboring nomadic warrior tribes, the early Chinese built
the Great Wall. It was not an impenetrable barrier, but it allowed a regulated commerce with
neighbors while providing an impediment to invasion and other unwanted influence. For two
thousand years it defined a boundary that helped the Chinese agricultural civilization to define
itself with less disruption from the chaos outside.
Although China might not have become so distinct a culture without the Great Wall, the Wall's
construction was immensely expensive and bankrupted at least one dynasty, probably contributing
to its fall. The benefits of isolation strategies must be balanced against their costs. There is a time
to be pragmatic and make measured revisions to the model, so that it can fit more smoothly with
foreign ones.
There is overhead involved in any integration, from full-on 
CONTINUOUS INTEGRATION
inside a single
BOUNDED CONTEXT
, through the lesser commitments of 
SHARED KERNELS
or 
CUSTOMER/SUPPLIER
DEVELOPER TEAMS
, to the one-sidedness of the 
CONFORMIST
and the defensive posture of the
ANTICORRUPTION LAYER
. Integration can be very valuable, but it is always expensive. We should be
sure it is really needed. . . .
[ Team LiB ]

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[ Team LiB ]
Separate Ways
We must ruthlessly scope requirements. Two sets of functionality with no indispensable
relationship can be cut loose from each other.