SKR 5302: Advanced Distributed Computing
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
Example of the Web : html
<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
| mailto:joe@anISP.net |
| http://net.pku.cn/ |
|---|
- HTTP URLs are the most widely used
- An HTTP URL has two main jobs to do:
- To identify which web server maintains the resource
- To identify which of the resources at that server
- http://servername[:port]//pathNameOnServer][?arguments]
- e.g.
| Server DNS name | Pathname on server | Arguments |
|---|---|---|
| www.cdk3.net | (default) | (none) |
| www.w3c.org | Protocols/Activity.html | (none) |
| e.pku.cn | cgi-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
- 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