About Extensible Markup Language (XML)
The Extensible Markup Language (XML) is a W3C-recommended general-purpose
markup language for creating special-purpose markup languages, capable of describing many
different kinds of data. It is a simplified subset of SGML. Its primary purpose is to
facilitate the sharing of data across different systems, particularly systems connected via
the Internet. Languages based on XML (for example, Geography Markup Language (GML), RDF/XML,
RSS, MathML, Physical Markup Language (PML), XHTML, SVG, MusicXML and cXML) are defined in a
formal way, allowing programs to modify and validate documents in these languages without
prior knowledge of their form.
History:
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. Jon Bosak argued that the W3C should sponsor an “SGML on
the Web” activity. After some resistance he was authorized to launch that activity in
mid-1996, albeit with little involvement by or support from the W3C leadership. Bosak was
well-connected in the small community of people who had experience both in SGML and the Web.
He received support in his efforts from Microsoft.
XML was designed by an eleven-member Working Group 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. 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 of 1996. 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.
Clarifications and minor changes were accumulated in published errata and
then incorporated into a Second Edition of the XML 1.0 Recommendation on October 6, 2000.
Subsequent errata were incorporated into a Third Edition on February 4, 2004.
Also published on the same day as XML 1.0 Third Edition was XML 1.1, a
variant of XML that encourages more consistency in how characters are represented and relaxes
restrictions on names, allowable characters, and end-of-line representations.
Both XML 1.0 Third Edition and XML 1.1 are considered current versions of
XML.
Features of XML:
XML provides a text-based means to describe and apply a tree-based structure
to information. At its base level, all information manifests as text, interspersed with
markup that indicates the information’s separation into a hierarchy of character data,
container-like elements, and attributes of those elements. In this respect, it is similar to
the LISP programming language’s S-expressions, which describe tree structures wherein each
node may have its own property list.
The fundamental unit in XML is the character, as defined by the Universal
Character Set. Characters are combined in certain allowable combinations to form an XML
document. The document consists of one or more entities, each of which is typically some
portion of the document’s characters, encoded as a series of bits and stored in a text
file.
The ubiquity of text file authoring software (word processors) facilitates
rapid XML document authoring and maintenance, whereas prior to the advent of XML, there were
very few data description languages that were general-purpose, Internet protocol-friendly,
and very easy to learn and author. In fact, most data interchange formats were proprietary,
special-purpose, “binary” formats (based foremost on bit sequences rather than characters)
that could not be easily shared by different software applications or across different
computing platforms, much less authored and maintained in common text editors.
By leaving the names, allowable hierarchy, and meanings of the elements and
attributes open and definable by a customizable schema, XML provides a syntactic foundation
for the creation of custom, XML-based markup languages. The general syntax of such languages
is rigid — documents must adhere to the general rules of XML, assuring that all XML-aware
software can at least read (parse) and understand the relative arrangement of information
within them. The schema merely supplements the syntax rules with a set of constraints.
Schemas typically restrict element and attribute names and their allowable containment
hierarchies, such as only allowing an element named “birthday” to contain 1 element named
“month” and 1 element named “day”, each of which has to contain only character data. The
constraints in a schema may also include data type assignments that affect how information is
processed; for example, the “month” element’s character data may be defined as being a month
according to a particular schema language’s conventions, perhaps meaning that it must not
only be formatted a certain way, but also must not be processed as if it were some other type
of data.
In this way, XML contrasts with HTML, which has an inflexible,
single-purpose vocabulary of elements and attributes that, in general, cannot be repurposed.
With XML, it is much easier to write software that accesses the document’s information, since
the data structures are expressed in a formal, relatively simple way.
XML makes no prohibitions on how it is used. Although XML is fundamentally
text-based, software quickly emerged to abstract it into other, richer formats, largely
through the use of datatype-oriented schemas and object-oriented programming paradigms (in
which the document is manipulated as an object). Such software might only treat XML as
serialized text when it needs to transmit data over a network, and some software doesn’t even
do that much. Such uses have led to “binary XML”, the relaxed restrictions of XML 1.1, and
other proposals that run counter to XML’s original spirit and thus garner an amount of
criticism.
Strengths and weaknesses:
Some features of XML that make it well-suited for data transfer are:
its simultaneously human- and machine-readable format;
it has support for Unicode, allowing almost any information in any human language to be
communicated;
the ability to represent the most general computer science data structures: records, lists
and trees;
the self-documenting format that describes structure and field names as well as specific
values;
the strict syntax and parsing requirements that allow the necessary parsing algorithms to
remain simple, efficient, and consistent.
XML is also heavily used as a format for document storage and processing, both online and
offline, and offers several benefits:
its robust, logically-verifiable format is based on international
standards;
the hierarchical structure is suitable for most (but not all) types of documents;
it manifests as plain text files, unencumbered by licenses or restrictions;
it is platform-independent, thus relatively immune to changes in technology;
it and its predecessor, SGML, have been in use since 1986, so there is extensive experience
and software available.
For certain applications, XML also has the following weaknesses:
Its syntax is fairly verbose and partially redundant. This can hurt human
readability and application efficiency, and yields higher storage costs. It can also make XML
difficult to apply in cases where bandwidth is limited, though compression can reduce the
problem in some cases. This is particularly true for multimedia applications running on cell
phones and PDAs which want to use XML to describe images and video.
Parsers should be designed to recourse arbitrarily nested data structures and must perform
additional checks to detect improperly formatted or differently ordered syntax or data (this
is because the markup is descriptive and partially redundant, as noted above). This causes a
significant overhead for most basic uses of XML, particularly where resources may be scarce —
for example in embedded systems. Furthermore, additional security considerations arise when
XML input is fed from untrustworthy sources, and resource exhaustion or stack overflows are
possible.
Some consider the syntax to contain a number of obscure, unnecessary features born of its
legacy of SGML compatibility. However, an effort to settle on a subset called “Minimal XML”
led to the discovery that there was no consensus on which features were in fact obscure or
unnecessary.
The basic parsing requirements do not support a very wide array of data types, so
interpretation sometimes involves additional work in order to process the desired data from a
document. For example, there is no provision in XML for mandating that “3.14159” is a
floating-point number rather than a seven-character string. XML schema languages add this
functionality.
Modeling overlapping (non-hierarchical) data structures requires extra effort.
Mapping XML to the relational or object oriented paradigms is often cumbersome.
Some have argued that XML can be used as a data storage only if the file is of low volume,
but this is only true given particular assumptions about architecture, data, implementation,
and other issues.
<< Back to .XML page
Links:
World Wide Web Consortium XML
homepage
The XML 1.0 specification
The XML 1.1 specification
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