Group items tagged

Expanding needs of business and scientific communities The ever-growing need of industry and government to integrate disparate IT (information technology) systems has resulted in interconnected networks that can no longer be constructed, managed, or enhanced centrally.

The overall stability of such systems requires that the individual components be loosely coupled so that they can be managed, modified, and replaced without threatening the operational integrity of the entire system.

This requirement has become equally important for both internal operations of large enterprises and business-to-business interactions between enterprises of differing size.

System architects realized early that many specialized languages suited to specific domains were required to represent the numerous bodies of data used in those domains.

Each language needed well-defined and self-consistent mechanisms for describing data

Data elements needed to be easy to transport, transform, search, combine, extract, filter, and view in different forms.

The definitions of the languages themselves needed to be easily shared and maintained by a large body of users.

Role of open source/open standards To address these common requirements, a committee (more commonly referred to as a working group) of the World Wide Web Consortium was formed in 1996.

XML is rarely used alone. A body of standards emerged around XML to support its use.

Companies participated in and adopted the XML open standards and worked on open-source technology for many reasons:

XML and its related standards enabled data interoperability, content manipulation, content sharing and reuse, document assembly, document security and access control, document filtering, and document formatting across all disciplines and for all types of devices and applications.

XML could not have happened without the World Wide Web. The Web has become a universal mechanism to deliver information to consumers and increasingly, to applications as well.

This new generation of XML-centric interactions led to the birth of the Web Services platform

It enabled information reuse by integrating text and data from different sources and by searching and linking across these sources, thereby breaking down traditional silos, which were barriers to information sharing

XML also could not have had such impact without a diverse collection of tremendous advances in computer science made over approximately a 50-year period.

The value of information hiding, generalization, encapsulation, and reuse in programming languages and methodologies

XML provides a rich data representation, with significant opportunities for high-value semantic tagging, which can provide superior support for information retrieval and related activities.

Lisp language introduced the concept of an attribute list

A standardized nonprocedural, high-performance approach to storage and retrieval of structured information—Relational databases7 possessing a powerful data model, underlying concurrency control, integrity and performance benefits, and a consistent Structured Query Language (SQL)8 interface were a great advance of the 1970s and 1980s.

The utility of simple key-value pair tagging and its application to providing metadata through annotations—With GML (Generalized Markup Language)13 in the late 1970s and then SGML, which was standardized in 1986,

A consensus on a layered-protocol stack for network communication, which standardizes not only layered protocols but also the interfaces to those layers

XML is clearly an advanced presentation layer protocol,

The necessity and practicality of sophisticated user interfaces programmed with very high-level techniques—Metaphor-based interfaces (i.e., those in which the target audience interacts with aesthetic concepts familiar to their area of expertise)

Performance and bandwidth—Relative to optimized binary formats, XML is expensive to process (parse) and transmit and would not have been practical without the many decades during which processor performance and network bandwidth have been accelerating according to Moore's law.

Connecting business to business with Web Services

The ubiquity of XML and its ability to be used as an underlying specification language enabled a new generation of application-to-application communication, supporting flexible integration of heterogeneous systems in a variety of domains.

XML is more accurately a technology for labeling information with descriptive names that can be consistently used and accessed in a multitude of applications

The Web Services technology suite is also an important enabler of the SOAs that are now being embraced by the entire IT industry. SOA is an abstract architectural concept founded on the idea of building software systems with uniformly described, discoverable services that interact in a loosely coupled way and can be composed.

The success of Web Services in this arena can be attributed to the nonproprietary nature of the underlying technologies as well as the loose coupling supported by the technology.

the participants in Web Services communications are loosely coupled and need only agree on the format of messages and their semantics. I

Web Services had their beginnings in mid to late 2000 with the introduction of the Simple Object Access Protocol (SOAP),45 Web Service Description Language (WSDL),46,47 and Universal Description, Discovery and Integration (UDDI).48

XML provides the interoperable format to describe message content between Web Services, and is the basic language in which Web Services specifications are defined.49

The difference between Web Services and traditional approaches is primarily in the use of self-describing, platform-independent messages to enable loose coupling of aspects of the architecture, making the approach more dynamic and adaptable to change.

HTTP provides a commonly used interoperable protocol for SOAP

SOAP provides only the protocol for exchanging self-describing messages between services, but by itself does not provide any information about the services. WSDL is a common grammar for providing design-time description of services and messages. It defines a template to encode the information required by service clients to access and interact with the service. It describes what a Web service does, where it resides, and how it should be invoked.

UDDI provides a mechanism for clients to dynamically find other Web services

It provides a uniform mechanism for exchanging structural and typed information encoded as XML.

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

orrect Style Sheet Declaration The root element that declares the document to be an XSL style sheet is <xsl:stylesheet> or <xsl:transform>.

Example study: How to transform XML into XHTML using XSLT.

XML Schema became a W3C Recommendation 02. May 2001.

the import element has optional namespace and schemaLocation attributes. If present, the schemaLocation attribute is understood in a way which parallels the interpretation of xsi:schemaLocation

The default value for both the minOccurs and the maxOccurs attributes is 1.

