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XML

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The Extensible Markup Language (XML) is a general-purpose markup language, classified as an extensible language because it allows its users to define their own elements. Its primary purpose is to facilitate the sharing of structured data across different information systems, particularly via the Internet. It is used both to encode documents and to serialize data. XML is a fee-free open standard and is recommended by the World Wide Web Consortium (W3C).


2.   XML started as a simplified subset of the Standard Generalized Markup Language (SGML). The versatility of SGML for dynamic information display was understood by early digital media publishers in the late 1980s prior to the rise of the Internet. By the mid-1990s, they believed that SGML offered solutions to some of the problems the Web was likely to face as it grew. Work began in mid-1996 when Jon Bosak developed a charter and received support from Microsoft. XML 1.0 became a W3C Recommendation on February 10, 1998.


3.   Currently, there are two versions of XML: XML 1.0 and XML 1.1. The two versions differ in the requirements of characters used for element and attribute names. The approach in XML 1.1 is that only certain characters are forbidden, and everything else is allowed, whereas in XML 1.0, only certain characters are explicitly allowed. Thus, XML 1.0 cannot accommodate the addition of characters in future versions of Unicode. XML 1.1, however, is not very widely implemented and is recommended for use only by those who need its unique features.


4.   There are two levels of correctness of an XML document:

  • Well-formed: A well-formed document conforms to all of XML's syntax rules. For example, if a start-tag appears without a corresponding end-tag, it is not well-formed, and a conforming parser is not allowed to process it.
  • Valid: A valid document additionally conforms to some semantic rules. These rules are either user-defined, or included as an XML schema or DTD. For example, if a document contains an undefined element, then it is not valid, in which case a validating parser is not allowed to process it.

5.   The XML specification requires that processors of XML support the pan-Unicode character encodings UTF-8 and UTF-16 (UTF-32 is not mandatory). The use of more limited encodings, such as those based on ISO/IEC 8859, is acknowledged and is widely used and supported.


6.   An example of a structured XML document is as follows: 

<recipe name="bread" prep_time="5 mins" cook_time="3 hours">
  <title>Basic bread</title>
  <ingredient amount="3" unit="cups">Flour</ingredient>
  <ingredient amount="0.25" unit="ounce">Yeast</ingredient>
  <ingredient amount="1.5" unit="cups" state="warm">Water</ingredient>
  <ingredient amount="1" unit="teaspoon">Salt</ingredient>
  <instructions>
    <step>Mix all ingredients together.</step>
    <step>Knead thoroughly.</step>
    <step>Cover with a cloth, and leave for one hour.</step>
    <step>Knead again.</step>
    <step>Place in a bread baking tin.</step>
    <step>Cover with a cloth, and leave for one hour.</step>
    <step>Bake in the oven at 350°F for 30 minutes.</step>
  </instructions>
</recipe>

Attribute values must always be quoted, using single or double quotes; and each attribute name should appear only once in any element. XML requires that elements be properly nested — elements may never overlap.  more... at Wikipedia



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