Introduction to Basic Concepts in Web

Web Application Programming Introduction to Basic Concepts Jussi Pohjolainen TAMK University of Applied Sciences

HTTP

Client and Server Interaction

Intro to HTTP

HTTP ( Hypertext transfer protocol ) is a stateless protocol, which is meant to transfer information on intranets and World Wide Web.

RFC2616:

http://www.w3.org/Protocols/rfc2616/rfc2616.html

HTTP is a request / response standard between client and server

Clients and Servers

Client

Client makes a http request .

Client can be a web browser, spider or other end-user tool

Client is referred as a user agent

Server

Stores information and makes them available to the client

Gives http response to the client

Resources

Server provides resources to clients

Resources can be accessed by HTTP using URLs ( Uniform Resource Locator )

Example

http://www.tamk.fi/~jack/document.doc

Request and Response Client User-agent: Firefox Client Apache HTTP Server request response

Request GET

HTTP defines eight methods that define a action which will be made to a resource

Most typical method is GET

By using the GET - method, client gets a representation of the resource.

Example of getting a file

URL: http://www.something.com/file.doc

1) Open connection to the server

2) Send following through the connection:

GET /path/to/file.doc HTTP/1.0

3) Read the response

More on Request GET

Typically HTTP client is a browser that creates the request automatically when user opens a url

Browser creates the request which contains addition to the GET lot of other information..

For example, information about user agent!

Request and Response Client User-agent: Firefox Client Apache HTTP Server example request GET / HTTP/1.1 Host: www.tamk.fi User-Agent: Mozilla/5.0 (Mac..) ... response

Response

When making a GET request to a certain URL, client gets a response

Response consists of

1) Header information

2) Resource itself

Browser makes decisions based on the header information..

Example of Response HTTP/1.1 200 OK Date: Fri, 12 Jan 2007 07:51:41 GMT Server: Apache/1.3.37 (Unix) Last-Modified: Wed, 10 Jan 2007 12:43:34 GMT ETag: "479a0-16a9-45a4df76" Accept-Ranges: bytes Content-Length: 5801 Connection: close Content-Type: text/html; charset=iso-8859-1 <html> <head> <title>... Header Resource

Request and Response Client User-agent: Firefox Client Apache HTTP Server example request GET / HTTP/1.1 Host: www.tamk.fi User-Agent: Mozilla/5.0 (Mac..) ... example response HTTP/1.1 200 OKDate: Fri, 12 Jan 2007 07:51:41 GMTServer: Apache/1.3.37 (Unix)... ... <html><head><title>...

Firefox and HTTP headers

WORLD WIDE WEB STANDARDS

Jukka Siltanen and Jussi Pohjolainen

Why standards?

The web is a very diverse environment

Different computer hardware & software

Different people with expectations and cultural backgrounds

People with disabilities

Computers as ‘users’

Lots of different kinds of media and levels of interaction

The ultimate goal of standards is to ensure that the contents of Internet is universally accessible to everyone

HTML-markup language

Hypertext Markup Language

Hypertext means documents which can be internally linked to other documents

Developed by Tim Berners-Lee in 1990

Won Millennium Technology Prize prize for his work in 2004

Originally HTML provided very little control over looks and no interaction apart from navigation

HTML-markup language

World Wide Web was standardized in 1991

HTML became the standard for www-documents by mid 90’s

W3C (World Wide Web Consortium) was founded in 1994 to coordinate the development of HTML and other web standards

HTML-standards

HTML 2.0 v. 1996

HTML 3.2 v. 1997

First W3C recommendation

Contained presentational elements

HTML 4.0 v. 1997

Introduced CSS-style sheets to define presentational elements

HTML 4.01 v. 1999

Included only fixes to 4.0 standard

XHTML 1.x v. 2000 ->

Current standard

XML compatible HTML

Content and presentation are separated

Problems with the HTML-standard

Browser wars in the end of the 90’s

Netscape and Microsoft developed non-standard features for their browsers to gain advantage over competition

The visual appearance of web pages became more important than content itself

Pages became overly complex to edit

Slow to download and often browser dependent

All of the newest browsers still do not comply to standards with causes problems in web page design

Most HTML-documents on the web won’t scale to new breeds of web clients (i.e. mobile phones, PDAs, screen readers etc.)

Other www-standards

CSS (Cascading Style Sheets): creates the look of a XHTML-document

All presentational elements are specified in CSS file leaving only content and structure to the XHTML document

W3C-recommendation from 1999

Two versions: CSS1 and CSS2

CSS3 in development

Still some (minor and major) browser compatibility issues

Using CSS is the best way to provide support for alternative web clients

Other www-stardards

XML (Extensible Markup Language):

General purpose language to provide structure for information

Source of XHTML’s syntax

JavaScript (officially EcmaScript) used for interaction on www-pages:

One of the biggest problems in the browser wars – still significant differences in browsers

Follows the DOM-model (Document Object Model), which defines the hierarchy of XHTML-elements

DHTML (Dynamic HTML): Marketing term coined by Microsoft and Netscape for interactive web pages which use HTML, CSS and Javascript

JavaScript was originally Netscape’s term but it has become general term for client-side HTML-scripting

Is however not related to Java-programming language

Structure of a modern www-document Structure Looks Behavior HTML XHTML XML JavaScript DOM CSS1 CSS2

More information…

Official standards on W3C web site:

http://www.w3.org

Difficult site for non-technically oriented

Simple tutorials:

W3Schools: http://www.w3schools.com/

Great articles:

http://www.webstandards.org

http://www.alistapart.com/

Search engines…

Introduction to Basic Concepts in Web

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