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Lesson#17
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CLIENT AND SERVER SIDE PROCESSING OF DATA
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CLIENT AND SERVER SIDE PROCESSING OF DATA
You are now familiar with the fact that internet presents a
two-way communication model, also called the
Client-Server Model as shown in Fig. 1 below. In the client
server model some data is processed on the
client side and certain data is processed by the server on the
server side.
Fig. 1
Client side processing
Data that is processed on the client side mainly includes the
HTML code, most of JavaScript, applets and
cookies (in most cases). As a matter of fact, the browser
installed on the client side has the capability to
interpret different HTML tags and display the result
accordingly. JavaScript code can be included in the
HTML document to be executed on the client side by the browser.
JavaScript is mainly used at the client
side for simple calculations and for pattern checking. We have
already learnt this concept in the examples of
calculator and registration form where JavaScript functions were
applied. Applets are compiled java
programs. Applet code is sent from server to the browser for
processing. Examples of applets include
animation files, java games spreadsheets etc. Typically applets
take more space on the window screen Applet
call example Consider the following HTML document where a
special <applet> tag has been used to make
a call for the applet code. The URL of the site where applet
code is stored has been assigned as a value for
the code attribute. Width and height attributes are used to
specify the space allocation for the display of the
applet result.
<HTML> <BODY>
<APPLET CODE=http://www.xyz.com/carfind.class width=100
height=200>DEMO</APPLET>
</BODY> </HTML>
Cookies
Cookies are text files stored on client’s hard disk sent by the
server side due to cookie related
programming in the code. Cookies are usually generated by
JavaScript or CGI scripts. In
JavaScript the processing for cookies takes place on the client
side. Consider the following
80
example where the property ‘cookie’ of the document object in
JavaScript is used to create a
cookie:
<HTML>
<SCRIPT LANGUAGE="JavaScript" TYPE="text/javaScript">
<!--
function updateCookie()
{
document.cookie=document.form1.cookie.value;
location.reload(true); } //--> </SCRIPT> <BODY> <SCRIPT
LANGUAGE="JavaScript"> <!--
document.write("Currently, your cookie is "+document.cookie);
//--> </SCRIPT> <FORM
NAME="form1">
<P>
This information would be treated as a Cookie: <INPUT
TYPE="TEXT"
NAME="cookie" size="50">
</P>
<INPUT TYPE="Button" name="setCookie" VALUE="Set Cookie!"
onclick="updateCookie()">
</FORM>
</SCRIPT>
</BODY>
</HTML>
In the above code, a text box called cookie is created. When a
user clicks the button Set Cookie, the control
is shifted to the function update Cookie which creates a cookie
in the hard disk of the client and opens or
reloads a fresh page for the user with the information of the
current cookie written on it. Suppose that I
type the word Ahmad in the text box and click the button Set
Cookie, then the cookie ‘Ahmad’ would be
consequently created and stored in the hard disk.
See figures 2-4 below.
Fig. 2
Fig. 3
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Fig. 4
Cookies can be used to track customers online. Typically, a
unique ID of the customer is stored in the
cookie file on the hard disk. Same ID is also stored in the
database along with the record of previously
purchased items by the customer. When the customer again visits
the online store, cookie (having customer
ID prepared against that URL) is transmitted by the browser to
the server side. There, it is matched with the
ID in the database. This enables the merchant side to know that
a particular customer (with a known buying
pattern) has come to visit again. By tracking the customer in
this manner, personalized recommendations
for certain items can be made to the customer in real time.
Cookies can also be used to maintain state between two broken
sessions. Thus, it is possible that some
information of the previous session is stored in the cookie
file, and this information is available (through
cookie) to the server side on starting a subsequent session. For
instance, in the code of a web form certain
information received form the user can be stored in the
variables as shown below:
firstName=document.form1.first.value
lastName=document.form1.last.value
email=document.form1.email.value
The coder can also define a variable to set the expiration date
of the cookie as follows:
expirationDate="Friday, 01-Dec-11 12:00:00 GMT"
Then using cookie property of the document object all such
information can be stored on the client hard
disk in a cookie file and can be used for maintaining state
between two sessions:
document.cookie="firstname="+firstname+";lastname="+lastname+";email="+email+
";expires="+expirationDate
Fat vs. Thin Client
Fat clients have a lot of processing done on the client side. On
the other hand, thin clients have very little
processing on client side, and most of the processing takes
place on the server side.
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