Basic Concept of Client-Server Computing

CLIENT SERVER MODEL

A simple definition of client server is "server software accepts request for data from client software and turns the result to the client".
Client application program running on the local machine requests a service from another application program-server-running on the remote machine.
Commonly server provides service to any client , not a particular client.
Generally, a client application program that requests a service should run only when it is needed. A server program providing service should run all the time, as it does not know when its services will be needed.
In client-server computing major focus is on SOFTWARE.
Element of client-server computing: Request-response may be repeated several times, the process is finite.  

                                          Client-Server Model

Client-server computing is distributed access,not a distributed computing.

 
Most typical use of technology in client-server is data base server.It accepts request for data, retrieves the data from its database (or request data from another node ) and passes the result back.

Server Side Programming/Dynamic HTML

Servlet

It is a java program which runs within the context of web servers, so only one process is there and no IPC mechanism
Run-time environment of a servlet is called servlet is called servlet container. There are two type of containers:
  (i) Generic servlet : any protocol.
  (ii) http servlet : http protocol

Servlet Methods:

1. init ( ) : It is called when the servlet is deployed. It is used for starting and initialising the servlet.
2. Service ( ) : It is called when a sarvlet gets a request. It there  are multiple request, service method will run in multiple threads.
3. Destroy( ) : It is called when the service is undeployed  or removed from the servlet.

Types of Contents on a website

1. Webpages (html) : WEbpages are primary things. Following types of pages are possible:
    (i) Static : HTML
    (ii) Dynamic : HTML + Programming language/server side script.
    (iii) Client side dynamic : HTML + Javascript
    (iv) Both sides dynamic : HTML + Programming language , server sid script + Javascript.
2. Images/Audio/Video : Audio/Video will play on the client side only.
3. CSS : Cascading Style Sheets are used for defining the styles of tags in html. CSS improves html.
4. Script : Script is embedded within html. Browser executes the script. Server side dynamic pages are returned using ASP , Servlet , JSP , PHP , ASP.NET whereas client side dynamic pages are returned using JavaScript, VBscript.

[Javascript can communicate with server to get some data or information for updation but not the complete html page.
Javascript can bypass the browser and send a request to a server side program, server side program will respond to program in from of XML and Javascript will update the data on a page.] 
 
5. XML : It is used for representing the data where HTML is used for formatting the data.
6. AJAX : Asynchronous Javascript XML , a new technique used to make pages dynamic on both sides. The purpose of AJAX is to achieve multitasking,since there is not only one script running all task rather multiple scripts are used.
7. Client side components
8. Server side components
[A component is a reusable piece of software which is reused at runtime , it provides its functionality through set of functions known as interface.]

Features of Web-server

1. Generate Response : If a request is made for a static page then the web-server will send the html contents of a file as response.
If a page is dynamic then web-server will execute the program , collects its output and send it as response.
2. Virtual Hosting / Shared Hosting /  Dedicated Hosting : A web-server should host more websites to reduce cost of web-hosting,The web-server will identify which website is desired.So, domain name or URL is not confined to DNS only , it is now used by web-server also.
3. Compression (gzip) : Every page which is transferred from client to server or vice versa is in form of compressed files.At the client/server site again it is used by decompressing the pages.
4. Authentication : Authentication users have their own pages , no one else can see it.
5. Secure Socket Layer (SSL) : SSL works only on network ( data is encrypted).
6. Web Statistic : Keeps an account of area from where people are visiting, number of people visiting, problems faced by them etc.