XML

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XML

Filename extension	.xml
Internet media type	application/xml, text/xml (deprecated)
Uniform Type Identifier	public.xml
Developed by	World Wide Web Consortium
Type of format	Markup language
Extended from	SGML
Extended to	XHTML, RSS, Atom, ...
Standard(s)	1.0 (Fifth Edition) 1.1 (Second Edition)

XML (Extensible Markup Language) is a general-purpose specification for creating custom markup languages.^[1] The term extensible is used to indicate that a markup-language designer has significant freedom in the choice of markup elements.^[2]

XML's goals^[3] emphasize representing documents with simplicity, generality, and usability over the Internet. XML has been used as the basis for a large number (at least hundreds) of custom-designed languages.^[4] Some of these, for example RSS, Atom, and XHTML, have become widely used on the Internet. XML is becoming the default file format for office-productivity software packages, including Microsoft Office, OpenOffice.org, AbiWord, and Apple's iWork.

XML is recommended by the World Wide Web Consortium (W3C). It is a fee-free open standard. The recommendation specifies lexical grammar and parsing requirements.

[edit] Meanings of the term

The term "XML" is used colloquially in the IT and marketing communities to describe a variety of technologies, including:

1. the syntax and rules defined by the W3C XML 1.* specification(s);
2. any vocabulary (markup language) based on the XML specification; and
3. XML technology in general, such as the collection of XML languages, parsers, tools and APIs applied to a particular problem set, as in "we are using XML for information exchange".

The majority of this article concerns the first meaning. Wikipedia contains numerous articles covering many of the specific XML languages and technologies. See the XML Category at the bottom of this page.

[edit] Key concepts and components

The material in this section is based on the XML Specification. This is not an exhaustive list of all the constructs which appear in XML; it provides an introduction to the key constructs most often encountered in day-to-day use.

Character, as defined in Unicode.

By definition, an XML document is a string of characters. Almost every legal Unicode character may appear in an XML document.

Processor and Application

Software which processes an XML document. It is expected that a processor works in the service of an application. There are certain very specific requirements about what an XML processor must do and not do, but none as to the behavior of the application. The processor (as the specification calls it) is often referred to colloquially as an XML parser.

Markup and Content

The characters which make up an XML document are divided into markup and content. Markup and content may be distinguished by the application of simple syntactic rules. All strings which constitute markup begin either with the character < and end with the next >, or begin with the character & and end with the character ;. Strings of characters which are not markup are content.

Tag

A markup construct that begins with < and ends with >. Tags come in three flavors: start-tags, for example <section>, end-tags, for example </section>, and empty-element tags, for example <line-break/>.

Element

A logical component of a document which either begins with a start-tag and ends with an end-tag, or consists only of an empty-element tag. The characters between the start- and end-tags, if any, are the element's content, and may contain markup, including other elements, which are called child elements. An example of an element is <Greeting>Hello, world.</Greeting>. Another is <line-break/>.

Attribute

A markup construct consisting of a name/value pair that exists within a start-tag or empty-element tag. In this example, the name of the attribute is number and the value is 3: <step number="3">Connect A to B.</step> This element has two attributes, src and alt: <img src="madonna.jpg" alt='by Raphael'/> An element may not have two attributes with the same name.

XML Declaration

This is a carefully-formatted string which is often found at the very beginning of XML documents. Here is an example:

<?xml version="1.0" encoding='ISO-8859-1'?>

Here is a small, complete XML document, which uses all of these constructs and concepts.

 <?xml version="1.0" encoding='ISO-8859-1'?>
 <painting>
  <img src="madonna.jpg" alt='Foligno Madonna, by Raphael'/>
  <caption>This is Raphael's "Foligno" Madonna, painted
           in <date>1511</date>-<date>1512</date>.</caption>
  </painting>

There are four elements in this example document. date is a child of caption which is a child of painting.

[edit] Characters and escaping

The design of XML relies heavily on the character repertoire provided by Unicode. Except for a small number of specifically identified "control characters", any character defined by Unicode may appear within the content of an XML document. The selection of characters which may appear within markup is somewhat more limited but still large.

XML includes facilities for identifying the encoding of the Unicode characters which make up the document, and for storing characters which, for one reason or another, cannot be used directly.

