Цели обучения

Узнать назначение SGML, HTML, and XML

Введение

Существуют 4 основные проблемы управления данными: сбор, хранение, поиск, и обмен. Цель этой книги в том чтобы представить XML как, технологию обмена данными. Главы в этой книге, разделяют различные модели подхода к XML. Так первые главы знакомят читателя с XML документами, XML схемами, и стилем XML, как единым целым. В следующих главах XML расширяется, множествами примеров и отношениями: один к одному, один к многим, многие к многим.

XML это устройство для обмена данными. Обмен данными был достаточно большим вопросом в интернет технологиях. Интернет увеличил потребность в таком обмене. Электронный обмен данными(EDI),традиционный метод обмена данными в больших компаниях, уступает XML, который может быть средством обмена данными в любых компаниях,независимо от их размера.

EDI поддерживающий электронный обмен стандартной бизнес-документации, является основным средством обмена данными в электронной коммерции. Структурированный формат используется в документообороте (например, счета-фактуры и транспортные заказы) между компаниями. К противоположности к свободной форме e-mail сообщений, EDI позволяет упростить рутинные транзакции. Это означает что эти транзакции могут быть краткими и точными. Главный стандарт используемый в США и Канаде это X.12, и главный международный стандарт это UN/EDIFACT. Фирмы поддерживаемые эти стандарты могут обмениваться данными в электронном виде.

The Internet is a global network potentially accessible by nearly every firm, with communication costs typically less than those of traditional EDI. Consequently, the Internet has become the electronic transport path of choice between trading partners. The simplest approach is to use the Internet as a means of transporting EDI documents. But because EDI was developed in the 1960s, another approach is to reexamine the technology of data exchange. A result of this rethinking is XML, but before considering XML we need to learn about SGML, the parent of XML.

SGML

For a typical U.S. firm, it is estimated that document management consumes up to 15 percent of its revenue, nearly 25 percent of its labour costs, and anywhere between 10 and 60 percent of an office worker’s time. The Standard Generalized Markup Language (SGML) is designed to reduce the cost and increase the efficiency of document management.

A markup language embeds information about a document within the document's text. In the following example, the markup tags indicate that the text contains details of a city. Note also that the city's name, state, and population are identified by specific tags. Thus, the reader—a person or a computer—is left in no doubt as to meaning of Athens, Georgia, or 100,000. Note also the latitude and location of the city are explicitly identified with appropriate tags. SGML’s usefulness is based upon both recording text and the meaning of that text.

Exhibit 1: Markup language

<city> 
       <cityname>Athens</cityname> 
       <state>GA</state>
       <description> Home of the University of Georgia</description>
       <population>100,000</population>
       <location>Located about 60 miles Northeast of Atlanta</location>
       <latitude>33 57' 39" N</latitude>
       <longitude>83 22' 42" W</longitude>
</city>

SGML is a vendor-independent International Standard (ISO 8879) that defines the structure of documents. Developed in 1986 as a meta language, SGML is the parent of both HTML and XML. Because SGML documents are standard text files, SGML provides cross-system portability. When technology is rapidly changing, SGML provides a stable platform for managing data exchange. Furthermore, SGML files can be transformed for publication in a variety of media. The use of SGML preserves textual information independent of how and when it is presented. Organizations reap long-term benefits when they can store documents in a single, independent standard that can then be converted for display in any desired media.

SGML has three major advantages for data management:

Reuse: Information can be created once and reused many times.
Flexibility: SGML documents can be published in any format. The same content can be printed, presented on the Web, or delivered with a text synthesis. Because SGML is content-oriented, presentation decisions can be delayed until the output format is decided.
Revision: SGML supports revision and version control. With content version control, a firm can readily track the changes in documents.

A short section of SGML demonstrates clearly the features and strength of SGML (see Exhibit 2). The tags surrounding a chunk of text describe its meaning and thus support presentation and retrieval. For example, the pair of tags <airline> and </airline> surrounding “Delta” identify the airline making the flight.

