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

Figure 14.9. Four contexts: no integration
Figure 14.10. Four contexts: minimal integration
As the blind men want to share more information about the elephant, the value of sharing a single
BOUNDED CONTEXT
goes up. But unifying the disparate models is a challenge. None of them is likely
to give up his model and adopt one of the others. After all, the man who touched the tail 
knows
the elephant is not like a tree, and that model would be meaningless and useless to him. Unifying
multiple models almost always means creating a new model.
With some imagination and continued discussion (probably heated), the blind men could eventually
recognize that they have been describing and modeling different parts of a larger whole. For many
purposes, a part-whole unification may not require much additional work. At least the first stage of
integration only requires figuring out how the parts are related. It may be adequate for some
needs to view an elephant as a wall, held up by tree trunks, with a rope at one end and a snake at
the other.
Figure 14.11. One context: crude integration

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The unification of the various elephant models is easier than most such mergers. Unfortunately, it
is the exception when two models purely describe different parts of the whole, although this is
often one aspect of the difference. Matters are more difficult when two models are looking at the
same part in a different way. If two men had touched the trunk and one described it as a snake
and the other described it as a fire hose, they would have had more difficulty. Neither can accept
the other's model, because it contradicts his own experience. In fact, they need a new abstraction
that incorporates the "aliveness" of a snake with the water-shooting functionality of a fire hose,
but one that leaves out the inapt implications of the first models, such as the expectation of
possibly venomous fangs, or the ability to be detached from the body and rolled up into a
compartment in a fire truck.
Even though we have combined the parts into a whole, the resulting model is crude. It is
incoherent, lacking any sense of following contours of an underlying domain. New insights could
lead to a deeper model in a process of continuous refinement. New application requirements can
also force the move to a deeper model. If the elephant starts moving, the "tree" theory is out, and
our blind modelers may break through to the concept of "legs."