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Markup language and file format
For other uses, see
XML (disambiguation)
XML (standard)
Extensible Markup Language
Abbreviation
XML
Status
Published,
W3C recommendation
Year started
1996
; 30 years ago
1996
First published
February 10, 1998
; 28 years ago
1998-02-10
Latest version
1.1 (2nd ed.)
September 29, 2006
; 19 years ago
2006-09-29
Organization
World Wide Web Consortium
(W3C)
Editors
Tim Bray
Jean Paoli
Michael Sperberg-McQueen
, Eve Maler, François Yergeau,
John W. Cowan
Base standards
SGML
Related standards
W3C XML Schema
Domain
Serialization
XML (file format)
Filename extension
.xml
Internet media type
application/xml
text/xml
Uniform Type Identifier (UTI)
public.xml
UTI conformation
public.text
Magic number
World Wide Web Consortium
Type of format
Markup language
Extended from
SGML
Extended to
Numerous languages
, including
XHTML
RSS
Atom
, and
KML
Standard
1.0, 5th ed.
(November 26, 2008
2008-11-26
1.1, 2nd ed.
(August 16, 2006
2006-08-16
Open format
Yes
Free format
Yes
Extensible Markup Language
XML
) 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
(APIs) to aid the processing of XML data.
Overview
edit
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.
10
XML tags represent the data structure and contain
metadata
. What is within the tags is data, encoded in the way the XML standard specifies.
10
An additional
XML schema
(XSD) defines the necessary metadata for interpreting and validating XML. (This is also referred to as the canonical schema.)
11
An XML document that adheres to basic XML rules is "well-formed"; one that adheres to its schema is "valid".
11
IETF
RFC 7303
(which supersedes the older
RFC 3023
), provides rules for the construction of
media types
for use in XML message. It defines three media types:
application/xml
text/xml
is an alias),
application/xml-external-parsed-entity
text/xml-external-parsed-entity
is an alias) 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
edit
XML has come into common use for the interchange of data over the Internet. Hundreds of document formats using XML syntax have been developed,
12
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 (AJAX)
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.
13
Key terminology
edit
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.
Character
An XML document is a string of
characters
Every
legal
Unicode
character (except Null) may appear in an (1.1) XML document (while some are discouraged).
Processor and application
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
(as the specification calls it) is often referred to colloquially as an
XML
parser
Markup and content
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
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.
Tag
tag
is a markup construct that begins with
and ends with
. There are three types of tag:
start-tag
, such as
end-tag
, such as
empty-element tag
, such as
Element
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
. Another is
Attribute
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
, where the names of the attributes are "src" and "alt", and their values are "madonna.jpg" and "Madonna" respectively. Another example is
, 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,
ii
a space-delimited list can be used. An example with space as a delimiter is
Welcome!
, where the attribute "class" both has the value "inner greeting-box" and also indicates the two
CSS
class names "inner" and "greeting-box".
XML declaration
XML documents may begin with an
XML declaration
that describes some information about themselves. An example is
Characters and escaping
edit
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
edit
Main article:
Valid characters in XML
Unicode code points in the following ranges are valid in XML 1.0 documents:
14
U+0009 (Horizontal Tab), U+000A (Line Feed), U+000D (Carriage Return): 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
(all surrogates, U+FFFE and U+FFFF are forbidden);
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.
15
At the same time, however, it restricts the use of C0 and
C1
control characters other than U+0009 (Horizontal Tab), U+000A (Line Feed), U+000D (Carriage Return), and U+0085 (Next Line) by requiring them to be written in escaped form (for example U+0001 must be written as
or its equivalent). 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
(Null) is the only character that is not permitted in any XML 1.1 document.
Encoding detection
edit
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
(which the XML standard recommends using, without a
BOM
) and
UTF-16
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.
17
Encodings other than UTF-8 and UTF-16 are not necessarily recognized by every XML parser (and in some cases not even UTF-16, even though the standard mandates it to also be recognized).
Escaping
edit
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.
18
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
) "А" and the
Latin capital letter A
A
) "A".
There are five
predefined entities
<
represents "<";
>
represents ">";
&
represents "&";
'
represents "
";
"
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
中
or
中
. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as
I <3 Jörg
�
is not permitted because the
null character
is one of the control characters excluded from XML, even when using a numeric character reference.
19
An alternative encoding mechanism such as
Base64
is needed to represent such characters.
