CLIENT AND SERVER SIDE PROCESSING OF DATA

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Lesson 17

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.

</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

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example where the property `cookie' of the document object in JavaScript is used to create a

cookie:

<HTML>

<!--

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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