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

Published Language
The translation between the models of two 
BOUNDED CONTEXTS
requires a common language.
When two domain models must coexist and information must pass between them, the translation process
itself can become complex and hard to document and understand. If we are building a new system, we will
typically believe that our new model is the best available, and so we will think in terms of translating
directly into it. But sometimes we are enhancing a set of older systems and trying to integrate them.
Choosing one messy model over the other may be choosing the lesser of two evils.
Another situation: When businesses want to exchange information with one another, how do they do it?
Not only is it unrealistic to expect one to adopt the domain model of the other, it may be undesirable for
both parties. A domain model is developed to solve problems for its users; such a model may contain
features that needlessly complicate communication with another system. Also, if the model underlying one
of the applications is used as the communications medium, it cannot be changed freely to meet new
needs, but must be very stable to support the ongoing communication role.
Direct translation to and from the existing domain models may not be a good solution. Those
models may be overly complex or poorly factored. They are probably undocumented. If one is
used as a data interchange language, it essentially becomes frozen and cannot respond to new
development needs.
The 
OPEN HOST SERVICE
uses a standardized protocol for multiparty integration. It employs a model of the
domain for interchange between systems, even though that model may not be used internally by those
systems. Here we go a step further and publish that language, or find one that is already published. By
publish
I simply mean that the language is readily available to the community that might be interested in
using it, and is sufficiently documented to allow independent interpretations to be compatible.
Recently, the world of e-commerce has become very excited about a new technology: Extensible Markup
Language (XML) promises to make interchange of data much easier. A very valuable feature of XML is
that, through the document type definition (DTD) or through XML schemas, XML allows the formal
definition of a specialized domain language into which data can be translated. Industry groups have begun
to form for the purpose of defining a single standard DTD for their industry so that, say, chemical formula
information or genetic coding can be communicated between many parties. Essentially these groups are
creating a shared domain model in the form of a language definition.
Therefore: