Resource Sharing

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