Exhibit 2: SGML example

  <flight>
      <airline>Delta</airline>
      <flightno>22</flightno>
      <origin>Atlanta</origin>
      <destination>Paris</destination>
      <departure>5:40pm</departure>
      <arrival>8:10am</arrival>
  </flight>

The preceding SGML code can be presented in several ways by applying a style sheet to the file. For example, it might appear as

Delta flight 22 flies from Atlanta to Paris leaving 5:40pm and arriving 8:10am

or as

Airline	Flight	Origin	Destination	Departure	Arrival
Delta	22	Atlanta	Paris	5:40pm	8:10am

If the data are stored in HTML format and rendered on a Web site (as in Exhibit 3), then the meaning of the data has to be inferred by the reader. This is generally quite easy for humans, but impossible for machines. Furthermore, the presentation format is fixed and can only be altered by rewriting the HTML. If you are not familiar with HTML, you should read the WikiBooks chapter on XHTML, an extension of HTML, before reading the next chapter.

Exhibit 3: HTML rendering example

    Delta flight 22 flies from Atlanta to Paris leaving 5:40pm and arriving 8:10am

Meaning and presentation should be independent, and this is an important reason why SGML is more powerful than HTML.

SGML is a markup language that defines the structure of documents and is preferred to HTML as it can be transformed into a variety of media.

XML

Many computer systems contain data in incompatible formats. A time-consuming challenge is to exchange data between such systems. XML is a generic data storage format that comes bundled with a number of tools and technologies that should make it easier to exchange specific XML 'applications' between incompatible systems. Since XML is open and generic, it is expected that as time progresses, more and more organizations and people will jump onto the XML bandwagon, both developers and data users. This should make XML the ultimate viable technology for certain types of data exchange.

XML is used not only for exchanging information, but also for publishing Web pages. XML's very strict syntax allows for smaller and faster Web browsers and as such is well suited for use with Personal Digital Assistants (PDAs) and cellphones. Web browsers that interpret HTML documents, on the other hand, are bloated with programming code to compensate for HTML’s not so strict coding.

The types of data generally well suited for encoding as XML are those where field lengths are unknown and unpredictable and where field contents are predominantly textual.

An XML schema allows for the exchange of information in a standardized structure. A schema defines custom markup tags that can contain attributes to describe the content that is enclosed by these tags. Information from the tagged data in the XML document can be extracted using an application called a “parser”, and with the use of an XML stylesheet the data can be formatted for a Web page.

XML's power lies in the combination of custom markup tags and content in a defined XML document. The purpose of eXtensible Markup Language (XML) is to make information self-describing. Based on SGML, XML is designed to support electronic commerce. The definition of XML, completed in early 1998 by the World Wide Web Consortium (W3C), describes it as a meta language — a language to generate languages. XML should steadily replace HTML on many Web sites because of some key advantages. The major differences between XML and HTML are captured in the following table.

Exhibit 4: XML vs HTML

XML	HTML
Information content	Information presentation
Extensible set of tags	Fixed set of tags
Data exchange language	Data presentation language
Greater hypertext linking	Limited hypertext linking

The eXtensible in XML means that a new data exchange language can be created by defining its structure and tags. For example, the OpenGIS Consortium designed a Geography Markup Language (GML) to facilitate the electronic exchange of geographic information. Similarly, the Open Tourism Consortium is working on the definition of TourML to support exchange of tourism information. The insurance industry uses data corresponding to the XML based standard ACORD for electronic data exchange. Another good example of XML in action is NewsML™.

In this text we will cover all the features of XML, but at this point let us introduce a few of the key features.

Applications of XML:

Before we start learning more about how an XML document is structured, lets point out what XML can be used for. The four major implementations of XML are:

Publication: Database content can be converted into XML and afterwards into HTML by using an XSLT stylesheet. Making use of this technique, complex websites as well as print media like PDF files can be generated. Information no longer has to be stored in different formats (i.e. RTF, DOC, PDF, HTML). Content can be stored in the neutral XML format and then, using appropriate layout style sheets and transformations, brochures, websites, or datalists can be generated (See more in Chapter 17.)

An example of the capability of XML and XSLT can be found at http://www.emimusic.de: This website contains approximately 20,000 pages with profiles of the artists, their products and the titles of the songs. These pages are generated using a XSLT script. Based on the script used it will also be possible to create a catalog in PDF format. Please see below for more details.

Interaction: XML can be used for accessing and changing data interactively. This man<->machine communication usually happens via a web browser (see Chapter 12).

Integration: Using XML, homogenous and heterogenous applications can be integrated. In this case, XML is used to describe data, interfaces, and protocols. This machine-machine communication helps integrate relational databases (i.e. by importing and exporting different formats).

