1. World Wide Web or The Web Web Browser USE R Web Server Documents INTERNET LAN Pieces of the Web (Source: Yeager and McGrath 1996)

2. The Web z“Boundless information world,” which is accessible to everyone through Web browsers (Berners-Lee at al. 1994). zAccording to Schatz and Hardin (1994), when using the Web, the “user navigates an information space of distributed items of information,” without regard to the physical hardware details.

3. The Web (contd.) zThe Web is a diverse collection of multimedia documents, computers, networks, and Web client and server software (Yeager and McGrath 1996). zKey Features of the Web are: y“Documents” are multimedia yHypertext embedded in Web documents makes the Web a “web”.

4. The Web (contd.) zKey Features of the Web (contd.) yBuilt “on top” of the Internet, which connects the client and the server. yThe Web client and Web server software. zA synthesis of many accomplishments in the computer and information sciences during the 70s, 80s, and 90s.

5. The Web (contd.) zA synthesis of yInformation Systems yNetwork technologies and services yThe Internet yHypermedia yDigital representation of information zAnd three new technologies - URLs, HTTP, and HTML.

6. The Web (contd.) zThe three key technologies that make the Web possible are yURLs - Universal Resource Locators (the address system) yHTTP - HyperText Transfer Protocol (the communication language of Web browsers and servers) yHTML - HyperText Markup Language (the programming language to create multimedia hypertext)

7. Components of the Web - The Documents zWeb document refers to the information delivered by a Web server. zWeb browser deciphers and displays several different digital encodings ytext (ASCII, Word processor encoding, or as an image) yimages (GIF, JPEG) yaudio yvideo

8. The Documents (contd.) zMIME (Multipurpose Internet Mail Extensions) defines a standard set of names for encoded pictures, text, sounds, and video. zThe mesh of the documents tied by “active links” is called hypertext. zHypertext is nonlinear text; users can follow different paths through the document using the links.

9. The Documents (contd.) zCreating good hypertext documents is an art. zHypertext existed long before the Web. yOriginal concept was proposed in 1945. yFirst hypertext system was built in 1968. yOther key developments: Nelson’s Xanadu (1980), Brown U’s Intermedia (1986), Apple’s Hypercard (1987).

10. Components of the Web: The Internet zA quick review! yInternetwork - network of networks yTCP/IP yIP address, Computer name, DNS yDNS is a service built on top of TCP/IP zExamples of network services that use TCP/IP yFile Transfer Protocol (FTP) yRemote login (TELNET)

11. Components of the Web: Browsers & Servers zThe Web browser and Web server collaborate using a client/server architecture. zA Web browser unifies different Internet services into a single service. zThe Web server delivers information and the Web browser uses/displays the information.

12. Web FTP TCP IP TCP FTP Web Physical network Source (browser) Destination (server) Web is a “top-level” protocol. (Source: Yeager and McGrath 1996) Browsers & Servers (contd.)

13. zWeb servers can execute programs and dynamically generate information. yInteractive applications with electronic fill-in forms zWeb servers can act as gateways to access other resources. zA Web server requires software to answer requests, produce forms, run scripts, etc. zThe Web server software must follow the rules of HTTP.

14. Web Server Components zWeb Server = Platform + Software + Information zWeb server’s job yreceive requests from web browsers for documents ydecipher request to determine which file is needed yfind the file if it is available ysend file to the web browser over the network connection zWeb server program runs continuously and is always listening for a request.

15. HTTP zHTTP is the protocol used by the Web. zHTTP is the set of rules that defines how Web browsers and Web servers communicate over a TCP/IP connection. zWeb server software is called httpd or HTTP daemon. zhttpd knows how to receive and reply to HTTP requests, and it knows how to locate the documents requested.

16. HTTP (contd.) zHTTP defines a simple request-response “conversation”. zThe HTTP rules define how to correctly phrase the request and the response. zEstablishing and managing network connections is done by lower-level protocols such as TCP/IP.

17. Universal Resource Locator (URL) zThe web uses URL to identify the object on which an HTTP request wants to execute a method. zURL Example. yhttp://www.okstate.edu/cocim/raqs/raqs.zip yURL tells what it is (raqs.zip), where it is (at www.okstate.edu in /cocim/raqs) and how to get it (use HTTP)

18. A Typical Request - Response Cycle zWeb server waits for a new request. zUser initiates a request yhttp://www.okstate.edu/cocim/raqs/raqs.zip. zNetwork connection is established to www.okstate.edu using TCP/IP. zA request arrives from client yGET /cocim/raqs/raqs.zip HTTP/1.0. zThe Web server parses the request.

19. Request - Response Cycle (contd.) zThe method requested (e.g. GET) is executed. If the document is found it is sent else an error message is sent. zThe httpd server closes the document file (if necessary) and terminates the network connection. zReturn to listening mode.

20. Handling Multiple Requests zCloning a copy of the httpd program for each request yWhen a request arrives the main program clones itself. yNew copy (child process) handles the request and the original (parent process) goes back to the listening mode. yUsed by many UNIX network services.

21. Handling Multiple Requests (contd.) zProgramming the httpd server to have multiple threads of execution. yMake progress on one request while another is waiting (usually for data to be read from disk storage). zMaking the httpd server a group of co- operating programs. yFor example, one program reads requests from the network and helpers do the rest.

22. Web Scripts zA Web script is a program that can be executed by the Web server in response to Web requests. zScripts are used to yaccess information from non-Web sources, such as databases. yfacilitate interaction between the user and the server. yconstruct custom documents dynamically.

23. Web Scripts (contd.) zScripts collect information through a special HTML document that is a fill-in form. zAny program can be a Web script - C, BASIC, Perl, Java, etc. zA script may call other programs or contact other servers.

24. Web Scripts (contd.) zA script may translate information from the client into a different format. zA gateway script may use the client input to construct a query for a database server and convert output into a HTML document.