What is it?
Extensible Markup Language (XML) is a metalanguage, which is
used to create other languages. It has been adopted almost
universally: organisations using it include Microsoft, IBM and the
open source Apache Foundation.
It has become the business interchange standard of choice. More
than half of large and medium-sized companies see it as part of
their strategic vision and are building new applications with it,
according to Resolution Market Research.
As a result, XML and web services developer skills are a prized
commodity, said Resolution.
But XML's strengths are also its weaknesses. It is
human-readable, which is great for improving communication between
IT and business, but this means it carries a huge performance
overhead. XML files can be 20 times the size of equivalent
proprietary formats.
Also, as XML is a metalanguage, alternative schemes and
approaches to querying data are proliferating.
XML is stable, but the glacial pace at which its custodians at
the Worldwide Web Consortium (W3C) reach decisions leaves urgent
problems unresolved. There is also a threat of patent-protected
proprietary implementations compromising XML's universality.
Where did it originate?
At the W3C, where Tim Bray and others were looking for a way for
internet-connected systems to exchange data. Work began in 1996 and
W3C approved the first XML standard in 1998. Like HTML, XML is
derived from the Standard Generalised Markup Language (SGML)
although, unlike HTML, it retains a lot of SGML's power and
flexibility while being easier to use.
What is it for?
Devised as a way of defining document formats, XML found most of
its early uses in publishing, but is now also applied to structured
data such as relational databases and spreadsheets.
XML has been used to create many application- and
industry-specific languages, from bible and music publishing to
chemical manufacturing, and document exchange standards for
e-business consortia.
What makes it special?
According to Resolution, users see the benefits of XML in terms
of improving efficiency and lowering support costs. Specific
examples include faster application development and deployment,
greater application portability and easier staff hires. It is
Unicode-compliant and can be read and edited using simple
text-editing tools. Although most programming languages are
developing ways of incorporating and working with XML, it is not
tied to any language or platform.
How difficult is it to master?
According to IBM's Developerworks site, XML involves a huge
variety of standards and technologies that interact in complex
ways. "It can be difficult for beginners to navigate the most
important aspects of XML, and for users to keep track of new
entries and changes in the space," it said. HTML developers should
make the change easily enough, because of the languages' common
ancestry, but will find XML a lot less lax and forgiving.
Bloor Research said, "It is very easy to design an XML message.
It is very much harder to design a coherent set of messages."
Rates of pay
Web service development roles using Microsoft products or Java
can earn from £25,000 to £40,000.
Training
There is a mass of free tutorial material. Try IBM's Alphaworks
and Developerworks sites, Microsoft's XML site, and the XML Cover
Pages, which also contain news about developments in XML. You could
take formal classroom training or learn from books published by
O'Reilly, Addison-Wesley and John Wiley, parts of which may be
available on the web. Consider also joining xmluk.org, the UK
chapter of the SGML and XML User Group.