XML, XLink, Namespace, DTD, Schema, CSS, XHTML,... If you are new to
XML, it may be hard know where to begin. This summary in 10 points
attempts to capture enough of the basic concepts to enable a beginner
to see the forest through the trees. And if you are giving a
presentation on XML, why not start with these 10 points? They are
hereby offered for your use.
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1. XML is a method for putting structured data in a text file
For "structured data" think of such things as spreadsheets, address
books, configuration parameters, financial transactions, technical
drawings, etc. Programs that produce such data often also store it on
disk, for which they can use either a binary format or a text format.
The latter allows you, if necessary, to look at the data without the
program that produced it. XML is a set of rules, guidelines,
conventions, whatever you want to call them, for designing text
formats for such data, in a way that produces files that are easy to
generate and read (by a computer), that are unambiguous, and that
avoid common pitfalls, such as lack of extensibility, lack of support
for internationalization/localization, and platform-dependency.
2. XML looks a bit like HTML but isn't HTML
Like HTML, XML makes use of tags (words bracketed by '<' and '>') and
attributes (of the form name="value"), but while HTML specifies what
each tag & attribute means (and often how the text between them will
look in a browser), XML uses the tags only to delimit pieces of data,
and leaves the interpretation of the data completely to the
application that reads it. In other words, if you see "<p>" in an XML
file, don't assume it is a paragraph. Depending on the context, it
may be a price, a parameter, a person, a p... (b.t.w., who says it
has to be a word with a "p"?)
3. XML is text, but isn't meant to be read
XML files are text files, as I said above, but even less than HTML
are they meant to be read by humans. They are text files, because
that allows experts (such as programmers) to more easily debug
applications, and in emergencies, they can use a simple text editor
to fix a broken XML file. But the rules for XML files are much
stricter than for HTML. A forgotten tag, or a an attribute without
quotes makes the file unusable, while in HTML such practice is often
explicitly allowed, or at least tolerated. It is written in the
official XML specification: applications are not allowed to try to
second-guess the creator of a broken XML file; if the file is broken,
an application has to stop right there and issue an error.
4. XML is a family of technologies
There is XML 1.0, the specification that defines what tags and
attributes are, but around XML 1.0, there is a growing set of
optional modules that provide sets of tags & attributes, or
guidelines for specific tasks. There is, e.g., Xlink (still in
development as of September 1999) which describes a standard way to
add hyperlinks and other inter-document relations to an XML file.
XPointer & XFragments (also still being developed) are syntaxes for
pointing to parts of an XML document. (An Xpointer is a bit like a
URL, but instead of pointing to documents on the Web, it points to
pieces of data inside an XML file.) CSS, the style sheet language, is
applicable to XML as it is to HTML. XSL (autumn 1999) is the advanced
language for expressing style sheets. It is based on XSLT, a
transformation language that is often useful outside XSL as well, for
rearranging, adding or deleting tags & attributes. The DOM is a
standard set of function calls for manipulating XML (and HTML) files
from a programming language. And there is Namespaces, a specification
that describes how you can associate a URL with every single tag and
attribute in an XML document. What that URL is used for is up to the
application that reads the URL, though. (RDF, W3C's standard for
metadata, uses it to link every piece of metadata to a file defining
the type of that data.) There are several more modules and tools
available or under development (DTD, Schema, Canonical XML,...). Keep
an eye on W3C's technical reports page.
5. XML is verbose, but that is not a problem
Since XML is a text format, and it uses tags to delimit the data, XML
files are nearly always larger than comparable binary formats. That
was a conscious decision by the XML developers. The advantages of a
text format are evident (see 3 above), and the disadvantages can
easily be solved at a different level. Disk spaces isn't as expensive
anymore as it used to be, and programs like zip and gzip can compress
files very well and very fast. Those programs are available for
nearly all platforms (and are usually free). In addition,
communication protocols such as modem protocols and HTTP/1.1 (the
core protocol of the Web) can compress data on the fly, thus saving
bandwith as effectively as a binary format.
6. XML is new, but not that new
Development of XML started in 1996 and it is a W3C standard since
February 1998, which may make you suspect that this is rather
immature technology. But in fact the technology isn't very new.
Before XML there was SGML, developed in the early '80s, an ISO
standard since 1986, and widely used for large documentation
projects. And of course HTML, whose development started in 1990. The
designers of XML simply took the best parts of SGML, guided by the
experience with HTML, and produced something that is no less powerful
than SGML, but vastly more regular and simpler to use. Some
evolutions, however, are hard to distinguish from revolutions... And
it must be said that while SGML is mostly used for technical
documentation and much less for other kinds of data, with XML it is
exactly the opposite.
7, 8, 9...
These I don't know yet.
10. XML is license-free, platform-independent and well-supported
By choosing XML as the basis for some project, you buy into a large
and growing community of tools (one of which may already do what you
need!) and engineers experienced in the technology. Opting for XML is
a bit like choosing SQL for databases: you still have to build your
own database and your own programs/procedures that manipulate it, but
there are many tools available and many people that can help you. And
since XML, as a W3C technology, is license-free, you can build your
own software around it without paying anybody anything. The large and
growing support means that you are also not tied to a single vendor.
XML isn't always the best solution, but it is always worth
considering.
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Bert Bos
Created 27 Mar 1999 (last update: $Date: 1999/09/20 17:44:51 $)