Comments
edit
Comments may appear anywhere in a document outside other markup. Comments cannot appear before the XML declaration. Comments begin with
. For compatibility with
SGML
, the string "--" (double-hyphen) is not allowed inside comments;
20
this means comments cannot be nested. The ampersand has no special significance within comments, so entity and character references are not recognized as such, and there is no way to represent characters outside the character set of the document encoding.
An example of a valid comment:
International use
edit
This example contains
Armenian
text.
Without proper
rendering support
, you may see
question marks, boxes, or other symbols
instead of Armenian letters.
XML 1.0 (Fifth Edition) and XML 1.1 support the direct use of almost any
Unicode
character in element names, attributes, comments, character data, and processing instructions (other than the ones that have special symbolic meaning in XML itself, such as the less-than sign, "<"). The following is a well-formed XML document including
Chinese
Armenian
and
Cyrillic
characters:
<俄语
լեզու=
"ռուսերեն"
данные
Syntactical correctness and error-handling
edit
Main article:
Well-formed document
The XML specification defines an XML document as a
well-formed
text, meaning that it satisfies a list of syntax rules provided in the specification. Some key points include:
The document 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 start-tag, end-tag, and empty-element tag that delimit elements are correctly nested, with
none missing
and none overlapping.
Tag names are case-sensitive; the start-tag and end-tag must match exactly.
Tag names cannot contain any of the characters
!"#$%&'()*+,/;<=>?@[\]^`{|}~
, nor a space character, and cannot begin with "-", ".", or a numeric digit.
A single root element contains all the other elements.
The definition of an XML document excludes texts that contain violations of well-formedness rules; they are simply not XML. An XML processor that encounters such a violation is required to report such errors and to cease normal processing.
21
22
This policy, occasionally referred to as "
draconian
error handling", stands in notable contrast to the behavior of programs that process
HTML
, which are designed to produce a reasonable result even in the presence of severe markup errors.
23
XML's policy in this area has been criticized as a violation of
Postel's law
("Be conservative in what you send; be liberal in what you accept").
24
The XML specification defines a
valid XML document
as a
well-formed XML document
which also conforms to the rules of a
Document Type Definition
(DTD).
25
Schemas and validation
edit
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 check XML documents for validity.
26
A processor that discovers a validity error must be able to report it, but may continue normal processing.
A DTD is an example of a
schema
or
grammar
. 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.
Document type definition
edit
Main article:
Document type definition
The oldest schema language for XML is the
document type definition
(DTD), inherited from SGML.
DTDs have the following benefits:
DTD support is ubiquitous due to its inclusion in the XML 1.0 standard.
DTDs are terse compared to element-based schema languages and consequently present more information in a single screen.
DTDs allow the declaration of
standard public entity sets
for publishing characters.
DTDs define a
document type
rather than the types used by a namespace, thus grouping all constraints for a document in a single collection.
DTDs have the following limitations:
They have no explicit support for newer
features
of XML, most importantly
namespaces
They lack expressiveness. XML DTDs are simpler than SGML DTDs and there are certain structures that cannot be expressed with regular grammars. DTDs only support rudimentary datatypes.
They lack readability. DTD designers typically make heavy use of parameter entities (which behave essentially as textual
macros
), which make it easier to define complex grammars, but at the expense of clarity.
They use a syntax based on
regular expression
syntax, inherited from SGML, to describe the schema. Typical XML APIs such as
SAX
do not attempt to offer applications a structured representation of the syntax, so it is less accessible to programmers than an element-based syntax may be.
Two peculiar features that distinguish DTDs from other schema types are the syntactic support for embedding a DTD within XML documents and for defining
entities
, which are arbitrary fragments of text or markup that the XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes.
DTD technology is still used in many applications because of its ubiquity.
Schema
edit
Main article:
XML Schema (W3C)
A newer 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.
xs:schema element that defines a schema:
"http://www.w3.org/2001/XMLSchema"
>
RELAX NG
edit
Main article:
RELAX NG
RELAX NG
(Regular Language for XML Next Generation) was initially specified by
OASIS
and is now a standard (Part 2:
Regular-grammar-based validation
of
ISO/IEC 19757 – 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 conversion tool—
Trang
—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.
Schematron
edit
Schematron
is a language for making
assertions
about the presence or absence of patterns in an XML document. It typically uses
XPath
expressions. Schematron is now a standard (Part 3:
Rule-based validation
of
ISO/IEC 19757 – DSDL
).
DSDL and other schema languages
edit
DSDL
(Document Schema Definition Languages) is a multi-part ISO/IEC standard (ISO/IEC 19757) that 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 the
infoset
augmentation facility and attribute defaults. RELAX NG and Schematron intentionally do not provide these.
Related specifications
edit
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 together with one or more of these other technologies that 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. Although XML Namespaces are not part of the XML specification itself, virtually all XML software also supports XML Namespaces.
XML Base
defines the
xml:base
attribute, which may be used to set the base for resolution of relative URI references within the scope of a single XML element.
XML Information Set
or XML Infoset is 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.
XSL
(Extensible Stylesheet Language) is a family of languages used to transform and render XML documents, split into three parts:
XSLT
(XSL Transformations), an XML language for transforming XML documents into other XML documents or other formats such as HTML, plain text, or XSL-FO. XSLT is very tightly coupled with XPath, which it uses to address components of the input XML document, mainly elements and attributes.
XSL-FO
(XSL Formatting Objects), an XML language for rendering XML documents, often used to generate PDFs.
XPath
(XML Path Language), a non-XML language for addressing the components (elements, attributes, and so on) of an XML document. XPath is widely used in other core-XML specifications and in programming libraries for accessing XML-encoded data.
XQuery
(XML Query) is an XML query language strongly rooted in XPath and XML Schema. It provides methods to access, manipulate and return XML, and is mainly conceived as a query language for
XML databases
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.
XML model (Part 11:
Schema Association
of
ISO/IEC 19757 – DSDL
) defines a means of associating any xml document with any of the schema types mentioned
above
Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including
XInclude
XLink
, and
XPointer
Programming interfaces
edit
The design goals of XML include, "It shall be easy to write programs which process XML documents."
Despite this, the XML specification contains almost no information about how programmers might go about doing such processing. The
XML Infoset
specification provides a vocabulary to refer to the constructs within an XML document, but 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
XML data binding
, which provides an automated translation between an XML document and programming-language objects.
Declarative transformation languages such as
XSLT
and
XQuery
Syntax extensions to general-purpose programming languages, for example
LINQ
and
Scala
Stream-oriented facilities require less memory and, for certain tasks based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and data-binding APIs 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
Simple API for XML
edit
Main article:
Simple API for XML
Simple API for XML
(SAX) is a
lexical
event-driven
API 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.
Pull parsing
edit
Pull parsing treats the document as a series of items 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 functions performing the parsing, or passed down (as function parameters) into lower-level functions, or returned (as function return values) to higher-level functions.
27
Examples of pull parsers include
Data::Edit::Xml
in
Perl
StAX
in the
Java
programming language, XMLPullParser in
Smalltalk
, XMLReader in
PHP
ElementTree.iterparse
in
Python
, SmartXML in
Red
System.Xml.XmlReader
in the
.NET Framework
, and the DOM traversal API (
NodeIterator
and
TreeWalker
).
A pull parser creates an iterator that sequentially visits the various elements, attributes, and data in an XML document. Code that uses this iterator can test the current item (to tell, for example, whether it is a start-tag or end-tag, 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 that are parent elements of the element being parsed. Pull-parsing code can be more straightforward to understand and maintain than SAX parsing code.
Document Object Model
edit
Main article:
Document Object Model
The
Document Object Model
(DOM) is an 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.
Data binding
edit
XML data binding
is a technique for simplifying development of applications that need to work with XML documents. It involves mapping the XML document to a hierarchy of strongly typed objects, rather than using the generic objects created by a DOM parser. The resulting code is often easier to read and maintain, and it can help to identify problems at compile time rather than run-time. XML data binding is particularly well-suited for applications where the document structure is known and fixed at the time the application is written. By creating a strongly typed representation of the XML data, developers can take advantage of modern integrated development environments (IDEs) that provide features like auto-complete, code refactoring, and code highlighting. This can make it easier to write correct and efficient code, and reduce the risk of errors and bugs. Example data-binding systems include the
Java Architecture for XML Binding
(JAXB), XML Serialization in
.NET Framework
28
and XML serialization in
gSOAP
XML as data type
edit
XML has appeared 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 node lists as distinct objects and use a dot-notation specifying parent-child relationships.
29
E4X is supported by the
Mozilla
2.5+ browsers (though now deprecated) and Adobe
Actionscript
but has not been widely adopted. Similar notations are used in Microsoft's
LINQ
implementation for Microsoft .NET 3.5 and above, and in
Scala
(which uses the Java VM). 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.
