XML Interview Questions and Answers

What is XML?
XML is the Extensible Markup Language. It improves the functionality of the Web by letting you identify your information in a more accurate, flexible, and adaptable way.
It is extensible because it is not a fixed format like HTML (which is a single, predefined markup language). Instead, XML is actually a metalanguage—a language for describing other languages—which lets you design your own markup languages for limitless different types of documents. XML can do this because it's written in SGML, the international standard metalanguage for text document markup (ISO 8879).

What is a markup language?
A markup language is a set of words and symbols for describing the identity of pieces of a document (for example ‘this is a paragraph’, ‘this is a heading’, ‘this is a list’, ‘this is the caption of this figure’, etc). Programs can use this with a style sheet to create output for screen, print, audio, video, Braille, etc.
Some markup languages (e.g. those used in word processors) only describe appearances (‘this is italics’, ‘this is bold’), but this method can only be used for display, and is not normally re-usable for anything else.

Where should I use XML?
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. Information structure
XML can therefore be used to store and identify any kind of (hierarchical) information structure, especially for long, deep, or complex document sets or data sources, making it ideal for an information-management back-end to serving the Web. This is its most common Web application, with a transformation system to serve it as HTML until such time as browsers are able to handle XML consistently. Publishing
The original goal of XML as defined in the quotation at the start of this section. Combining the three previous topics (identity, storage, structure) means it is possible to get all the benefits of robust document management and control (with XML) and publish to the Web (as HTML) as well as to paper (as PDF) and to other formats (e.g. Braille, Audio, etc) from a single source document by using the appropriate style sheets. Messaging and data transfer
XML is also very heavily used for enclosing or encapsulating information in order to pass it between different computing systems which would otherwise be unable to communicate. By providing a lingua franca for data identity and structure, it provides a common envelope for inter-process communication (messaging). Web services
Building on all of these, as well as its use in browsers, machine-processable data can be exchanged between consenting systems, where before it was only comprehensible by humans (HTML). Weather services, e-commerce sites, blog newsfeeds, AJAX sites, and thousands of other data-exchange services use XML for data management and transmission, and the web browser for display and interaction.

Why is XML such an important development?

It removes two constraints which were holding back Web developments:
1. dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. the complexity of full SGML, whose syntax allows many powerful but hard-to-program options.
XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

Describe the role that XSL can play when dynamically generating HTML pages from a relational database.
Even if candidates have never participated in a project involving this type of architecture, they should recognize it as one of the common uses of XML. Querying a database and then formatting the result set so that it can be validated as an XML document allows developers to translate the data into an HTML table using XSLT rules. Consequently, the format of the resulting HTML table can be modified without changing the database query or application code since the document rendering logic is isolated to the XSLT rules.

What is SGML?
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.

Aren't XML, SGML, and HTML all the same thing?
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++.

Who is responsible for XML?
XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is supervised by an XML Working Group. A Special Interest Group of co-opted contributors and experts from various fields contributed comments and reviews by email.
XML is a public format: it is not a proprietary development of any company, although the membership of the WG and the SIG represented companies as well as research and academic institutions. The v1.0 specification was accepted by the W3C as a Recommendation on Feb 10, 1998.

Why is XML such an important development?
It removes two constraints which were holding back Web developments:
1. dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. the complexity of full question A.4, SGML, whose syntax allows many powerful but hard-to-program options.
XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

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