World Wide Web or The Web

Web

BrowserUSERWeb

ServerDocuments

INTERNET

LANLAN

Pieces of the Web(Source: Yeager and McGrath 1996)

The Web

“Boundless information world,” which is accessible to everyone through Web browsers (Berners-Lee at al. 1994).

According 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.

The Web (contd.)

The Web is a diverse collection of multimedia documents, computers, networks, and Web client and server software (Yeager and McGrath 1996).

Key Features of the Web are: “Documents” are multimedia Hypertext embedded in Web documents

makes the Web a “web”.

The Web (contd.)

Key Features of the Web (contd.) Built “on top” of the Internet, which

connects the client and the server. The Web client and Web server software.

A synthesis of many accomplishments in the computer and information sciences during the 70s, 80s, and 90s.

The Web (contd.)

A synthesis of Information Systems Network technologies and services The Internet Hypermedia Digital representation of information

And three new technologies - URLs, HTTP, and HTML.

The Web (contd.)

The three key technologies that make the Web possible are URLs - Universal Resource Locators (the

address system) HTTP - HyperText Transfer Protocol (the

communication language of Web browsers and servers)

HTML - HyperText Markup Language (the programming language to create multimedia hypertext)

Components of the Web - The Documents

Web document refers to the information delivered by a Web server.

Web browser deciphers and displays several different digital encodings text (ASCII, Word processor encoding, or as

an image) images (GIF, JPEG) audio video

The Documents (contd.)

MIME (Multipurpose Internet Mail Extensions) defines a standard set of names for encoded pictures, text, sounds, and video.

The mesh of the documents tied by “active links” is called hypertext.

Hypertext is nonlinear text; users can follow different paths through the document using the links.

The Documents (contd.)

Creating good hypertext documents is an art.

Hypertext existed long before the Web. Original concept was proposed in 1945. First hypertext system was built in 1968. Other key developments: Nelson’s Xanadu

(1980), Brown U’s Intermedia (1986), Apple’s Hypercard (1987).

Components of the Web: The Internet

A quick review! Internetwork - network of networks TCP/IP IP address, Computer name, DNS DNS is a service built on top of

TCP/IPExamples of network services that

use TCP/IP File Transfer Protocol (FTP) Remote login (TELNET)

Components of the Web: Browsers & Servers

The Web browser and Web server collaborate using a client/server architecture.

A Web browser unifies different Internet services into a single service.

The Web server delivers information and the Web browser uses/displays the information.

Web

FTP

TCP

IP IP

TCP

FTP

Web

Physical network

Source (browser) Destination (server)

Web is a “top-level” protocol.(Source: Yeager and McGrath 1996)

Browsers & Servers (contd.)

Browsers & Servers (contd.)

Web servers can execute programs and dynamically generate information. Interactive applications with electronic fill-in

formsWeb servers can act as gateways to

access other resources. A Web server requires software to

answer requests, produce forms, run scripts, etc.

The Web server software must follow the rules of HTTP.

Web Server Components

Web Server = Platform + Software + Information

Web server’s job receive requests from web browsers for

documents decipher request to determine which file is needed find the file if it is available send file to the web browser over the network

connectionWeb server program runs continuously and is

always listening for a request.

HTTP

HTTP is the protocol used by the Web.HTTP is the set of rules that defines how

Web browsers and Web servers communicate over a TCP/IP connection.

Web server software is called httpd or HTTP daemon.

httpd knows how to receive and reply to HTTP requests, and it knows how to locate the documents requested.

HTTP (contd.)

HTTP defines a simple request-response “conversation”.

The HTTP rules define how to correctly phrase the request and the response.

Establishing and managing network connections is done by lower-level protocols such as TCP/IP.

Universal Resource Locator (URL)

The web uses URL to identify the object on which an HTTP request wants to execute a method.

URL Example. http://www.okstate.edu/cocim/raqs/

raqs.zip URL tells what it is (raqs.zip), where it is

(at www.okstate.edu in /cocim/raqs) and how to get it (use HTTP)

A Typical Request - Response Cycle

Web server waits for a new request.User initiates a request

http://www.okstate.edu/cocim/raqs/raqs.zip.Network connection is established to

www.okstate.edu using TCP/IP.A request arrives from client

GET /cocim/raqs/raqs.zip HTTP/1.0.The Web server parses the request.

Request - Response Cycle (contd.)

The method requested (e.g. GET) is executed. If the document is found it is sent else an error message is sent.

The httpd server closes the document file (if necessary) and terminates the network connection.

Return to listening mode.

Handling Multiple Requests

Cloning a copy of the httpd program for each request When a request arrives the main

program clones itself. New copy (child process) handles the

request and the original (parent process) goes back to the listening mode.

Used by many UNIX network services.

Handling Multiple Requests (contd.)

Programming the httpd server to have multiple threads of execution. Make progress on one request while

another is waiting (usually for data to be read from disk storage).

Making the httpd server a group of co-operating programs. For example, one program reads requests

from the network and helpers do the rest.

Web Scripts

A Web script is a program that can be executed by the Web server in response to Web requests.

Scripts are used to access information from non-Web sources,

such as databases. facilitate interaction between the user and

the server. construct custom documents dynamically.

Web Scripts (contd.)

Scripts collect information through a special HTML document that is a fill-in form.

Any program can be a Web script - C, BASIC, Perl, Java, etc.

A script may call other programs or contact other servers.

Web Scripts (contd.)

A script may translate information from the client into a different format.

A gateway script may use the client input to construct a query for a database server and convert output into a HTML document.