A clear description of XML:
XML explained...
XML 1: What is XML by the way? A look at the
Web "Lingua Franca"
Wed, 10 Nov 1999 12:01:01 GMT
Dave Wilby
We've all heard that XML is the next big thing for the web,
but the many white papers available out there are a little
intense. Here's a straight-forward explanation on the web
language of love...
XML stands for eXtensible Markup Language. It is a subset of
SGML, Standard Generalised Markup language, and was
designed by the W3C to make it easier for Internet users to
interchange structured documents.
Structured documents are documents that contain content --
including words and pictures -- together with explanations as to
what role each piece of content plays. XML files clearly indicate
where the start and end of each logical part of a given document
occurs. This means for example, that content in a header,
introduction, photo caption, conclusion, table or whatever, can be
given a different meaning and way of acting and displaying itself
than each other element.
This is one major reason why XML is considered to be more
personal and intelligent than HTML, for example. XML was
designed from the ground-up NOT to code text in a standard
way. Indeed it is fair to say that no language can truly suit all
applications. So XML simply describes the component parts of
the document and gives that information to other computer
systems. This means that XML is inherently flexible, making it ideal
for describing any block of content at pretty much any scale, from
a mail or news story, to an encyclopaedia or entire database.
As mentioned, XML is defined as an application profile of SGML
(ISO 8879). SGML has been the standard, vendor-independent
definer of structured documents since the eighties, but it is an
unwieldy tool for serving documents over web. A great tool, and
very powerful, but you wouldn't use a chainsaw to open an
envelope, and this is where XML comes into play. Any SGML
conformant system can read XML documents, but XML
documents don't have to have a full understanding of the
inner-mechanisms of SGML. In essence then, XML is a restricted
version of SGML.
One of the coolest things about XML is its liberality and lack of
semantics. If you want to embolden a sentence you could call the
tag "nice and bold" if you so wish. This is extremely useful when
developing style sheets and so forth, and allows for feature rich
documents with complicated structures to be created reasonably
simply. HTML is easier at a basic level, but you have to learn a
tag-set and how those tags are delimited from normal text and in
which order they may be used. XML systems can be defined to
make the user aware of valid tag choices at any given stage of a
document, which can be manually or dynamically validated. Users
familiar with HTML will be pleased to see tags used in angle
brackets.
XML documents are made up of a series of elements, things, or
objects. The language works as a formal syntax for describing the
relationship between these objects that make up the document,
and can be used to tell the computer how to deal with each.
Because XML tag sets are more logical in structure they are
perhaps easier to understand than basic mark-up schemes.
When the W3C decided to knock up XML it had Ten
Commandments, or more correctly "development goals" that it
wanted to achieve. XML is a little easier to grasp when you know
what it was intended to help you do.
1) Firstly, it had to be easy to use over the Internet, with
documents that could be as viewed as easily as existing HTML
efforts.
2) It had to be able to support a wide variety of applications for
authoring, browsing and so on.
3) It had to be compatible with SGML, so as not to put out the
many development houses with huge resource investments in
SGML.
4) It should be easy to write programs that process XML
documents, with an average programmer being capable of bashing
out such a program in around a fortnight.
5) Options were to be kept to a minimum, and hopefully zero, so
as not to create unnecessary compatibility errors between
disparate users and systems.
6) XML documents should be "human" and clear, so that you
could view XML source code in an everyday text-editor and still
figure out its basic purpose.
7) The whole XML design process had to be completed quickly,
before the problems it was intended to solve built up and became
more complicated problems.
8) The design had to be formal and to the point, unlike SGML,
which is overtly complicated to the untrained eye.
9) Documents must be easy to create without sophisticated
editors.
10) And lastly, it was decided that terseness in XML markup was
unimportant, with the belief that clarity was preferable to less
characters, especially when shortcuts can easily be implemented.
Sounds like a cool language, doesn't it? Well, in many ways it is.
XML is easy to maintain, as there is no sprawl of
incomprehensible mass of markup to wade through when
troubleshooting documents. Unnecessary complication has been
torn out of XML, leaving an elegant, human development
environment that is open and universally compatible. Above all
XML is friendly, a rare quality in the world of programming.
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