Язык разметки XML: Структурирование и обмен данными

Markup language by the W3C for encoding of data

Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary 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.

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. 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 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 is in the transfer of Operational meteorology (OPMET) information based on IWXXM standards.

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.

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. There are many other text encodings that predate Unicode, such as ASCII and various ISO/IEC 8859; their character repertoires are in every case subsets of the Unicode character set. XML allows the use of any of the Unicode defined encodings and any other encodings whose characters also appear in Unicode. XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used. Encodings other than UTF 8 and UTF 16 are not necessarily recognized by every XML parser (and in some cases not even UTF 16, even though the standard mandates it to also be recognized).

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 in the fairly lengthy list 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. 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. 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"). 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).

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. 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.

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 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 (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.

The design goals of XML include, "It shall be easy to write programs which process XML documents." 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 (DOM) is an API 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.

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, and XML serialization in gSOAP.

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. 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. The Resource Description Framework defines a data type rdf:XMLLiteral to hold wrapped, canonical XML. 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.

XML is an application profile of SGML (ISO 8879). 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. 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, 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. 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 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, when the first Working Draft of an XML specification was published. Further design work continued through 1997, and XML 1.0 became a W3C Recommendation on February 10, 1998.

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. 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. 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.). 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.

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, 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.

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 a specification for a significantly reduced subset of XML.

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 XML Information Set. 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 format (ITU T Rec. X.891 and ISO/IEC 24824 1) to avoid confusion.

XML and its extensions have regularly been criticized for verbosity, complexity and redundancy. 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 (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) 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.