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Chapter: Distributed Systems - Introduction

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Resource Sharing

Is the primary motivation of distributed computing

RESOURCE SHARING

 

o   Is the primary motivation of distributed computing

o   Resources types

o   Hardware, e.g. printer, scanner, camera

o   Data, e.g. file, database, web page

o   More specific functionality, e.g. search engine, file

o   Service

 

o   manage a collection of related resources and present their functionalities to users and applications

 

o   Server

 

o   a process on networked computer that accepts requests from processes on other computers to perform a service and responds appropriately

 

o   Client

o   the requesting process

o   Remote invocation

 

A complete interaction between client and server, from the point when the client sends its request to when it receives the server’s response

 

·        Motivation of WWW

o   Documents sharing between physicists of CERN

 

o   Web is an open system: it can be extended and implemented in new ways without disturbing its existing functionality.

 

o   Its operation is based on communication standards and document standards

o   Respect to the types of ‘resource’ that can be published and shared on it.

·        HyperText Markup Language

o   A language for specifying the contents and layout of pages

 

·        Uniform Resource Locators

o   Identify documents and other resources

·        A client-server architecture with HTTP

 

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

 

HTML

 


·         HTML text is stored in a file of a web server.

 

·         A browser retrieves the contents of this file from a web server.

 

-The browser interprets the HTML text

 

-The server can infer the content type from the filename extension.

 

URL

 

·         HTTP URLs are the most widely used

 

·         An HTTP URL has two main jobs to do:

 

o   To identify which web server maintains the resource

o   To identify which of the resources at that server

 

Web servers and web browsers


 

HTTP URLs


 

HTTP

 

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

 

·        Main features

o   Request-replay interaction

 

o   Content types. The strings that denote the type of content are called MIME (RFC2045,2046)

 

o   One resource per request. HTTP version 1.0

o   Simple access control

 

More features-services and dynamic pages

 

·        Dynamic content

 

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

 

·        Downloaded code

o   JavaScript

o   Applet

 

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

 

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