[edit] Encoding detection

The Unicode character set can be encoded into bytes for storage in a variety of different ways, called "encodings". Unicode itself defines encodings which can encode the entire repertoire; well-known ones include UTF-8 and UTF-16.^[5] There are many other text encodings which pre-date Unicode. such as ASCII and ISO/IEC 8859; their character repertoires in almost every case have been included in 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.^[6]

[edit] Escaping

There are a variety of reasons why it may be difficult or impossible to include some character directly in an XML document. For example, the characters < and & are key syntax markers and may never appear in content. Also, it is legal to encode an XML document in ASCII, but ASCII has no encoding for many Unicode characters, for example é.

For these reasons, XML provides facilities for escaping problematic characters. There are five predefined entities: &lt; represents <, &gt; represents >, &amp; represents &, &apos represents ', and &quot; represents ". Also, any Unicode character may be identified with a numeric character reference. Consider the Unicode character 中, whose numeric code is hexadecimal 4E2D, or decimal 20,013. It may be referred to as &#20013; or &#x4e2d;.

Thus, the string "I <3 Jörg" could be encoded for inclusion in an XML document as "I &lt;3 J&#xF6;rg".

[edit] Well-formedness and error-handling

The XML specification defines an XML document as a text which is well-formed, i.e satisfies a list of syntax rules provided in the specification. The list is fairly lengthy; some key points are:

• It contains only properly-encoded legal Unicode characters.
• None of the special syntax characters such as < and & appear except when performing their markup-delineation roles.
• The begin, end, and empty-element tags which delimit the elements are correctly nested, with none missing and none overlapping.
• There is a single "root" element which contains all the other elements.

The definition of an XML document excludes texts which contain violations of well-formedness rules; they are simply not XML. An XML processor which encounters such a violation is required to report such errors and to cease normal processing. This policy, occasionally referred to as Draconian, stands in notable contrast to the behavior of programs which process HTML, which are designed to produce a reasonable result even in the presence of severe markup errors. XML's policy in this area has been criticized as a violation of Postel's law.^[7]

[edit] Schemas and validation

In addition to being well-formed, an XML document may be valid. This means that it contains a reference to a Document Type Definition (DTD), and that its elements and attributes are declared in that DTD and follow the grammatical rules for them that the DTD specifies.

XML processors are classified as validating or non-validating depending on whether or not they process DTDs and check XML documents for validity. A processor which discovers a validity error is required to report it but may continue normal processing.

A DTD is an example of a schema language. Since the initial publication of XML 1.0, there has been substantial work in the area of schema languages for XML. Such schema languages typically constrain the set of elements that may be used in a document, which attributes may be applied to them, the order in which they may appear, and the allowable parent/child relationships.

[edit] DTD

The oldest schema format for XML is the Document Type Definition (DTD), inherited from SGML. While DTD support is ubiquitous due to its inclusion in the XML 1.0 standard, it is seen as limited for the following reasons:

• It has no support for newer features of XML, most importantly namespaces.
• It lacks expressiveness. Certain constraints commonly applied to XML documents cannot be expressed in a DTD.
• It uses a custom non-XML syntax, inherited from SGML, to describe the schema.

DTD is still used in many applications because of its ubiquity.

[edit] XML Schema

A newer XML schema language, described by the W3C as the successor of DTDs, is XML Schema, often referred to by the initialism for XML Schema instances, XSD (XML Schema Definition). XSDs are far more powerful than DTDs in describing XML languages. They use a rich datatyping system and allow for more detailed constraints on an XML document's logical structure. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them.

[edit] RELAX NG

RELAX NG was initially specified by OASIS, RELAX NG is now also an ISO international standard (as part of DSDL). RELAX NG schemas may be written in either an XML based syntax or a more compact non-XML syntax; the two syntaxes are isomorphic and James Clark's Trang conversion tool can convert between them without loss of information. RELAX NG has a simpler definition and validation framework than XML Schema, making it easier to use and implement. It also has the ability to use datatype framework plug-ins; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes.

[edit] ISO DSDL and other schema languages

The ISO DSDL (Document Schema Description Languages) standard brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes RELAX NG full and compact syntax, Schematron assertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based routing of document fragments to different validators. DSDL schema languages do not have the vendor support of XML Schemas yet, and are to some extent a grassroots reaction of industrial publishers to the lack of utility of XML Schemas for publishing.

Some schema languages not only describe the structure of a particular XML format but also offer limited facilities to influence processing of individual XML files that conform to this format. DTDs and XSDs both have this ability; they can for instance provide attribute defaults. RELAX NG and Schematron intentionally do not provide these; for example the infoset augmentation facility.

[edit] Related specifications

A cluster of specifications closely related to XML have been developed, starting soon after the initial publication of XML 1.0. It is frequently the case that the term "XML" is used to refer to XML and a few of these other technologies which have come to be seen as part of the XML core.

