I think that you already imagine why you need Html(Yes, Html). It is needed to present data in the browser. That is, there is Html code and corresponding to this HTML code a certain kind. However, modern trends require not only the display of data, but also their competent internal structure.

That's it for create a structure and there is an XML language... Simple example:

Green apple

For us people, everything becomes immediately clear. An image immediately appears in my head " green apple"however, how to explain to the computer that this is an apple, not an orange, a man or our galaxy? Here again comes to the rescue XML, where we can create any tags, making it clear where is the apple, where is the orange, where is the person, and where is our galaxy. I hope I explained it clearly.

Now about the most important thing. The main feature of XML is its versatility.... That is XML understands any modern language. And since XML is a text file, then you can work with it in a regular notebook. Now specifically to practice, where is XML used:

• Settings file... Settings in XML file very easy to read and write. For this reason, there are hundreds of XML files.
• Communication bridge between programs written in different languages. A very important feature resulting from the versatility of the language, and it is regularly used in complex systems.
• Data storage... In fact, this is a kind of analogue of a database, but does not require DBMS(For example, MySQL). And thanks to the query language XPath it becomes possible to communicate easily with this " database".

And, finally, from my own practice I can give you the simplest example. I have an XML sitemap on my site. There are links to all pages of the site. This is a very convenient and important thing for good site indexing, however, it is inconvenient to manually add a new page there every time. Therefore, thanks to knowledge of working with XML, I easily automated this business. So that XML is a useful language, which any programmer should know at least in general terms.

The Extensible Markup Language (XML) was developed by the World Wide Web Consortium (W3C) XML Working Group. Here's how the creators describe it:

“The Extensible Markup Language (XML) is an integral part of the SGML language ... It is designed to facilitate the use of SGML on the Web and to accomplish the tasks currently performed using HTML. XML is designed to improve the use and interoperability of SGML and HTML. "

This is an excerpt from the XML 1.0 specification, created by the XML Working Group in February 1998. The entire document can be found on the W3C site at http://www.w3.org/TR/REC/-xml.

XML is a markup language designed specifically for posting information on the World Wide Web, similar to the Hypertext Markup Language (HTML), which originally became the standard language for creating Web pages. Since HTML fully satisfies all of our needs, the question arises: Why did you need a completely new language for the Web? What are its advantages and advantages? How does it interact with HTML? Will it replace HTML, or just improve it? Finally, what is SGML, of which XML is a part, and why can't you use SGML proper for your Web pages? In this chapter, I will try to answer all of these questions.

XML purpose

HTML provides a fixed set of elements that you can use to place components on a typical Web page. Examples of such elements are headings, paragraphs, lists, tables, images, and links. For example, HTML is great for

creating a personal home page. Below is the description of the home page in HTML codes:

Michael Young "s Home Page

Welcome to my Web site!

Web Site Contents

Please choose one of the following topics:

• Writing
• Family
• Photo Gallery

Other Interesting Web Sites

Click one of the following to explore another Web site:

• "one. XML should become a language for direct use on the Internet. "

  As you can imagine, XML was designed primarily for storing and distributing information on the Web.

  "2. XML will support a wide variety of applications. "

  Although its main purpose is to distribute information on the Web through servers and browsers, XML is also designed to be used by other programs. For example, XML is used to exchange information between financial programs, to distribute and update software products, and to write voice scripts when delivering information over the phone.

  "3.XML will be SGML compliant."

  XML is a specialized branch of SGML. The advantage here is that SGML software can be easily adapted to work with HTML.

  "4. It will be easier to write programs that process XML documents. "

  Practical use of XML requires that it be easy to write browsers and other programs that process XML documents. In fact, the main reason for separating XML from SGML was the availability of writing programs to process XML documents.

  "5. The number of additional features in XML should be kept to a minimum, ideally zero. "

  The minimum number of additional functions in XML makes it easy to write programs to process XML documents. The abundance of additional pluggable features in SGML has become a major reason why it is practically unsuitable for presenting Web documents. Additional SGML features require tag delimiter characters to be overridden (usually ) and skip the end tag to allow the processor to detect the end of the element. When strictly writing an SGML document processing program, you must consider the possibility of all additional functions, even if they are rare.

  "6. XML documents should be understandable and clear to the user. "

  XML aims to become the lingua franca (universal language) for the exchange of information among users and programs around the world. According to this concept, users, as well as specialized programs, should be able to create and read XML documents. Accessibility and transparency for the user sets XML apart from most other formats used in building databases and text documents.

  The XML document can be easily read by the user because it is described in plain text and has a logical hierarchical structure in the form of a tree. You can simplify XML documents by assigning meaningful names to elements, attributes, and objects, and adding helpful comments. (This is discussed later in this chapter.)

  "7. XML development should be completed quickly enough. "

  XML will become an accepted standard only if programmers and users accept it. This standard should be created before society accepts alternative standards that are increasingly being created by software companies.

  "eight. XML should be formal and concise. "

  The XML specification is written in a formal language used to represent computer languages, with a notation known as the Extended Backus-Naur Form (EBNF). This formal language, although difficult to understand, is devoid of ambiguity and greatly facilitates the writing of XML documents, and especially the programs for processing them.

  "9. XML documents will be easier to create. "

  Practical use of XML as a markup language for Web documents simplifies not only the writing of processing programs, but also the process of creating the XML documents themselves.

  "10. The compressed form is not important in XML markup. "

  In accordance with clause 6 (the XML document must be clear and understandable for the user), the XML markup should not be overly condensed in order not to conflict with the stated purpose.

