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 manage a collection of related resources and present their functionalities to users and applications
o a process on networked computer that accepts requests from processes on other computers to perform a service and responds appropriately
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 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.
· 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
· 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
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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