PSI_ESEI_08

either single or double quotes can be used

<gangster name='George "Shotgun" Ziegler'>

XML Attributes Must be Quoted

Use of Elements vs. Attributes Data can be stored in child elements or in attributes.

Avoid XML Attributes? Some of the problems with using attributes are: attributes cannot contain multiple values (elements can) attributes cannot contain tree structures (elements can) attributes are not easily expandable (for future changes) Attributes are difficult to read and maintain. Use elements for data. Use attributes for information that is not relevant to the data.

Should you avoid using attributes? Some of the problems with using attributes are: attributes cannot contain multiple values (child elements can) attributes are not easily expandable (for future changes) attributes cannot describe structures (child elements can) attributes are more difficult to manipulate by program code attribute values are not easy to test against a Document Type Definition (DTD) - which is used to define the legal elements of an XML document

Try to use elements to describe data. Use attributes only to provide information that is not relevant to the data.

< < less than > > greater than & & ampersand  ' ' apostrophe " " quotation mark

Apostrophes, quotation marks and greater than signs are legal, but it is a good habit to replace them.

CDATA Everything inside a CDATA section is ignored by the parser.

Nested CDATA sections are not allowed.

A CDATA section starts with "<![CDATA[" and ends with "]]>":

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:

<f:table>

XML Namespaces - The xmlns Attribute When using prefixes in XML, a so-called namespace for the prefix must be defined. The namespace is defined by the xmlns attribute in the start tag of an element.

<h:table xmlns:h="http://www.w3.org/TR/html4/">

xmlns:prefix="URI".

<f:table xmlns:f="http://www.w3schools.com/furniture">

xmlns:namespace-prefix="namespaceURI"

Namespaces can be declared in the elements where they are used or in the XML root element:

Note that the address used to identify the namespace is not used by the parser to look up information. The only purpose is to give the namespace a unique name.

The namespace URI is not used by the parser to look up information. The purpose is to give the namespace a unique name

Uniform Resource Identifier (URI) A Uniform Resource Identifier (URI) is a string of characters which identifies an Internet Resource. The most common URI is the Uniform Resource Locator (URL) which identifies an Internet domain address.

not so common type of URI is the Universal Resource Name (URN)

Default Namespaces Defining a default namespace for an element saves us from using prefixes in all the child elements.

<table xmlns="http://www.w3.org/TR/html4/">

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.