These mechanisms:

• GET, HEAD, POST
• URL pathname
• URL query-part extension

are all that is used to support the interactions between browser and server required for

• normal link following
• search
• forms
• image maps

1. Simple link anchor requests - GET

Simple link requests just use GET as shown earlier.

The response is a status line and a document for display.

3 types of Web document

Comer [chapter 28] sees 3 types of Web document

1. static
2. dynamic
3. active

Static documents

• document is stored as a file on server disk
• same content is delivered every time that the URL is Got

Advantages

Disadvantages

Dynamic documents

• document is created by a program
  (compiled program or command script - cgi script)
  every time that URL is Got
• content may be different in whole or in part each time

Advantages

Disadvantages

The generator script may be very simple (insert date/time into doc body...)
or as complex as a front-end to a database management system.

This form of giving Web access to non-Web information is very powerful,
- may include updates as well as accesses
- can provide a standard or simplified wrapper giving access via forms
or a client-side application etc.
to legacy software

Active documents

Document contains executable elements that are executed by the client on arrival
- possibly execute repeatedly, whenever page is being viewed.

Executable elements are in various script languages or mobile code

javascript

active X

Java

applets

Advantages

Disadvantages

- cost of establishing and maintaining the active program

- complexity - executable code needs to run everywhere, cross-platform

- potential security risks from untrustworthy servers, untrustworthy code

2. Queries and <ISINDEX> tag

Browser - client

A document containing an <ISINDEX> tag is displayed with a text box for user input.

When RETURN or ENTER is typed, the browser sends a GET request with the contents of the box appended to the original document's URL as a query.

E.g. document from http://iwaki/cgi-bin/staff.html

-- if user types in the search text box

			 Freda Nurke? who cares

then the browser sends

GET http://cgi-bin/cgi-bin/staff.html?Freda+Nurke%3F+who+cares

The typed text is encoded:
spaces are encoded as "+"

or (like all other URL-significant punctuation)
as a hex-code of their ASCII representation

% hex-digit hex-digit

Server

The server parses the URL and splits it:
delivers the query part as environment variable to an execution of a program (if any) or discards it (if a plain file is named).

Delivers dynamic document
- which may be just a selection from a collection of static documents.

HTML note: Strictly, the ISINDEX tag is a head element, but many browsers accept it in the body part and display it at that point.

The text associated with ISINDEX is standard to the browser and cannot be set by the document.

3. Forms

Forms are a collection of HTML tags:

Browser

The various widgets in the FORM are displayed for user input and selection.

All widgets are reset if a "reset" button is selected.

When a "submit" button is selected, the data in that form is encoded as for query strings, tagged with the name of each item in the form
and is sent as contents of a POST message (or as a query string, if method=GET)
to the server specified in the form's ACTION attribute.

Server

The program referred to in the POST request pathname is executed, with the message body attached as standard input
(or the information in the query part of the URL supplied as environment variable QUERY_STRING, if the request was received was a GET).

The program generates a response to standard output
- which the server packages as the return message.

Example of a Form - Elisa

Note that this is 3 forms in one document. The action in each specifies a different server to send the Query string added to its URL.
Note the hidden fields which are included in the query automatically by the browser. The HTML source in part is:

 
<CENTER>	
<TABLE BORDER=1>

<!-- ALTA VISTA -->
<TR>
	<TD ALIGN=center>
	<A HREF="http://www.altavista.yellowpages.com.au/">
	<IMG SRC="/graphics/alta.gif" ALT="Alta Vista" BORDER=0><BR>
	<IMG SRC="/graphics/altaoz2.gif" ALT="Australian Mirror" BORDER=0></A>
	</TD>
	
	<TD>
		<FORM method=GET action="http://www.altavista.yellowpages.com.au/cgi-bin/telstra">
		<INPUT TYPE=hidden NAME=pg VALUE=q>
		<INPUT TYPE=hidden NAME=what VALUE=web>
		<INPUT TYPE=hidden NAME=fmt VALUE=".">
		<INPUT NAME=q size=35 maxlength=200 VALUE="">
		<INPUT TYPE=submit VALUE="Search">
		</FORM>
		
... part omitted ...
	</TD></TR>
	
<!-- INFOSEEK -->
<TR>
	<TD ALIGN=center VALIGN=center>
	<A HREF="http://www.infoseek.com/">
	<IMG SRC="/graphics/infoseek.gif" ALT="infoseek" BORDER=0></A>
	</TD>
	
	<TD>
		<FORM method="get" action="http://guide-p.infoseek.com/WW/IS/Titles">
		<INPUT NAME="qt" SIZE=35 VALUE="" MAXLENGTH=80>
		<INPUT TYPE=submit VALUE="Search">
		<input type=hidden name="cat" VALUE="">
		<input type=hidden NAME=col VALUE=WW>
		</form>
	</TD>
</TR>
</table></center>

Last modified: Tue Mar 30 11:24:08 EST 1999
Queries to : infs2052.iwaki.anu.edu.au