30
The
Resource Description Framework
defines a data type
rdf:XMLLiteral
to hold wrapped,
canonical XML
31
Facebook has produced extensions to the
PHP
and
JavaScript
languages that add XML to the core syntax in a similar fashion to E4X, namely
XHP
and
JSX
respectively.
History
edit
XML is an application
profile
of
SGML
(ISO 8879).
32
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.
22
33
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.
34
XML was compiled by a
working group
of eleven members,
35
supported by a (roughly) 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.
36
James Clark
served as Technical Lead of the Working Group, notably contributing the empty-element
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).
37
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.
38
The XML Working Group communicated primarily through email and weekly teleconferences. The major design decisions were reached in a short burst of intense work between August and November 1996,
39
when the first Working Draft of an XML specification was published.
40
Further design work continued through 1997, and XML 1.0 became a
W3C
Recommendation on February 10, 1998.
Sources
edit
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. The SGML declaration was removed; thus, XML has a fixed delimiter set and adopts
Unicode
as the document
character set
Other sources of technology for XML were the
TEI
(Text Encoding Initiative), which defined a profile of SGML for use as a "transfer syntax" and
HTML
. The ERCS (Extended Reference Concrete Syntax) project of the SPREAD (Standardization Project Regarding East Asian Documents) project of the ISO-related China/Japan/Korea Document Processing expert group was the basis of XML 1.0's naming rules; SPREAD also introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters. To support ERCS, XML and HTML better, the SGML standard IS 8879 was revised in 1996 and 1998 with WebSGML Adaptations.
Ideas that developed during discussion that are novel in XML included 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, although it had been implemented successfully in the Electronic Book Technology "Dynatext" software;
41
the software from the University of Waterloo New Oxford English Dictionary Project; the RISP LISP SGML text processor at Uniscope, Tokyo; the US Army Missile Command IADS hypertext system; Mentor Graphics Context; Interleaf and Xerox Publishing System.
Versions
edit
1.0 and 1.1
edit
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,
42
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.
43
The main changes are to enable the use of line-ending characters used on
EBCDIC
platforms, and the use of scripts and characters absent from Unicode 3.2. XML 1.1 is not very widely implemented and is recommended for use only by those who need its particular features.
44
Prior to its fifth edition release, XML 1.0 differed from XML 1.1 in having stricter requirements for characters available for use in element and attribute names and unique identifiers: in the first four editions of XML 1.0 the characters were exclusively enumerated using a specific version of the
Unicode
standard (Unicode 2.0 to Unicode 3.2.) The fifth edition substitutes the mechanism of XML 1.1, which is more future-proof but reduces
redundancy
. The approach taken in the fifth edition of XML 1.0 and in all editions of XML 1.1 is that only certain characters are forbidden in names, and everything else is allowed to accommodate suitable name characters in future Unicode versions. In the fifth edition, XML names may contain characters in the
Balinese
Cham
, or
Phoenician
scripts among many others added to Unicode since Unicode 3.2.
43
Almost any Unicode code point can be used in the character data and attribute values of an XML 1.0/1.1 document, even if the character corresponding to the code point is not defined in the current version 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 (and #x7F through #x9F, which had been allowed in XML 1.0, are in XML 1.1 even required to be expressed as numeric character references
43
). Among the supported control characters in XML 1.1 are two line break codes that must be treated as whitespace characters, which are the only control codes that can be written directly.
2.0
edit
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
skunkworks
), which one of the original developers of XML has written,
45
contains some proposals for what an XML 2.0 might look like, including elimination of DTDs from syntax, as well as integration of
XML namespaces
XML Base
and
XML Information Set
into the base standard.
MicroXML
edit
In 2012,
James Clark
(technical lead of the XML Working Group) and
John Cowan
(editor of the XML 1.1 specification) formed the MicroXML Community Group within the W3C and published MicroXML, a specification for a significantly reduced subset of XML.
46
MicroXML provides a much simpler core syntax by stripping away many features of full XML, such as document type declarations and CDATA sections,
21
while ensuring XML namespace validity by disallowing names conflicting with namespace prefixing.
Binary XML
edit
Main article:
Binary XML
Due to the verbosity of textual XML, various binary formats have been proposed as compact representations for XML:
Fast Infoset
, based on
ASN.1
, was published as an international standard by the
ITU-T
in 2005, and later by
ISO
Efficient XML Interchange
(EXI), a binary XML format originally developed by AgileDelta, was adopted as a W3C recommendation in 2011, with a second edition published in 2014.
Criticism
edit
XML and its extensions have regularly been criticized for verbosity, complexity and redundancy.
