[XML] What is XML?

XML is one of the most popular and widely used file types in IT. It is somewhat verbose, so can be user-friendly (when the file size is small). But in general XML is not optimized for machine-processing. The good news is that there are good tools and APIs to generate or parse XML documents nowadays.


1. XML

XML (Extensible Markup Language) is a specification developed by the W3C. XML is a meta-markup language, like SGML(ISO Standard), but it is specially designed to use over the Web. It allows designers to create their own customized tags, enabling the definition, transmission, validation, and interpretation of data between applications and between organizations.

XML is an application profile or restricted form of SGML(the Standard Generalized Markup Language [ISO 8879]) XML documents are made up of storage units called entities, which contain either parsed or unparsed data.

  • Parsed data is made up of characters, some of which form character data, and some of which form markup
  • Markup encodes a description of the document’s storage layout and logical structure
  • XML provides a mechanism to impose constraints on the storage layout and logical structure

XML offers cross-platform, long-term data formats

  • simple, well documented, and straightforward
  • can be read by any software that can read a text file

XML is a metadata language

  • Metadata means data about data
  • XML enables the author to add information about the data


2. What XML is NOT:

  • NOT a programming language
  • NOT a network transport protocol
  • NOT a database
  • NOT a solution for every IT problem


3. Extensible

Extensible means that XML has no fixed set of tags that everybody should use. It’s the big difference from HTML. HTML is not extensible. HTML contains a standard set of tags. In XML, you can create your own tags.


4. Structured Documents

A Document is a term that describes any set of information presented to a reader.

  • Structure: the content of a document
  • Format: the way a document looks

Structured Model

  • In the structured model, each part of element of the document has a relationship to the others.
  • The meaning of an element is separate from the visual appearance of the element.

Constructing a Structure

  • Step 1 : Planning and Analysis
  • Step 2 : DTD(Document Type Definition)/Schema Writing
  • Step 3 : Marking up the Document
  • Step 4 : Tying Structure to Display


5. Origin / History

XML was developed by an XML Working Group formed under the auspices of the World Wide Web Consortium(W3C) in 1996.

  • Feb. 1998 : W3C Recommendation, Extensible Markup Language (XML) 1.0
  • Nov. 2008 : XML 1.0 5th edition

XML 1.1 is not very widely implemented and is recommended only for special cases.

  • Feb 2004: XML 1.1
  • Aug. 2006: XML 1.1 2nd edition


6. Terminology

  • Attribute : A part of an element that provides additional information about that element.
  • DTD : Document Type Definition. A DTD provides a list of the elements, attributes, comments, notes, and entities contained in the document, as well as their relationships to one another.
  • Element : The central building block of any XML document
  • Nesting : Placing one element inside another. When two tags are opened, they must be closed in the reverse order.
  • Tag : The markup characters that indicate the start or end of an element – but not the element content itself.
  • Valid : An XML document that is verified correct against a DTD or schema.
  • Well-formed : An XML document that follows the rules set forth by the XML specification, including having an XML declaration, correct comments, all tags are closed, all attributes are quoted, every document has one “container” element.

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