What is 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 "" 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 recurse 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.
• Modelling 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 for 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.

About this Terminology
This terminology is from The Wikipedia which is published under the GNU Free Documentation License.