Transaction: XML helps to process transactions in applications like online marketplaces, supply chain management, and e-procurement systems.

Key features of XML

Elements have both an opening and a closing tag
Elements follow a strict hierarchy, with documents containing only one root element
Elements cannot overlap other elements
Element names must obey XML naming conventions
XML is case sensitive

XML will improve the efficiency of data exchange in several important ways, which include:

write once and format many times: Once an XML file is created it can be presented in multiple ways by applying different XML stylesheets. For instance, the information might be displayed on a web page or printed in a book.
hardware and software independence: XML files are standard text files, which means they can be read by any application.
write once and exchange many times: Once an industry agrees on a XML standard for data exchange, data can be readily exchanged between all members using that standard.
Faster and more precise web searching: When the meaning of information can be determined by a computer (by reading the tags), web searching will be enhanced. For example, if you are looking for a specific book title, it is far more efficient for a computer to search for text between the pair of tags <booktitle> and </booktitle> than search an entire file looking for the title. Furthermore, spurious results should be eliminated.

10 reasons why to use XML

XML is a widely accepted open standard.
XML allows to clearly separate from content and form (appearance).
XML is text-oriented.
XML is extensible.
XML is self-describing.
XML is universal; meaning internationalization is no problem.
XML is independent from platforms and programming languages.
XML provides a robust and durable format for information storage.
XML is easily transformable.
XML is a future-oriented technology.

The major XML elements

The major XML elements are:

XML document: An XML file containing XML code.
XML schema: An XML file that describes the structure of a document and its tags.
XML stylesheet: An XML file containing formatting instructions for an XML file.

In the next few chapters you will learn how to create and use each of these elements of XML.

Creating a markup file

Any text editor can be used to create a markup file (e.g. an HTML file). In this book, we use the text editor within NetBeans, an open source Integrated Development Environment (IDE) for Java, because NetBeans supports editing and validation of XML files. Before proceeding, you should download and install NetBeans from http://www.NetBeans.org/.

The examples in this book use NetBeans to illustrate proper XML code. For an alternative to NetBeans, see XML: Managing Data Exchange/Exchanger XML Lite as an alternative to NetBeans Chapter 37

Case Studies in XML Implementation

XML at United Parcel Service (UPS)

“UPS is a service company and it is all about scale and speed,” says Geoff Chalmers, Project Leader at UPS eSolutions Department. In 2003, UPS had $33.5 billion annual revenue and 357,000 employees worldwide. Six percent of the United States' Gross Domestic Product (GDP) on any given day is in the UPS system.

UPS uses technology extensively. The Information Systems department employs 4,000 people. The company's web site has 166 different country home pages and is supported by 44 applications.

UPS delivers around 13 million packages every day, and customers can track these shipments via the UPS Web site, which receives around 200 million hits daily. Nineteen of the applications within ups.com are XML OnLine Tool (Web services) applications.

UPS’s online tools are developed specifically to be integrated with customers’ applications. This makes the customer’s task simpler, easier, and faster. UPS verified the importance of simplicity and speed, via “CampusShip,” a product that has been one of the UPS’s most successful in the last 10 years. UPS CampusShip® is a Web-based, UPS-hosted shipping system. Using an Internet connection, employees can ship their own packages and letters from any desktop, while management maintains overall control of shipping activities. UPS CampusShip® allows simultaneous shipper autonomy and managerial cost-control within the organization. This product has been successful because no installation or software maintenance is required and it is quick to implement. XML Online Tools enabled cheap and fast evolution of CampusShip®.

UPS favors XML especially because it is agnostic; platform and language independent. These features make XML very flexible and powerful. It is also decoupled and scalable. XML has enabled UPS to target a broader market and reduce customer interaction, and thus the cost of customer service. Another positive feature of XML is that it is backward compatible. The adoption of XML has reduced maintenance, implementation, and usage costs significantly within UPS.

However these advantages don’t come without a price. “XML is inefficient in so many ways” says Chalmers. XML unfortunately takes more CPU and bandwidth than the other technologies. Yet bandwidth and CPU are cheap and getting cheaper everyday, so this is a gradually disappearing problem.

Nevertheless, Chalmers also thinks that XML doesn’t work well in databases. He says that it is too wordy and it is an exchange medium rather than a database medium. There were some early attempts to tightly integrate XML and databases. Because databases do supply structure and identification to data as does XML, the value-add of XML-database integration is limited to applying hierarchical structure. On the other hand, if data is to be stored as a blob, then XML makes sense. Another problem that he points out about XML is that business rules cannot be expressed in XML schemas.

