Transcript WWW (World Wide Web)

Prepared by
Ahmed Saad Kareem
The World Wide Web (WWW) is a repository 
of information linked together from points all
over the world. The WWW has a unique
combination of flexibility, portability, and userfriendly features that distinguish it from other
services provided by the Internet. The WWW
project was initiated by CERN (European
Laboratory for Particle Physics) to create a
system to handle distributed resources
necessary for scientific research [1].
The initial proposal for a web of linked documents
came from CERN physicist Tim Berners-Lee in
March 1989. The first (text-based) prototype was
operational 18 months later. In December 1991, a
public demonstration was given at the Hypertext
'91 conference in San Antonio, Texas. In 1994,
CERN and M.I.T. signed an agreement setting up
the World Wide Web Consortium (sometimes
abbreviated as W3C), an organization devoted to
further developing the Web, standardizing
protocols, and encouraging interoperability
between sites. Berners-Lee became the director.
Since then, several hundred universities and
companies have joined the consortium [2].

The WWW is a distributed client/server 
service, in which a client using a browser can
access a service using a server. However, the
service provided is distributed over many
locations called sites, as shown in Figure 1.1.
A variety of vendors offer commercial browsers
that interpret and display a Web document,
and all use nearly the same architecture. Each
browser usually consists of three parts:
1- A controller. 
2- Client protocol. 
3- and interpreters. 
The Web page is stored at the server. Each time
a client request arrives, the corresponding
document is sent to the client. To improve
efficiency, servers normally store requested
files in a cache in memory; memory is faster to
access than disk. A server can also become
more efficient through multithreading or
multiprocessing. In this case, a server can
answer more than one request at a time[1].
A client that wants to access a Web page needs
the address. To facilitate the access of
documents distributed throughout the world,
HTTP uses locators. The uniform resource
locator (URL) is a standard for specifying any
kind of information on the Internet. The URL
defines four things: protocol, host computer,
port, and path as shown in figure 1.3
The documents in the WWW can be grouped
into three broad categories: static, dynamic,
and active.
1 Static Document
Static documents are fixed-content documents
that are created and stored in a server. The
client can get only a copy of the document[1].
In other word Web pages are static, that is, are
just files sitting on some server waiting to be
retrieved. In this context, even a video is a
static Web page because it is just a file.
Web pages are currently written in a language
called HTML (HyperText Markup Language).
HTML allows users to produce Web pages that
include text, graphics, and pointers to other
Web pages. HTML is a markup language, a
language for describing how documents are to
be formatted [2].
A dynamic document is created by a Web
server whenever a browser requests the
document. When a request arrives, the Web
server runs an application program or a script
that creates the dynamic document. The server
returns the output of the program or script as a
response to the browser that requested the
document.

Active Documents
For many applications, we need a program or
a script to be run at the client site. These are
called active documents for example a program
that creates animated graphics on the screen or
a program that interacts with the user [1].

A web service is typically an application programming
interface (API) or web API that is accessed via Hyper
Text Transfer Protocol (HTTP) and executed on a
remote system, hosting the requested service.
The W3C defines a "web service" as "a software system
designed to support interoperable machine to machine
interaction over a network. It has an interface
described in a machine-processable format (specifically
Web Services Description Language WSDL). Other
systems interact with the web service in a manner
prescribed by its description using SOAP messages,
typically conveyed using HTTP with an XML
serialization in conjunction with other Web-related
standards.
The W3C defines a "web service" as "a software
system designed to support interoperable
machine to machine interaction over a network.
It has an interface described in a machineprocessable format (specifically Web Services
Description Language WSDL). Other systems
interact with the web service in a manner
prescribed by its description using SOAP
messages, typically conveyed using HTTP with
an XML serialization in conjunction with other
Web-related standards

SOAP, originally defined as Simple Object Access 
Protocol, is a protocol specification for exchanging
structured information in the implementation of Web
Services in computer networks. It relies on Extensible
Markup Language (XML) for its message format, and
usually relies on other Application Layer protocols, most
notably Remote Procedure Call (RPC) and Hypertext
Transfer Protocol (HTTP), for message negotiation and
transmission. SOAP can form the foundation layer of a web
services protocol stack, providing a basic messaging
framework upon which web services can be built. This XML
based protocol consists of three parts: an envelope, which
defines what is in the message and how to process it, a set of
encoding rules for expressing instances of applicationdefined datatypes, and a convention for representing
procedure calls and responses.
As an example of how SOAP procedures can 
be used, a SOAP message could be sent to a
web-service-enabled web site, for example, a
real-estate price database, with the parameters
needed for a search. The site would then return
an XML-formatted document with the
resulting data, e.g., prices, location, features.
Because the data is returned in a standardized
machine-parseable format, it could then be
integrated directly into a third-party web site
or application.