1. WEB PROGRAMMING  Introduction to World Wide Web  Lecture #: 1

2. Introduction to the World Wide Web  Outline  Definition.  Internet Vs. WWW  Basic Architectural Bases  H ow the web works  Basic c omponents of the web

3. the World Wide Web Definition  The World Wide Web (WWW, or simply Web) is an information space in which the items of interest, referred to as resources, are identified by global identifiers called Uniform Resource Identifiers (URI).  Examples such as the following travel scenario are used throughout this document to illustrate typical behavior of Web agents—people or software acting on this information space. A user agent acts on behalf of a user. Software agents include servers, proxies, spiders, browsers, and multimedia players.  The World Wide Web was created in 1989 by Sir Tim Berners-Lee, working at CERN in Geneva, Switzerland. Since then, Berners-Lee has played an active role in guiding the development of Web standards (such as the markup languages in which Web pages are composed), and in recent years has advocated his vision of a Semantic Web. Robert Cailliau, also at CERN, was an early evangelist for the project.

4. the World Wide Web Example

5. the World Wide Web Example  The browser recognizes that what was typed is a URI.  The browser performs an information retrieval action in accordance with its configured behavior for resources identified via the "http" URI scheme.  The authority responsible for "weather.example.com" provides information in a response to the retrieval request.  The browser interprets the response, identified as XHTML by the server, and performs additional retrieval actions for inline graphics and other content as necessary.  The browser displays the retrieved information, which includes hypertext links to other information. Nadia can follow these hypertext links to retrieve additional information.

6. Basic Architectural Bases Identification, Interaction, Format  Identification: URIs are used to identify resources. In this travel scenario, the resource is a periodically updated report on the weather in Oaxaca, and the URI is “http://weather.example.com/oaxaca”.  Interaction: Web agents communicate using standardized protocols that enable interaction through the exchange of messages which adhere to a defined syntax and semantics. By entering a URI into a retrieval dialog or selecting a hypertext link, X tells his browser to perform a retrieval action for the resource identified by the URI.  In this example, the browser sends an HTTP GET request (part of the HTTP protocol) to the server at "weather.example.com", via TCP/IP port 80, and the server sends back a message containing what it determines to be a representation of the resource as of the time that representation was generated.

7. Basic Architectural Bases Identification, Interaction, Format  Formats: Most protocols used for representation retrieval and/or submission make use of a sequence of one or more messages, which taken together contain a payload of representation data and metadata, to transfer the representation between agents. The choice of interaction protocol places limits on the formats of representation data and metadata that can be transmitted. HTTP, for example, typically transmits a single octet stream plus metadata, and uses the "Content- Type" and "Content-Encoding" header fields to further identify the format of the representation.

8. World Wide Web Vs. The Internet  Do not make the rather common mistake of confusing the Web with the Internet itself: the Internet simply provides the medium for the Web to run on, just as a telephone line provides the medium for telephone conversations. What the Web does is provide the technology for publishing, sending and obtaining information over the expanse of the Internet. How the Internet actually works may be a matter of interest, but the Web user does not need to know about it in any detail.

9. How the web works Steps  Type the URI of the page into a Web browser, or by following a hyperlink to that page or resource. The Web browser then initiates a series of communication messages, behind the scenes, in order to fetch and display it.  First, the server-name portion of the URL is resolved into an IP address using the global, distributed Internet database known as the domain name system, or DNS.  The browser then requests the resource by sending an HTTP request to the Web server at that particular address. In the case of a typical Web page, the HTML text of the page is requested first and parsed immediately by the Web browser, which will then make additional requests for images and any other files that form a part of the page.  Having received the required files from the Web server, the browser then renders the page onto the screen as specified by its HTML, CSS, and other Web languages. Any images and other resources are incorporated to produce the on-screen Web page that the user sees.

10. Basic components of the web  Web Servers.  Servers.  Web clients.  Web Protocols  Web Browser.

11. Basic components of the web Web Servers  A computer program that is responsible for accepting HTTP requests from clients, which are known as web browsers, and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and linked objects (images, etc.).  HTTP.  Logging.  Authentication.  Dynamic content.  HTTPS.  Bandwidth throttling.

12. Basic components of the web Servers  Server or server computer is a designation for computer models intended for use running server applications, often under heavy workloads, unattended, for extended time.  A server computer usually has special features intended to make it more suitable. Distinctions often include faster processor and memory, more RAM, larger hard drives, higher reliability, redundant power supplies, redundant hard drives (RAID), compact size and shape, modular design (e.g., blade servers often used in server farms), rack or cabinet mountability, serial console redirection, etc.

13. Basic components of the web Web Client  which can be PCs, Macintoshes and other computers that are connected to the Internet and which can retrieve information from Web servers. A Web client is the computer on your desk. PCs, Macintoshes, UNIX workstations and even simple terminals can run client software. Different client software is marketed (or is given away free) for different platforms. Thus, Mosaic has both a Macintosh and a PC implementation.

14. Basic components of the web Web Protocol  A protocol consists of salutations exchanged between computers: `good morning', `be with you in a tick', `file coming down the line now' and so on. Each service on the Internet has its own protocol: its own personal way of sending files around the system. The protocol for the Web is HTTP. File Transfer Protocol (FTP) is another common protocol of which you may have heard.

15. Basic components of the web Web Browser  software, which is needed by a Web client for displaying text, images, video clips and so on. This is supplied under the umbrella name `browser'.  Browser software gives you the ability to scan information retrieved from Web servers, as you would browse through a book. It also gives you facilities for saving and printing information obtained on the Web.  Examples?

16. Thanks… Break…