Metadata, Resources, and the Resource Description Framework
attributes are limited to those that make sense in the world of books, and not
for a more general set of knowledge representation, or even for magazines.
The knowledge representation community, on the other hand, was the hot
thing of the late 1980 s, expected to become the next huge thing in the software
industry. It fizzled for different reasons, but they came at the problem from a
different direction. An expert system is a set of rules, into which you can fit
knowledge as input, and get different knowledge sets as output (for instance, if
the input is the symptoms of a patient, you can get a recommendation for treat-
ment as output). But this means that you want to find a way of representing any
and all knowledge that fits into the system.
This brings a different set of problems. If you just think about metadata within
one single domain, it makes sense to have a table with the attribute names in
one column, and the attribute values in the other. But if you want to compare
the attributes to something else, how do you know what they represent? What
is the unit of measurement if thickness is 1, for instance? Inches or centime-
ters? Or millimeters? Or meters? Or miles? This seemingly simple problem
made the Mars Surveyor crash, and it can easily make someone else misunder-
stand what you want to present, especially if you want to present it to a com-
puter program. A computer is stupid, and if you do not tell it exactly what you
want to say, you cannot expect the receiving software to take any relevant
RDF, the Resource Description Framework, was developed by the World Wide
Web Consortium (W3C) to create a format for making assertions that leverage
the XML format to represent and transport information. XML, however, is not a
markup language; it is a set of rules for creating markup languages. I will talk
more about this in Chapter 2, but suffice to say that using XML brings us as
near to a universal data format as is possible nowadays.
However, XML gives only the rules for how the byte strings should be cobbled
together to form a coherent whole, which can be used by a widely spread set of
computer programs. How an XML data set should be interpreted is determined
by the Document Type Description (DTD), which essentially creates a schema
for an application of XML. But XML does not say anything about the informa-
tion itself, only the way it is structured.
The level above XML determines how the information is interpreted, and this is
where RDF exists. It builds to a very large extent on experiences from the
knowledge representation community, creating a way of comparing different
knowledge representations by making the user define them in a specific way
and enabling the comparison to use a format which is defined in a branch of
mathematics called graph theory. RDF is a way to express relations between
objects, something XML does not allow you to do.
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