XML


Extensible Markup Language is a markup language and file format for storing, transmitting, and reconstructing data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The World Wide Web Consortium's XML 1.0 Specification of 1998 and several other related specifications—all of them free open standards—define XML.
The design goals of XML emphasize simplicity, generality, and usability across the Internet. It is a textual data format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures, such as those used in web services.
Several schema systems exist to aid in the definition of XML-based languages, while programmers have developed many application programming interfaces to aid the processing of XML data.

Overview

The main purpose of XML is serialization, i.e. storing, transmitting, and reconstructing arbitrary data. For two disparate systems to exchange information, they need to agree upon a file format. XML standardizes this process. It is therefore analogous to a lingua franca for representing information.
As a markup language, XML labels, categorizes, and structurally organizes information. XML tags represent the data structure and contain metadata. What is within the tags is data, encoded in the way the XML standard specifies. An additional XML schema defines the necessary metadata for interpreting and validating XML. An XML document that adheres to basic XML rules is "well-formed"; one that adheres to its schema is "valid".
IETF RFC 7303, provides rules for the construction of media types for use in XML message. It defines three media types: application/xml, application/xml-external-parsed-entity and application/xml-dtd. They are used for transmitting raw XML files without exposing their internal semantics. RFC 7303 further recommends that XML-based languages be given media types ending in +xml, for example, image/svg+xml for SVG.
Further guidelines for the use of XML in a networked context appear in RFC 3470, also known as IETF BCP 70, a document covering many aspects of designing and deploying an XML-based language.

Applications

XML has come into common use for the interchange of data over the Internet. Hundreds of document formats using XML syntax have been developed, including RSS, Atom, Office Open XML, OpenDocument, SVG, COLLADA, and XHTML. XML also provides the base language for communication protocols such as SOAP and XMPP. It is one of the message exchange formats used in the Asynchronous JavaScript and XML programming technique.
Many industry data standards, such as Health Level 7, OpenTravel Alliance, FpML, MISMO, and the National Information Exchange Model are based on XML and the rich features of the XML schema specification. In publishing, Darwin Information Typing Architecture is an XML industry data standard. XML is used extensively to underpin various publishing formats.
One of the applications of XML in science is the representation of operational meteorology information based on IWXXM standards.

Key terminology

The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs that appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use.
;: An XML document is a string of characters. Every legal Unicode character may appear in an XML document.
;: The processor analyzes the markup and passes structured information to an application. The specification places requirements on what an XML processor must do and not do, but the application is outside its scope. The processor is often referred to colloquially as an XML parser.
;: The characters making up an XML document are divided into markup and content, which may be distinguished by the application of simple syntactic rules. Generally, strings that constitute markup either begin with the character < and end with a >, or they begin with the character & and end with a ;. Strings of characters that are not markup are content. However, in a CDATA section, the delimiters <!CDATA[ and > are classified as markup, while the text between them is classified as content. In addition, whitespace before and after the outermost element is classified as markup.
;: A tag is a markup construct that begins with < and ends with >. There are three types of tag:
;: An element is a logical document component that either begins with a start-tag and ends with a matching end-tag or consists only of an [empty-element tag. The characters between the start-tag and end-tag, if any, are the element's content, and may contain markup, including other elements, which are called child elements. An example is <greeting>Hello, world!</greeting>. Another is <line-break />.
;: An attribute is a markup construct consisting of a name–value pair that exists within a start-tag or empty-element tag. An example is <img src="madonna.jpg" alt="Madonna" />, where the names of the attributes are "src" and "alt", and their values are "madonna.jpg" and "Madonna" respectively. Another example is <step number="3">Connect A to B.</step>, where the name of the attribute is "number" and its value is "3". An XML attribute can only have a single value and each attribute can appear at most once on each element. In the common situation where a list of multiple values is desired, this must be done by encoding the list into a well-formed XML attribute with some format beyond what XML defines itself. Usually this is either a comma or semi-colon delimited list or, if the individual values are known not to contain spaces, a space-delimited list can be used. An example with space as a delimiter is <div class="inner greeting-box">Welcome!</div>, where the attribute "class" both has the value "inner greeting-box" and also indicates the two CSS class names "inner" and "greeting-box".
;: XML documents may begin with an XML declaration that describes some information about themselves. An example is <?xml version="1.0" encoding="UTF-8"?>.

Characters and escaping

XML documents consist entirely of characters from the Unicode repertoire. Except for a small number of specifically excluded control characters, any character defined by Unicode may appear within the content of an XML document.
XML includes facilities for identifying the encoding of the Unicode characters that make up the document, and for expressing characters that, for one reason or another, cannot be used directly.

Valid characters

Unicode code points in the following ranges are valid in XML 1.0 documents:
  • U+0009, U+000A, U+000D : these are the only C0 controls accepted in XML 1.0;
  • U+0020–U+D7FF, U+E000–U+FFFD: this excludes some noncharacters in the BMP ;
  • U+10000–U+10FFFF: this includes all code points in supplementary planes, including noncharacters.
XML 1.1 extends the set of allowed characters to include all the above, plus the remaining characters in the range U+0001–U+001F. At the same time, however, it restricts the use of C0 and C1 control characters other than U+0009, U+000A, U+000D, and U+0085 by requiring them to be written in escaped form. In the case of C1 characters, this restriction is a backwards incompatibility; it was introduced to allow common encoding errors to be detected.
The code point U+0000 is the only character that is not permitted in any XML 1.1 document.

Encoding detection

The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings". Unicode itself defines encodings that cover the entire repertoire; well-known ones include UTF-8 and UTF-16. There are many other text encodings that predate Unicode, such as ASCII and various ISO/IEC 8859; their character repertoires are in every case subsets of the Unicode character set.
XML allows the use of any of the Unicode-defined encodings and any other encodings whose characters also appear in Unicode. XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used. Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser.

Escaping

XML provides escape facilities for including characters that are problematic to include directly. For example:
  • The characters "<" and "&" are key syntax markers and may never appear in content outside a CDATA section. It is allowed, but not recommended, to use "<" in XML entity values.
  • Some character encodings support only a subset of Unicode. For example, it is legal to encode an XML document in ASCII, but ASCII lacks code points for Unicode characters such as "é".
  • It might not be possible to type the character on the author's machine.
  • Some characters have glyphs that cannot be visually distinguished from other characters, such as the nonbreaking space " " and the space " ", and the Cyrillic capital letter A "А" and the Latin capital letter A "A".
There are five predefined entities:
  • &lt; represents "<";
  • &gt; represents ">";
  • &amp; represents "&";
  • &apos; represents "";
  • &quot; represents .
All permitted Unicode characters may be represented with a numeric character reference''. Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as &#20013; or &#x4e2d;. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as I &lt;3 J&#xF6;rg.
&#0; is not permitted because the null character is one of the control characters excluded from XML, even when using a numeric character reference. An alternative encoding mechanism such as Base64 is needed to represent such characters.