Extensible Markup Language
An activity of the World Wide Web Consortium (W3C) organized and led by Sun Microsystems
Objective: move the Web to its next stage of evolution by adapting existing ISO standards for markup, linking, and formatting
Primary effects:
Will create new data-centric Web applications
Will fundamentally change publishing on the web and publishing in general
Two aspects of Web evolution demanded a technology beyond HTML.
Internationalized electronic publishing
Platform-independent
Language-independent
Media-independent
New data-centric Web applications
Database exchange
Distribution of processing to clients
Client-side manipulation of views into the data
Customization of information by intelligent agents
Management of document collections
HTML was optimized for easy learning
One tag set for all applications
Predefined semantics for each tag
Predefined data structures
No formal validation
HTML trades power for ease of use
HTML is well suited to simple applications, but poorly suited to more demanding applications
Large or complex collections of data
Data that must be used in different ways
Data with a long life cycle
Data intended to drive scripts or Java applets
XML provides key features needed for a new generation of Web applications:
Extensibility: Users can define new tags as needed
Structure: Hierarchical data can be modeled to any level of complexity
Validation: Data can be checked for structural correctness
Media independence: The same content can be published in multiple media
In industry, we knew from electronic publishing experience that HTML would not work for publishing in the general case.
We also knew that future Web applications would require a method of encoding that could drive arbitrarily complex distributed processes.
It was clear that if an open standard like XML was not
created, HTML would be replaced by a more powerful
Strategically, we had to have XML in order to keep Web data open and portable. We needed XML to do for data what Java does for programs.
The XML 1.0 Rec is being widely deployed
XML is being widely adopted as a framework for the definition of domain-specific languages
It is now generally agreed that Web content will be managed using standards based on XML
Key predictions:
XML will be the basis for future Web standards.
XML will become the universal format for data exchange in heterogenous environments.
XML will almost certainly become the basis for international publishing.
The combination of XML and XSL may replace all existing word processing and desktop publishing formats.
The W3C activity:
Standards and drafts:
Markup technology in general:
The XML family of languages moves the web to a new level of evolution suitable for electronic commerce and other industrial-strength applications.
XML (Extensible Markup Language): A subset of SGML (ISO 8879) designed for easy implementation
Will replace HTML markup in industrial contexts
XLink/XPointer: A set of standard hypertext mechanisms based on HyTime (ISO/IEC 10744) and the Text Encoding Initiative (TEI)
Will replace HTML linking in industrial contexts
XSL (Extensible Stylesheet Language): A standard stylesheet language for structured information based on DSSSL (ISO/IEC 10179) and CSS
Will replace CSS in industrial contexts
A simplified subset of SGML (ISO 8879)
Very powerful -- no limits on namespace or structural depth
But easy to implement and small enough for Web browsers
Not a language but a metalanguage
Designed to support the definition of an unlimited number of vertical-market languages for specific industries
All XML languages can be processed by a single lightweight parser built into every Web browser
XML allows industries to design specific tag languages to solve specific problems.
Examples featured in Robin Cover's SGML/XML News page in one recent 30-day period (3/15 to 4/15, 1999):
SVG (Scalable Vector Graphics)
XMLNews (for the news industry)
XCI (XML Court Interface)
DocBk XML (for software documentation)
XMI (XML Metadata Interface Format -- OMG)
WAP (Wireless Application Protocol)
SIF (Schools Interoperability Framework)
Key: An unlimited number of domain-specific tag languages can all be processed by a single parser.
"Syntax, not semantics"
Tags have no predefined meaning
XML by itself conveys only content and structure, not presentation or behavior (unlike HTML)
There are important applications for XML alone: interprocess communication, object serialization, metadata, database exchange
But associating
Downloadable programs, applets, or scripts designed for a specific tag set (grammar)
Tag-sensitive components (e.g., Java beans)
Industry agreements on the processing of specific grammars (example: HTML)
Stylesheets (XSL or CSS)
A document is data that you can
Documents are a
The basic problem with documents is that we need to display them
in lots of
XML allows you to specify the content and structure of a document in a way that lets you generate particular presentations as needed.
Legal XML documents are called
A well-formed document describes a
If a well-formed document conforms to an optional set of
constraints (a DTD), it is also
A well-formed XML document:
A valid XML document:
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Media-independent publishing works!
HTML can handle the online version (for the moment), but not the print version
The language for formatting specifications (stylesheets)
must support
Separating content and structure from presentation and behavior makes possible
Reusable information
Media-independent publishing
One-on-one marketing
Intelligent downstream document processing
Large-scale information management
XML has been based on Unicode from Day One
There is nothing in an XML file but Unicode characters
Unicode is used for both content and markup (so you can mix languages, even in tag names)
XML tools
UTF-8: 1-5 bytes; Latin-1 is upward-compatible
UTF-16: 2 bytes; fixed overhead
The widespread adoption of XML for data management and electronic commerce will probably make Unicode support universal
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The catalog example shows that the distinction between data exchange and publishing is ultimately an artificial one (the same source would also be used to create the printed catalog)
The rendition in each case occurs
The database owner can publish
Consider the alternative:
Generation of a different HTML output stream for
Much greater load on the server
No user autonomy
In electronic commerce, XML documents will be assembled on the fly from a wide variety of sources using different tag vocabularies (DTDs)
Must prevent collisions between elements (or attributes) with the same name but different meanings
For example, the element <RING> would have very different meanings in a jewelry catalogue, a chemistry textbook, and a mathematical journal
Must also allow re-use of common data elements (dates, currencies, measurements) across different XML tag languages
Ultimately, we will need a system for associating
XML Namespaces (http://www.w3.org/TR/) is a small first step toward solving this problem
An XML namespace is a collection of XML element and/or attribute names that are guaranteed to be unique
Basic trick: use DNS (Domain Name Service) to ensure uniqueness
DNS is the service that controls the ownership of domain names. It also provides the mechanism whereby names are resolved to actual resources, but DNS resolution is not necessary to make XML namespaces work.
Here the element name "price" is not unique:
Prefix the element name with a URI such as "http://ecommerce.org/schema"; now the name is unique (although verbose and syntactically illegal):
By substituting a namespace prefix for the URI we get a structure that is both elegant and legal:
Namespace scoping ensures that "edi:" means the same as "{http://ecommerce.org/schema}" only upon and within the element <x> on which it is declared.
Namespace prefixes are just
A namespace URI does not necessarily point to a web resource (although it may).
If there is a resource, it is as likely to be a prose description as a machine-processable schema.
Namespace scoping is cool but complicated.
Namespaces make traditional DTD validation highly problematic if not downright useless. The solution to this lies in the XML schema work.
We need much more namespace implementation experience before this technique can be considered fully cooked.