1. Chapter 1: A Review : Distributed System

1.7. Example of the Web

  • HyperText Markup Language

A language for specifying the contents and layout of pages 

  • Uniform Resource Locators

Identify documents and other resources

  • A client-server architecture with HTTP

By with browsers and other clients fetch documents and other resources from web servers

Web servers and web browsers



Example of the Web : html


Example

<IMG SRC = http://www.cdk3.net/WebExample/Images/earth.jpg>

<P>

Welcome to Earth! Visitors may also be interested in taking a look at the

<A HREF = “http://www.cdk3.net/WebExample/moon.html">Moon</A>.

<P>

(etcetera)


  • HTML text is stored in a file of a web server.
  • A browser retrieves the contents of this file from a web server.
    • To identify which web server maintains the resource
    • To identify which of the resources at that server

Example of the Web : URL


  • Scheme: scheme-specific-location
Example
 mailto:joe@anISP.net 
 

ftp://ftp.downloadIt.com/software/aProg.exe
http://net.pku.cn/

Server DNS namePathname on serverArguments
 www.cdk3.net (default) 

(none)

www.w3c.orgProtocols/Activity.html 

(none)

e.pku.cncgi-bin/allsearch           word=distributed+system

  • Publish a resource remains unwieldy


Example of the Web : HTTP

  • Defines the ways in which browsers and any other types of client interact with web servers (RFC2616)

  • Main features
    • Request-replay interaction
    • Content types. The strings that denote the type of content are called MIME (RFC2045,2046)
    • One resource per request. HTTP version 1.0
    • Simple access control

  • Dynamic content
    • Common Gateway Interface: a program that web servers run to generate content for their clients

  • Student : Downloaded code JavaScript or Applet that shows web functions and services.

Discussion of Web

  • Dangling: a resource is deleted or moved, but links to it may still remain
  • Find information easily: e.g. Resource Description Framework which standardize the format of metadata about web resources
  • Exchange information easily: e.g. XML – a self describing language
  • Scalability: heavy load on popular web servers
  • More applets or many images in pages increase in the download time