  Standard XML Applications

  You can use XML for more than just describing an individual document. An individual user, company, or standards committee can define the required set of XML elements and document structure to apply to a particular class of documents. Such a set of elements and a description of the structure of a document is called an XML application or XML vocabulary.

  For example, an organization can define an XML application for creating documents that describe molecular structures, human resources, multimedia presentations, or contain vector graphics. At the end of the chapter, there is a list of some common XML applications that have already been created and are planned to be created.

  An XML application is typically defined by creating a Document Type Descriptor (DTD), which is a valid XML document component. A DTD is structured around a database schema: it sets and defines the names of elements that can be used in a document, the order in which elements can appear, the attributes of elements that can be applied, and other features of the document. For practical use of an XML application, you usually include its DTD in your XML document; the presence of a DTD in a document limits the elements and structures that you will use, so your document meets the standards of this application. The XML document descriptions discussed earlier in this chapter did not include DTDs. You will learn how to define and use DTDs in Chapter 5.

  The advantages of using standard XML applications to design your documents are that you can share documents with all other users of the application, and the document can be processed and displayed using the software that is already built for the application.

  XML applications that enhance the quality of XML documents

  In addition to XML applications for describing specific classes of documents, there are several XML applications that you can use within any type of XML document. These applications make it easier to create a document and improve its quality. Below are examples of such applications.

  • Extensible Stylesheet Language (XSL) allows you to create powerful style sheets using XML syntax.
  • XML Schema allows you to design detailed schemas for your XML documents using standard XML syntax, which is a more powerful alternative to using DTDs.
  • XML Linking Language (XLink) gives you the ability to link your XML documents. It supports multiple targeted links and other useful features, providing more freedom than HTML link organizing mechanism.
  • XML Pointer Language (XPointer) allows you to define flexible target links. When you use XPointer and XLink together, you can organize links to anywhere in the target document — not just jump to highlighted items.

  XLS is covered in Chapter 10. Other XML applications are not yet ready to go and are not covered in this book. (XLink and XPointer are not supported in Internet Explorer 5).

  As you can see, XML is not only a useful tool for describing documents, but also serves as a basis for building applications and extensions that may be needed as the Internet develops.

  Real use of XML

  While the concept of XML is quite interesting, you might wonder how to put it into practice. This section provides a list of examples of such uses of XML, both widely used and promising. If there are relevant XML applications for practical use, they will be shown in parentheses. For example, you might find out that the MathML XML application will allow you to format mathematical formulas.

  Link... For a more complete list of current and emerging XML applications, including their detailed descriptions, see the Oasis SGML / XML Web page (http: //www.oasis-open.arg/cover/ocml.htmW applications).

  • Working with databases. Like traditional databases, XML can be used to assign a label to each field of information within each database record. (For example, you can tag each name, address, and phone number inside address list entries.) You can then display the data in a variety of ways and organize searches, sorting, filtering, and other processing of the data.
  • Structuring documents. The hierarchical structure of XML documents is ideal for marking up the structure of documents such as novels, scientific papers, and plays. For example, you can use XML to mark up a play with acts, scenes, characters, storylines, scenery, and so on. XML markup enables programs to display or print a document in the required format; find, extract or manipulate information in a document; generate tables of contents, summaries and annotations; process information in other ways.
  • Working with vector graphics (VML - Vector Markup Language).
  • Multimedia presentations (SMIL - Synchronized Multimedia Integration Language, HTML + TIME - HTML Timed Interactive Multimedia Extensions).
  • Description of channels. Channels are Web pages that are automatically sent to subscribers. (CDF - Channel Definition Format).
  • Description of software packages and their relationships. Such descriptions provide distribution and updating of software products on the network (OSD - Open Software Description).
  • Interaction of applications over the Web using XML co-communications. These messages are independent of operating systems, object models, and computer languages ​​(SOAP - Simple Object Access Protocol).
  • Sending electronic business cards via e-mail.
  • Financial information exchange. The exchange of information in an open and understandable format is carried out between financial programs (such as Quicken and Microsoft Money) and financial institutions (banks, public funds) (OFX - Open Financial Exchange).
  • Creation, management and use of complex digital forms for commercial Internet transactions. Such forms may include digitized signatures that make them legally recognized (XFDL - Extensible Forms Description Language).
  • Exchange of requests for employment and resumes (HRMML - Human Resource Management Markup Language).
  • Formatting mathematical formulas and scientific information in
  • Web (MathML - Mathematical Markup Language).
  • Description of molecular structures (CML - Chemical Markup Language).
  • Coding and display of information about DNA, RNA and strands (BSML - Bioinformatic Sequence Markup Language).
  • Genealogical Data Markup Language (GeDML).
  • Astronomical Markup Language (AML) exchange.
  • Creation of musical scores (MusicML -Music Markup Language).
  • Working with voice scripts to deliver information over the phone. Voice scripts can be used, for example, to generate voice messages, product availability inquiries, and weather forecasts (VoxML).
  • Processing and delivery of information by courier services. Federal Express, for example, already uses XML for this purpose.
  • Presentation of advertising in the press in digital format (AdMarkup).
  • Legal filing and electronic exchange of legal information (XCL - XML ​​Court Interface).
  • Weather Observation Markup Format (OMF) coding.
  • Exchange of information on transactions with real estate (RETS - Real Estate Transaction Standard).
  • Exchange of insurance information.
  • Exchange of news and information using open Web-standards (XMLNews).
  • Presentation of religious information and markup of texts of worship (ThML - Theological Markup Language, LitML - Liturgical Markup Language).