XML, eXtensible Markup Language
A short introduction
XML stands for eXtensible Markup
Language and is actually a
meta-language because it just gives
a framework for a specific XML based
language (i.e. an XML application).
Creating an XML application means
that data has been structured in a
particular way. This could be
compared to HTML where the focus is
on how to present data to the user.
Both XML applications and HTML use
tags for markup i.e. a start tag
describing the data to come and an
end tag signaling the end of data.
An example shows the idea. Assume
that we want to define a structure
that makes it possible to relate
vehicles with owners. Thus it is
necessary to define person data,
vehicle data and the relation
between person and vehicle e.g. if a
person owns more than one vehicle.
We can then create an instance of
the XML application, i.e. a specific
record, for person A owning vehicle
1 and 2, another instance for person
B owning vehicle 3 etc. This
information could then be sent as
files e.g. to authorities that will
extract relevant pieces of
information.
Getting started with XML is very
easy since the knowledge threshold
is low. On the other hand since it
is so low, many have actually
started and created a vast number of
standards (more than 450 so far)
related to XML. Due to the high
number another form of complexity
emerges. One example is that when
approaching the XML world it seems
that everything depends on something
else making it difficult to find a
starting point. This is especially
true for the core functionality of
XML.
One strength of XML is that it allows
the creation of an application specific
grammar, i.e. a list of rules how to
structure data, which can be used by a
parser to validate a specific instance
of an XML application. Two major types
of grammars exist, the DTD and the XML
Schema, where the latter is probably the
winner in the long run since it is more
capable. By using a grammar we could
check if data has been recorded
correctly otherwise an error is
generated.
For more information about XML please
visit
World Wide Web Consortium, W3C.
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