• XML Namespaces enable the same document to contain XML elements and attributes taken from different vocabularies, without any naming collisions occurring. Essentially all software which is advertised as supporting XML also supports XML Namespaces.
• The XML Information Set or XML infoset describes an abstract data model for XML documents in terms of information items. The infoset is commonly used in the specifications of XML languages, for convenience in describing constraints on the XML constructs those languages allow.
• XPath defines a syntax named XPath expressions which identify one or more of the internal components (elements, attributes, and so on) included in an XML document. XPath is widely used in other core-XML specifications and in programming libraries for accessing XML-encoded data.
• XSLT is an XML-based syntax for declarative description of transformations to be applied to XML documents.
• XQuery is an XML-oriented query language strongly rooted in XPath and XML Schema. It provides methods to access, manipulate and return XML.
• xml:id allows an author to confer ID-ness (in the sense used in a DTD) on an attribute, by naming it in an xml:id attribute.
• XML Signature defines syntax and processing rules for creating digital signatures on XML content.
• XML Encryption defines syntax and processing rules for encrypting XML content.

Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including XInclude, XLink, and XPointer.

[edit] Use on the Internet

It is common for XML to be used in interchanging data over the Internet. RFC 3023 gives rules for the construction of Internet Media Types for use when sending XML. It also defines the types "application/xml" and "text/xml", which say only that the data is in XML, and nothing about its semantics. The use of "text/xml" has been criticized as a potential source of encoding problems but is now in the process of being deprecated.^[8] RFC 3023 also recommends that XML-based languages be given media types beginning in "application/" and ending in "+xml"; for example "application/svg+xml" for SVG.

Further guidelines for the use of XML in a networked context may be found in RFC 3470, also known as IETF BCP 70; this document is very wide-ranging and covers many aspects of designing and deploying an XML-based language.

[edit] Programming interfaces

The design goals of XML include "It shall be easy to write programs which process XML documents."^{[citation needed]} Despite this fact, the XML specification contains almost no information about how programmers might go about doing such processing. The XML Infoset provides a vocabulary to refer to the constructs within an XML document, but once again does not provide any guidance on how to access this information. A variety of APIs for accessing XML have been developed and used, and some have been standardized.

Existing APIs for XML processing tend to fall into these categories:

• Stream-oriented APIs accessible from a programming language, for example SAX and StAX.
• Tree-traversal APIs accessible from a programming language, for example DOM and XOM.
• XML data binding, which provides an automated translation between an XML document and programming-language objects.
• Declarative transformation languages such as XSLT and XQuery.

Stream-oriented facilities require less memory and, for certain tasks which are based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and data-binding APIs are typically require the use of much more memory, but are often found more convenient for use by programmers; some include declarative retrieval of document components via the use of XPath expressions.

XSLT is designed for declarative description of XML document transformations, and has been widely implemented both in server-side packages and Web browsers. XQuery overlaps XSLT in its functionality, but is designed more for searching of large XML databases.

[edit] Simple API for XML (SAX)

SAX is a lexical, event-driven interface in which a document is read serially and its contents are reported as "callbacks" to various methods on a handler object of the user's design. SAX is fast and efficient to implement, but difficult to use for extracting information at random from the XML, since it tends to burden the application author with keeping track of what part of the document is being processed. It is better suited to situations in which certain types of information are always handled the same way, no matter where they occur in the document.

[edit] Pull parsing

Pull parsing^[9] treats the document as a series of items which are read in sequence using the Iterator design pattern. This allows for writing of recursive-descent parsers in which the structure of the code performing the parsing mirrors the structure of the XML being parsed, and intermediate parsed results can be used and accessed as local variables within the methods performing the parsing, or passed down (as method parameters) into lower-level methods, or returned (as method return values) to higher-level methods. Examples of pull parsers include StAX in the Java programming language, SimpleXML in PHP and System.Xml.XmlReader in .NET.

A pull parser creates an iterator that sequentially visits the various elements, attributes, and data in an XML document. Code which uses this 'iterator' can test the current item (to tell, for example, whether it is a start or end element, or text), and inspect its attributes (local name, namespace, values of XML attributes, value of text, etc.), and can also move the iterator to the 'next' item. The code can thus extract information from the document as it traverses it. The recursive-descent approach tends to lend itself to keeping data as typed local variables in the code doing the parsing, while SAX, for instance, typically requires a parser to manually maintain intermediate data within a stack of elements which are parent elements of the element being parsed. Pull-parsing code can be more straightforward to understand and maintain than SAX parsing code.