47
Mapping the basic tree model of XML to
type systems
of programming languages or databases can be difficult, especially when XML is used for exchanging highly structured data between applications, which was not its primary design goal. However,
XML data binding
systems allow applications to access XML data directly from objects representing a
data structure
of the data in the programming language used, which ensures
type safety
, rather than using the
DOM
or
SAX
to retrieve data from a direct representation of the XML itself. This is accomplished by automatically creating a mapping between elements of the XML schema
XSD
of the document and members of a class to be represented in memory.
Other criticisms attempt to refute the claim that XML is a
self-describing
language
48
(though the XML specification itself makes no such claim).
JSON
YAML
, and
S-Expressions
are frequently proposed as simpler alternatives (see
Comparison of data-serialization formats
49
that focus on representing highly structured data rather than documents, which may contain both highly structured and relatively unstructured content. However, W3C-standardized XML schema specifications offer a broader range of structured
XSD
data types compared to simpler serialization formats and offer modularity and reuse through
XML namespaces
See also
edit
AIDX
Binary XML
Comparison of data-serialization formats
EBML
Extensible programming
List of XML markup languages
List of types of XML schemas
Simple XML
WBXML
XML Protocol
Notes
edit
i.e., embedded quote characters would be a problem
A common example of this is
CSS
class or identifier names.
References
edit
Citations
edit
Thompson, H.; Lilley, C. (July 2014).
XML Media Types
. Internet Engineering Task Force.
doi
10.17487/RFC7303
RFC
7303
Bray et al. (2008)
Bray, T.; Paoli, J.; Sperberg-McQueen, C. M., eds. (10 February 1998).
"Extensible Markup Language (XML) 1.0"
(W3C Recommendation)
(1st ed.). W3C.
Bikakis, N.; Tsinaraki, C.; Gioldasis, N.; Stavrakantonakis, I.; Christodoulakis, S. (2013).
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319–
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ISBN
978-3-642-28977-4
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Fennell, Philip (June 2013).
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XML London 2013
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80–
86.
doi
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ISBN
978-0-9926471-0-0
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on Mar 1, 2023.
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TechTarget WhatIs
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2017
World Meteorological Organization
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Manual on Codes, Volume I.3 – International Codes, Annex II to the WMO Technical Regulations, Part D – Representations derived from data models
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Bray et al. (2006)
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Bray, T. (April 26, 2003).
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Tbray.org
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16 November
2017
Bray et al. (2008)
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Bray et al. (2008)
, section 2.3.
Texin, Tex; Yergeau, François (6 September 2003).
"W3C I18N FAQ: HTML, XHTML, XML and Control Codes"
W3C Internationalization
. W3C
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16 November
2017
Bray et al. (2008)
, section 2.5.
Stührenberg, Maik (February 2013).
Quo vadis XML? History and possible future directions of the Extensible Markup Language
XML Prague 2013
(PDF)
. Prague. pp.
141–
162.
ISBN
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Gray, Jim (1 February 2005).
"A Conversation with Tim Bray: Searching for ways to tame the world's vast stores of information"
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Pilgrim, Mark (6 August 2010).
HTML5: Up and Running: Dive into the Future of Web Development
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"There are No Exceptions to Postel's Law [dive into mark]"
DiveIntoMark.org
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on 2011-05-14
. Retrieved
22 April
2013
Harold & Means (2002)
, p. 29.
Harold & Means (2002)
, p. 8.
DuCharme, Bob.
"Push, Pull, Next!"
Xml.com
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16 November
2017
Obasanjo, Dare (30 June 2006).
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Microsoft Developer Network
. Retrieved
31 July
2009
"Processing XML with E4X"
Mozilla Developer Center
. Mozilla Foundation. Archived from
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on 2011-05-01
. Retrieved
2010-07-27
"XML Shell: Core Syntax"
Xmlsh.org
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on 2010-05-03
. Retrieved
22 August
2010
Klyne, G.; Carroll, J. J., eds. (10 February 2004).
"Resource Description Framework (RDF): Concepts and Abstract Syntax"
(W3C Recommendation)
. W3C. section 5.1.
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(PDF)
ISO
IEC
. 1 June 2006. p. vi
. Retrieved
January 1,
2025
Ambron, Sueann & Hooper, Kristina, eds. (1988). "Publishers, multimedia, and interactivity".
Interactive multimedia
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ISBN
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Kimber, W. Eliot (November 26, 2006).
