Metadata, Resources, and the Resource Description Framework
The XML aspect is important for several reasons. XML is as close to a global,
universal data format as we come today (because it uses Unicode, it is more
universal than ASCII, which is restricted to the Latin alphabet). How to handle
it in databases, how to transport it over the network, and how to build applica-
tions that use it are all well understood. And it has a large installed base, with
plenty of applications.
RDF builds on XML to create descriptions, and descriptions are metadata: data
about data. It can be very hard to understand, and there are basically two ways
of explaining it: As object properties, or as profiles. Which is more useful to you
depends on your background.
First, let us start with the concept of descriptions. A description of a document
is a document in its own right. Documents are nothing but a sequence of frag-
ments, elements of information, and the order and structure of the fragments
constitute a metadescription of the document. Of course, there can be other
descriptions as well, such as what the document is about, how it should be pre-
sented in different formats, and anything that pertains to the document but is
not the document itself.
In RDF, you always identify the object you are describing by a unique address,
the Universal Resource Identifier. The descriptions can be object identifiers as
well (URIs). This means that in object-oriented terms, you can describe which
classes an object belongs to and then compare the listing of classes for this
particular object to other objects and so find out what they have in common.
The descriptions are also data. Data about data is data, too. And metadata is
nothing but data about data. It can be embedded in the document, or exist sep-
arately from it, as a document or as headers in a protocol, for instance. But
there is nothing that limits the use of metadata to documents. It is possible to
describe any object using metadata. And anything can be an object, from the
collection of all information in the universe down to the letters on this page.
When the metadata about the object is structured to provide a description, and
the structure is common for all instances of the same type of object, it is a pro-
file. The profile can have different values for different instances, but the struc-
ture is always the same. So, all the books in the library can be described using
the same library cards, but each library card will have different content, even
though they all include book title, author, and so forth.
What falls under one classification to one man, however, is something else to
another. No object falls unambiguously into a single classification. Not even in
physics can we find unambiguous ways to describe the objects we talk about.
What is a quark? Is an electron really a wave or a particle? And it just keeps
going uphill from there.
Language, from the point of view of the language philosopher, is based on a
social contract between the sender and receiver. If I am speaking Japanese and
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