XML

t's amazing how many developers claim to be proficient programming with XML, yet do not understand the basic differences between XML and HTML. Anyone with a fundamental grasp of XML should be able describe some of the main differences outlined in the table below.

Differences Between XML and HTML

XML

► User definable tags

► Content driven

► End tags required for well formed documents

► Quotes required around attributes values

► Slash required in empty tags

HTML

► Defined set of tags designed for web display

► Format driven

► End tags not required

► Quotes not required

► Slash not required

Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.

Despite early attempts, browsers never allowed other SGML, only HTML (although there were plugins), and they allowed it (even encouraged it) to be corrupted or broken, which held development back for over a decade by making it impossible to program for it reliably. XML fixes that by making it compulsory to stick to the rules, and by making the rules much simpler than SGML.

But XML is not just for Web pages: in fact it's very rarely used for Web pages on its own because browsers still don't provide reliable support for formatting and transforming it. Common uses for XML include:

Information identification

because you can define your own markup, you can define meaningful names for all your information items. Information storage because XML is portable and non-proprietary, it can be used to store textual information across any platform. Because it is backed by an international standard, it will remain accessible and processable as a data format.

There are literally thousands of applications that can benefit from XML technologies. The point of this question is not to have the candidate rattle off a laundry list of projects that they have worked on, but, rather, to allow the candidate to explain the rationale for choosing XML by citing a few real world examples. For instance, one appropriate answer is that XML allows content management systems to store documents independently of their format, which thereby reduces data redundancy.

Another answer relates to B2B exchanges or supply chain management systems. In these instances, XML provides a mechanism for multiple companies to exchange data according to an agreed upon set of rules. A third common response involves wireless applications that require WML to render data on hand held devices.

Not quite; SGML is the mother tongue, and has been used for describing thousands of different document types in many fields of human activity, from transcriptions of ancient Irish manuscripts to the technical documentation for stealth bombers, and from patients' clinical records to musical notation. SGML is very large and complex, however, and probably overkill for most common office desktop applications.

XML is an abbreviated version of SGML, to make it easier to use over the Web, easier for you to define your own document types, and easier for programmers to write programs to handle them. It omits all the complex and less-used options of SGML in return for the benefits of being easier to write applications for, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files may still be processed in the same way as any other SGML file (see the question on XML software).

HTML is just one of many SGML or XML applications?the one most frequently used on the Web.

Technical readers may find it more useful to think of XML as being SGML-- rather than HTML++.

SGML is the Standard Generalized Markup Language (ISO 8879:1986), the international standard for defining descriptions of the structure of different types of electronic document. There is an SGML FAQ from David Megginson at http://math.albany.edu:8800/hm/sgml/cts-faq.htmlFAQ; and Robin Cover's SGML Web pages are at http://www.oasis-open.org/cover/general.html. For a little light relief, try Joe English's ?Not the SGML FAQ? at http://www.flightlab.com/~joe/sgml/faq-not.txtFAQ.

SGML is very large, powerful, and complex. It has been in heavy industrial and commercial use for nearly two decades, and there is a significant body of expertise and software to go with it.

XML is a lightweight cut-down version of SGML which keeps enough of its functionality to make it useful but removes all the optional features which made SGML too complex to program for in a Web environment.

HTML was already overburdened with dozens of interesting but incompatible inventions from different manufacturers, because it provides only one way of describing your information.

XML allows groups of people or organizations to question C.13, create their own customized markup applications for exchanging information in their domain (music, chemistry, electronics, hill-walking, finance, surfing, petroleum geology, linguistics, cooking, knitting, stellar cartography, history, engineering, rabbit-keeping, question C.19, mathematics, genealogy, etc).

HTML is now well beyond the limit of its usefulness as a way of describing information, and while it will continue to play an important role for the content it currently represents, many new applications require a more robust and flexible infrastructure.

Here are a few reasons for using XML (in no particular order). Not all of these will apply to your own requirements, and you may have additional reasons not mentioned here (if so, please let the editor of the FAQ know!).

► XML can be used to describe and identify information accurately and unambiguously, in a way that computers can be programmed to ?understand? (well, at least manipulate as if they could understand).

► XML allows documents which are all the same type to be created consistently and without structural errors, because it provides a standardised way of describing, controlling, or allowing/disallowing particular types of document structure. [Note that this has absolutely nothing whatever to do with formatting, appearance, or the actual text content of your documents, only the structure of them.]

► XML provides a robust and durable format for information storage and transmission. Robust because it is based on a proven standard, and can thus be tested and verified; durable because it uses plain-text file formats which will outlast proprietary binary ones.

C and C++ (and other languages like FORTRAN, or Pascal, or Visual Basic, or Java or hundreds more) are programming languages with which you specify calculations, actions, and decisions to be carried out in order:

mod curconfig[if left(date,6) = "01-Apr",

t.put "April googlel!",

f.put days('31102005','DDMMYYYY') -

days(sdate,'DDMMYYYY')

" more shopping days to Samhain"];

XML is a markup specification language with which you can design ways of describing information (text or data), usually for storage, transmission, or processing by a program. It says nothing about what you should do with the data (although your choice of element names may hint at what they are for):

<part num="DA42" models="LS AR DF HG KJ"

update="2001-11-22">

<name>Camshaft end bearing retention circlip</name>

<image drawing="RR98-dh37" type="SVG" x="476"

y="226"/> <maker id="RQ778">Ringtown Fasteners Ltd</maker>

<notes>Angle-nosed insertion tool <tool

id="GH25"/> is required for the removal

and replacement of this part.</notes>

</part>

On its own, an SGML or XML file (including HTML) doesn't do anything. It's a data format which just sits there until you run a program which