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

Example
A P
UBLISHED
 L
ANGUAGE
 for Chemistry
Innumerable programs are used to catalog, analyze, and manipulate chemical formulas in industry and
academia. Exchanging data has always been difficult, because almost every program uses a different
domain model to represent chemical structures. And of course, most of them are written in languages,
such as FORTRAN, that do not express the domain model very fully anyway. Whenever anyone wanted to
share data, they had to unravel the details of the other system's database and work out some sort of
translation scheme.
Enter the Chemical Markup Language (CML), a dialect of XML intended as a common interchange language
for this domain, developed and managed by a group representing academics and industry (Murray-Rust et
al. 1995).
Chemical information is very complex and diverse, and it changes all the time with new discoveries. So
they developed a language that could describe the basics, such as the chemical formulas of organic and
inorganic molecules, protein sequences, spectra, or physical quantities.
Now that the language has been published, tools can be developed that would never have been worth the
trouble to write before, when they would have only been usable for one database. For example, a Java
application, called the JUMBO Browser, was developed that creates graphical views of chemical structures
stored in CML. So if you put your data in the CML format, you'll have access to such visualization tools.
In fact, CML gained a double advantage by using XML, a sort of "published meta-language." The learning
curve of CML is flattened by people's familiarity with XML; the implementation is eased by various off-the-
shelf tools, such as parsers; and documentation is helped by the many books written on all aspects of
handling XML.
Here is a tiny sample of CML. It is only vaguely intelligible to nonspecialists like myself, but the principle is
clear.
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.ENT=CML.THE.AOEPOPS>

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1.17947 0.95091 0.97175 1.00000 1.17947 0.95090 0.97174 1.00000
1.17946 0.98215 0.94049 1.00000 1.17946 0.95091 0.97174 1.00000
1.17946 0.95091 0.97174 1.00000 1.17946 0.98215 0.94049 1.00000
0.89789 0.89790 0.89789 0.89789 0.89790 0.89788

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