[edit] Document Object Model (DOM)

DOM (Document Object Model) is an interface-oriented Application Programming Interface that allows for navigation of the entire document as if it were a tree of "Node" objects representing the document's contents. A DOM document can be created by a parser, or can be generated manually by users (with limitations). Data types in DOM Nodes are abstract; implementations provide their own programming language-specific bindings. DOM implementations tend to be memory intensive, as they generally require the entire document to be loaded into memory and constructed as a tree of objects before access is allowed.

[edit] Data binding

Another form of XML Processing API is data binding, where XML data is made available as a hierarchy of custom, strongly typed classes, in contrast to the generic objects created by a Document Object Model parser. This approach simplifies code development, and in many cases allows problems to be identified at compile time rather than run-time. Example data binding systems include the Java Architecture for XML Binding (JAXB),^[10] XML Serialization in .NET,^[11] Liquid XML Data Binder for C++, Java, VB & .Net ^[12] and CodeSynthesis XSD for C++.^[13][14]

[edit] XML as data type

XML is beginning to appear as a first-class data type in other languages. The ECMAScript for XML (E4X) extension to the ECMAScript/JavaScript language explicitly defines two specific objects (XML and XMLList) for JavaScript, which support XML document nodes and XML document lists as distinct objects and use a dot-notation specifying parent-child relationships. E4X is supported by the Mozilla 2.5+ browsers and Adobe Actionscript, but has not been adopted more universally. A somewhat similar notation is used in Microsoft's LINQ implementation for Microsoft .NET 2.0 and above, while XJ is an IBM implementation of an XML data object implementation for Java, developed in 2005. The open-source xmlsh application, which provides a Linux-like shell with special features for XML manipulation, similarly treats XML as a data type, using the <[ ]> notation.^[15]

[edit] History

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.^[16][17] By the mid-1990s some practitioners of SGML had gained experience with the then-new World Wide Web, and believed that SGML offered solutions to some of the problems the Web was likely to face as it grew. Dan Connolly added SGML to the list of W3C's activities when he joined the staff in 1995; work began in mid-1996 when Sun Microsystems engineer Jon Bosak developed a charter and recruited collaborators. Bosak was well connected in the small community of people who had experience both in SGML and the Web.

XML was compiled by a working group of eleven members,^[18] supported by an (approximately) 150-member Interest Group. Technical debate took place on the Interest Group mailing list and issues were resolved by consensus or, when that failed, majority vote of the Working Group. A record of design decisions and their rationales was compiled by Michael Sperberg-McQueen on December 4, 1997.^[19] James Clark served as Technical Lead of the Working Group, notably contributing the empty-element "<empty/>" syntax and the name "XML". Other names that had been put forward for consideration included "MAGMA" (Minimal Architecture for Generalized Markup Applications), "SLIM" (Structured Language for Internet Markup) and "MGML" (Minimal Generalized Markup Language). The co-editors of the specification were originally Tim Bray and Michael Sperberg-McQueen. Halfway through the project Bray accepted a consulting engagement with Netscape, provoking vociferous protests from Microsoft. Bray was temporarily asked to resign the editorship. This led to intense dispute in the Working Group, eventually solved by the appointment of Microsoft's Jean Paoli as a third co-editor.

The XML Working Group never met face-to-face; the design was accomplished using a combination of email and weekly teleconferences. The major design decisions were reached in twenty weeks of intense work between July and November 1996, when the first Working Draft of an XML specification was published.^[20] Further design work continued through 1997, and XML 1.0 became a W3C Recommendation on February 10, 1998.

XML 1.0 achieved the Working Group's goals of Internet usability, general-purpose usability, SGML compatibility, facilitation of easy development of processing software, minimization of optional features, legibility, formality, conciseness, and ease of authoring. Like its antecedent SGML, XML allows for some redundant syntactic constructs and includes repetition of element identifiers. In these respects, terseness was not considered essential in its structure.

[edit] Sources

XML is a profile of an ISO standard SGML, and most of XML comes from SGML unchanged. From SGML comes the separation of logical and physical structures (elements and entities), the availability of grammar-based validation (DTDs), the separation of data and metadata (elements and attributes), mixed content, the separation of processing from representation (processing instructions), and the default angle-bracket syntax. Removed were the SGML Declaration (XML has a fixed delimiter set and adopts Unicode as the document character set).