Be sure that if you specify a value for only the minOccurs attribute, it is less than or equal to the default value of maxOccurs,

Attributes may appear once or not at all, but no other number of times

The existence of a substitution group does not require any of the elements in that class to be used

Several facets can be applied to list types: length, minLength, maxLength, pattern, and enumeration

<xsd:simpleType name="USStateList"> <xsd:list itemType="USState"/> </xsd:simpleType> <xsd:simpleType name="SixUSStates"> <xsd:restriction base="USStateList"> <xsd:length value="6"/> </xsd:restriction> </xsd:simpleType>

Note that it is possible to derive a list type from the atomic type string. However, a string may contain white space, and white space delimits the items in a list type, so you should be careful using list types whose base type is string.

The include mechanism enables you to use externally created schema components "as-is", that is, without any modification.

<complexType name="Address">

base="ipo:Address"

</redefine>

The redefine element acts very much like the include element as it includes all the declarations and definitions from the address.xsd file

Outside of the redefine element, any such attempt to define a complex type with the same name (and in the same namespace) as the base from which it is being derived would cause an error.

Now that Address has been redefined, the extension applies to all schema components that make use of Address

Substitution GroupsXML Schema provides a mechanism, called substitution groups, that allows elements to be substituted for other elements. More specifically, elements can be assigned to a special group of elements that are said to be substitutable for a particular named element called the head element. (Note that the head element as well as the substitutable elementsmust be declared as global elements

Elements in a substitution group must have the same type as the head element, or they can have a type that has been derived from the head element's type

<element name="shipComment" type="string" substitutionGroup="ipo:comment"/>

substitutionGroup="ipo:comment"/>

<ipo:shipComment>

<ipo:customerComment>

global declarations cannot contain the attributes > minOccurs > , > maxOccurs > , or > use > . >

Abstract Elements and TypesXML Schema provides a mechanism to force substitution for a particular element or type. When an element or type is declared to be "abstract", it cannot be used in an instance document. When an element is declared to be abstract, a member of that element's substitution group must appear in the instance document. When an element's corresponding type definition is declared as abstract, all instances of that element must use xsi:type to indicate a derived type that is not abstract.

Declaring an element as abstract requires the use of a substitution group. Declaring a type as abstract simply requires the use of a type derived from it (and identified by the xsi:type attribute) in the instance document

the redefine mechanism we describe here enables you to redefine simple and complex types, groups, and attribute groups that are obtained from external schema files. Like the include mechanism, redefine requires the external components to be in the same target namespace as the redefining schema,

abstract="true"

abstract="true"

extension base

extension

extension

xsi:type="Car"

Controlling the Creation & Use of Derived Types

schema authors will sometimes want to control derivations of particular types, and the use of derived types in instances.

to specify that for a particular complex type, new types may not be derived from it, either (a) by restriction, (b) by extension, or (c) at all.

substitutionGroup=

The restriction value of the final attribute prevents derivations by restriction. Preventing derivations at all, or by extension, are indicated by the values #all and extension respectively

an optional finalDefault attribute on the schema element whose value can be one of the values allowed for the final attribute. The effect of specifying the finalDefault attribute is equivalent to specifying a final attribute on every type definition and element declaration in the schema.

When a simple type is defined, the fixed attribute may be applied to any of its facets to prevent a derivation of that type from modifying the value of the fixed facets.

fixed="true"

a mechanism that controls which derivations and substitution groups may be used in instance documents

eplacement by derived types can be controlled using the block attribute in a type definition

final="restriction"

final="restriction"

Preventing replacement by derivations at all, or by derivations-by-extension, are indicated by the values #all and extension respectively. As with final, there exists an optional blockDefault attribute on the schema element whose value can be one of the values allowed for the block attribute.

block="restriction"

final="restriction"

mechanism that enables schema components from different target namespaces to be used together, and hence enables the schema validation of instance content defined across multiple namespaces.

Importing Types

only global schema components can be imported:

Only named complex types can be imported; local, anonymously defined types cannot.

each namespace must be identified with a separate import element.

The import elements themselves must appear as the first children of the schema element

each namespace must be associated with a prefix, using a standard namespace declaration

import elements optionally contain a schemaLocation attribute to help locate resources associated with the namespaces.

ref="xipo:

Deriving Types by Extension

we indicate that the content models of the new types are complex, i.e. contain elements, by using the complexContent element, and we indicate that we are extending the base type Address by the value of the base attribute on the extension element.

xsi:type="ipo:UKAddress">

xsi:type="ipo:USAddress">

Deriving Complex Types by Restriction

the values represented by the new type are a subset of the values represented by the base type (as is the case with restriction of simple types).

<complexContent> <restriction base=

Table 3 shows several examples of how element and attribute declarations within type definitions may be restricted

A "Valid" XML document also conforms to a DTD. A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):

Valid XML Documents A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):

XML Schema is an XML based alternative to DTD.

XML Schema  W3C supports an XML based alternative to DTD called XML Schema:

XSLT is used to transform an XML document into another XML document, or another type of document that is recognized by a browser, like HTML and XHTML. Normally XSLT does this by transforming each XML element into an (X)HTML element.

What is XSLT? XSLT stands for XSL Transformations XSLT is the most important part of XSL XSLT transforms an XML document into another XML document XSLT uses XPath to navigate in XML documents XSLT is a W3C Recommendation