"XML: Ten Year Aniversary"
Dr. Macro's XML Rants
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16 November
2017
The working group was originally called the "Editorial Review Board". The original members and seven who were added before the first edition was complete, are listed at the end of the first edition of the XML Recommendation (
Bray, Paoli & Sperberg-McQueen 1998
).
Sperberg-McQueen, C. M., ed. (4 December 1997).
"Reports From the W3C SGML ERB to the SGML WG And from the W3C XML ERB to the XML SIG"
. W3C
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2009
Warwick, C.; Pritchard, E. (2000).
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ASLIB Proceedings
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Sun Developer Network
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. Retrieved
16 November
2017
Bray, T.; Sperberg-McQueen, C. M., eds. (14 November 1996).
"Extensible Markup Language (XML)"
(W3C Working Draft)
. W3C
. Retrieved
31 July
2009
Bosak, Jon
(12 July 2006).
Closing Keynote
. XML 2006. Archived from
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on 2007-07-11
. Retrieved
31 July
2009
Bray, T.; Paoli, J.; Sperberg-McQueen, C. M.; Maler, E.; Yergeau, F, eds. (4 February 2004).
"Extensible Markup Language (XML) 1.0"
(W3C Recommendation)
(3rd ed.). W3C.
Bray et al. (2006)
, section 1.3.
Harold, Elliotte Rusty (2004).
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. Addison-Wesley. pp.
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ISBN
0-321-15040-6
Bray, Tim (10 February 2002).
"Extensible Markup Language, SW (XML-SW)"
Textuality
"MicroXML Community Group"
W3C
. 2012-10-01
. Retrieved
2023-08-05
"XML: The Angle Bracket Tax"
Codinghorror.com
. 11 May 2008. Archived from
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. Retrieved
16 November
2017
"The Myth of Self-Describing XML"
(PDF)
Workflow.HealthBase.info
. September 2003
. Retrieved
16 November
2017
"What usable alternatives to XML syntax do you know?"
StackOverflow.com
. Retrieved
16 November
2017
Bibliography
edit
Bray, T.; Paoli, J.; Sperberg-McQueen, C. M.; Maler, E.; Yergeau, F, eds. (26 November 2008).
"Extensible Markup Language (XML) 1.0"
(W3C Recommendation)
(5th ed.). W3C.
Bray, T.; Paoli, J.; Sperberg-McQueen, C. M.; Maler, E.; Yergeau, F; Cowan, J., eds. (16 August 2006).
"Extensible Markup Language (XML) 1.1"
(W3C Recommendation)
(2nd ed.). W3C.
Dykes, Lucinda (2005).
XML for Dummies
(4th ed.). Hoboken, N.J.: Wiley.
ISBN
978-0-7645-8845-7
Harold, E. R.; Means, W. S. (2002).
XML in a Nutshell
(2nd ed.). Sebastopol, CA: O'Reilly.
ISBN
978-0-5960-0292-3
Further reading
edit
Annex A of ISO 8879:1986 (SGML)
Cunningham, L. A. (2005). "Language, Deals and Standards: The Future of XML Contracts".
Washington University Law Review
84
(2):
313–
373.
SSRN
900616
Bosak, Jon; Bray, Tim (May 1999).
"XML and the Second-Generation Web"
Scientific American
280
(5): 89.
Bibcode
1999SciAm.280e..89B
doi
10.1038/scientificamerican0599-89
(inactive 12 July 2025). Archived from
the original
on 1 October 2009.
{{
cite journal
}}
: CS1 maint: DOI inactive as of July 2025 (
link
Kelly, Sean (February 6, 2006).
"Making Mistakes with XML"
Developer.com
. Archived from
the original
on 13 April 2021.
St. Laurent, Simon (February 12, 2003).
"Five Years Later, XML."
O'Reilly XML Blog
O'Reilly Media
. Retrieved
26 October
2010
"W3C XML is Ten!"
World Wide Web Consortium
. 12 February 2008
. Retrieved
26 October
2010
Geneves, Pierre
(October 2012).
"Introduction to XML"
(PDF)
Course Slides
. Project WAM.
Archived
(PDF)
from the original on 2015-10-16.
External links
edit
Wikimedia Commons has media related to
XML
Wikibooks has a book on the topic of:
Subject:XML
Official website
, World Wide Web Consortium (W3C)
XML 1.0 Specification
Retrospective on Extended Reference Concrete Syntax
Archived
2019-11-18 at the
Wayback Machine
by
Rick Jelliffe
XML, Java and the Future of the Web
(1997) by
Jon Bosak
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