Other sources of technology for XML were the Text Encoding Initiative (TEI), which defined a profile of SGML for use as a 'transfer syntax'; HTML, in which elements were synchronous with their resource, the separation of document character set from resource encoding, the xml:lang attribute, and the HTTP notion that metadata accompanied the resource rather than being needed at the declaration of a link. Largely in response to discussion with the XML committee, SGML added the Extended Reference Concrete Syntax (ERCS), from which XML 1.0's naming rules were then taken, and introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters.

Ideas that developed during discussion which were novel in XML, were the algorithm for encoding detection and the encoding header, the processing instruction target, the xml:space attribute, and the new close delimiter for empty-element tags. The notion of well-formedness as opposed to validity (which enables parsing without a schema) was first formalized in XML, though Bosak^[21] notes that it traces to Steven DeRose, an XML committee member who in 1989 implemented the same notion in DynaText, an early SGML browser and index system.

[edit] Versions

There are two current versions of XML. The first, XML 1.0, was initially defined in 1998. It has undergone minor revisions since then, without being given a new version number, and is currently in its fifth edition, as published on November 26, 2008. It is widely implemented and still recommended for general use. The second, XML 1.1, was initially published on February 4, 2004, the same day as XML 1.0 Third Edition, and is currently in its second edition, as published on August 16, 2006. It contains features — some contentious — that are intended to make XML easier to use in certain cases^[22] - mainly enabling the use of line-ending characters used on EBCDIC platforms, and the use of scripts and characters absent from Unicode 2.0. XML 1.1 is not very widely implemented and is recommended for use only by those who need its unique features. ^[23]

Prior to its fifth edition release, XML 1.0 differed from XML 1.1 in the requirements of characters used for element and attribute names: the first four editions of XML 1.0 only allowed characters which are defined in Unicode 2.0, which includes most world scripts, but excludes those which were added in later Unicode versions. Among the excluded scripts are Mongolian, Cambodian, Amharic, Burmese, and others.

Almost any Unicode character can be used in the character data and attribute values of an XML 1.1 document, even if the character is not defined, aside from having a code point, in the current version of Unicode. The approach in XML 1.1 is that only certain characters are forbidden, and everything else is allowed, whereas in older editions of XML 1.0, only certain characters were explicitly allowed, and thus prior to its fifth edition XML 1.0 could not accommodate the addition of characters in future versions of Unicode.

In character data and attribute values, XML 1.1 allows the use of more control characters than XML 1.0, but, for "robustness", most of the control characters introduced in XML 1.1 must be expressed as numeric character references. Among the supported control characters in XML 1.1 are two line break codes that must be treated as whitespace. Whitespace characters are the only control codes that can be written directly.

There has been discussion of an XML 2.0, although no organization has announced plans for work on such a project. XML-SW (SW for skunk works), written by one of the original developers of XML, contains some proposals for what an XML 2.0 might look like: elimination of DTDs from syntax, integration of namespaces, XML Base and XML Information Set (infoset) into the base standard.

The World Wide Web Consortium also has an XML Binary Characterization Working Group doing preliminary research into use cases and properties for a binary encoding of the XML infoset. The working group is not chartered to produce any official standards. Since XML is by definition text-based, ITU-T and ISO are using the name Fast Infoset for their own binary infoset to avoid confusion (see ITU-T Rec. X.891 | ISO/IEC 24824-1).

[edit] Criticism

[edit] Standards

In addition to the ISO standards mentioned above, other related document include:

[edit] See also

Wikibooks has more on the topic of XML

• Binary XML
• XML Protocol
• List of XML markup languages
• Category:XML-based standards
• Comparison of layout engines (XML)
• Comparison of data serialization formats
• List of computer standards

[edit] References

[edit] Further reading

• Annex A of ISO 8879:1986 (SGML)
• Lawrence A. Cunningham (2005). "Language, Deals and Standards: The Future of XML Contracts". Washington University Law Review. SSRN 900616. 
• "XML and the Second-Generation Web". Scientific American. 1999.  Online at XML and the Second-Generation Web

[edit] External links

• W3C XML homepage
• Introduction to Generalized Markup by Charles Goldfarb
• Making Mistakes with XML by Sean Kelly
• The Multilingual WWW by Gavin Nicol
• Retrospective on Extended Reference Concrete Syntax by Rick Jelliffe
• XML, Java and the Future of the Web by Jon Bosak
• XML tutorials in w3schools
• XML.gov
• Thinking XML: The XML decade by Uche Ogbuji
• XML: Ten year anniversary by Elliot Kimber
• Closing Keynote, XML 2006 by Jon Bosak
• Five years later, XML... by Simon St. Laurent
• 23 XML fallacies to watch out for by Sean McGrath
• W3C XML is Ten!, XML 10 years press release