Finally, raw XML programming and debugging can be challenging. Therefore, UPS’s enterprise customers are starting to explore the code generators and embedded facilities to be found in .NET and BEA. However, hand coding by experienced in-house engineers is a must for the high availability, scalability, and performance that UPS requires for the UPS OnLine Tools.

XML at EMI Music

How is it used?

EMI Music Germany GmbH & Co. KG, a famous German record label, displays information about the artists it is affiliated with on its website. Visitors are able to explore all their audio or video productions. The whole website consists of nearly 20,000 pages that contain information about artists and their products (CD, DVD, LP). Everything is properly linked and systematically grouped.

After all, there is data to be provided for every artist, albums, samples, pictures, descriptions or article codes. The site is updated on a daily basis and is subject to change by a web editor whenever it’s necessary. Now this is a fairly complex and large amount of data to be handled.

This is where XML comes into play. The data, which is stored in a database, has been transformed into XML code. Now an XSLT stylesheet converts this data into HTML code, which can be easily read by any web browser (e.g. Internet Explorer or Firefox).

What's the benefit?

The advantage of XML is that the programming effort is considerably lower as compared to other formats. This is because XML lies at the point of intersection of XSLT and HTML.

It’s also no problem for the web editor to update the website. Using XML makes it easy for the person in charge to deal with this large amount of data.

Going beyond… On the basis of the XML scripts thus far produced by EMI Music, the company could easily produce a PDF-formatted catalog or design i-Mode pages for the current mobile phone generation. Thanks to XML, this can be done with little extra effort.

A brief history of XML

In the late 60s Charles Goldfarb, Raymond Lorie and Edward Mosher all working for IBM started to develop GML (Generalized Markup Language), a text formatting language. The language was successfully applied for internal documentation procedures. As it used to be common, the document editing was performed in the batch-mode. GenCode, another procedure to define generic formatting codes for the typesetting systems of various software producers, was developed by the GCA (Graphic Communications Association) at about the same time. Both of these technologies, GML syntactically and GenCode semantically, served as basis for the development of SGML (Standard Generalized Markup Language). The process of standardization started at the U.S. Standardization institute ANSI in the early 80s and in 1996 SGML finally passed as ISO standard ISO2879:1986.

SGML is reckoned to be a complex and comprehensive language (the specification extends 500 pages). However, the success of HTML (Hyper Text Markup Language) approved that the concepts of SGML were appropriate. SGML-based HTML was developed by Tim Berner-Lee in Geneva, in the early 90s in order to illustrate and link documents in the Internet. Meanwhile HTML developed being the most successful format for all electronical documents. The Internet was originally designed as a space for human-human and human-machine communication but lately the machine-machine communication gained tremendously importance, putting a completely new challenge on the computer languages used.

HTML is a descriptive language for the presentation of documents. The main focus is on the presentation meaning that a HTML-document mixes the presented data and its formatting instruction. A human being may recognize the displayed semantic by means of the presentation and the context meaning, a machine or better-said software is unable to.

In 1996 a team under the guidance of Jos Bosak attending the W3C-consortium was established to make SGML web-suitable. The result was a 30-page specification, which received in February 1998 the status of a "W3C-recommendation" and was named "Extensible Markup Language (XML)".

The most important goals developing XML were:

XML should be compatible with SGML
XML should be easy to use in the Internet
The number of optional characteristics should be minimized
XML-documents should be easy to generate and human-readable
XML should be supported by a variety of application
It should be easy to write programs for XML
XML should be put into practice on time

In the terminology of markup languages, a description formulated in XML is called a XML-document, albeit the content has nothing to do with text processing.

Why is this book not an XML document?

If you have accepted the ideas presented in this chapter, the question is very pertinent. The simple answer is that we have been unable to find the technology to support the creation of an open text book in XML. We need several pieces of technology

An XML language for describing a book. DocBook is such a language, but the structure of a book is quite complex, and DocBook (reflecting this complexity) cannot be quickly mastered
A Wiki that works with a language such as DocBook
A XML stylesheet that converts XML into HTML for displaying the book's content

There is a project to create WikiMl (Wiki MarkupLanguage), and this might be used at some point.

References

Initiating author Richard T. Watson, University of Georgia