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Simple Web Theory

When we installed ASP.NET in Chapter 1, the installation was broken down into stages because we installed several different pieces of software. One of these pieces of software was the web server, whose main job is to make your web pages available to everyone. Another job of the web server is to provide an area (typically in a directory or folder structure) in which to organize and store your web pages, or whole web site.

 

When you use the Web to view a web page, you will automatically make contact with a web server. The process of submitting your URL is called 'making a Request' to the server. The server interprets the URL, locates the corresponding page, and sends back the code to create the page as part of what is called the Response to the browser. The browser then takes the code it has received from the web server and compiles a viewable page from it. The browser is referred to as a client in this interaction, and the whole interaction as a client-server relationship.

 

The term client-server describes the workings of the Web, by outlining the distribution of tasks. The server (the web server) stores, interprets, and distributes data (that is compiled into web-pages), and the client (browser) accesses the server to get at the data. From now on, whenever we use the term client, we are just referring to the browser.

 

To understand what is going on in greater detail, we need to briefly discuss how the client and server communicate over the Internet using the HTTP protocol.


 

The HTTP Protocol

The Internet is a network of interconnected nodes. It is designed to carry information from one place to another. When the user tells the browser to fetch a web page, a message is sent from the browser to the web server.

 

This message is sent using Hypertext Transfer Protocol (or HTTP). HTTP is the protocol used by the World Wide Web in the transfer of information from one machine to another. When you see a URL prefixed with http://, you know that the Internet protocol being used is HTTP, as HTTP is the default protocol used by web browsers. This means if we type www.wrox.com, the browser will automatically use the HTTP protocol and search for http://www.wrox.com.

 

The message passed from the browser to the web server asking for a particular web page is known as an HTTP Request. When the web server receivesthis request, it checks its stores to find the appropriate page. If the web server finds the page, it bundles up the HTML in an HTTP Response, and sends this back across the network to the browser. If the web server cannot find the requested page, it issues a response that features an appropriate error message, and dispatches that page to the browser.

 

Here's an illustration of the process, as we understand it so far:

 

 

HTTP is known as a stateless protocol. This is because it doesn't know whether the request that has been made is part of an ongoing correspondence or just a single message, just the same way your postman won't know whether your letter is the first asking your local hi-fi company for a refund, or the fifteenth.


The reason HTTP is stateless is that it was only intended to retrieve a single web page for display. Its purpose was to handle simple transactions, where a user requests a web page, the browser connects to the requisite web server, retrieves that web page, and then shuts down the connection. The Internet would be very slow and might even collapse if permanent connections needed to be maintained between browsers and servers as people moved from one page to another. Think about the extra work HTTP would have to do if it had to worry about whether you had been connected for one minute or whether you had been idle for an hour, and needed disconnecting. Then multiply that by a million for all the other users. Instead, HTTP makes the connection and delivers the request, and then returns the response and disconnects. However, the downside of this is that HTTP can't distinguish between different requests, and can't assign different priorities, so it won't be able to tell whether a particular HTTP Request is the request of a user, or the request of a virus infected machine, that might have been set up, for instance, to hit a government web server 1,000 times an minute. It will treat all requests equally, as there are no ways for HTTP to determine where the request originated.

 

If a request is successful, the HTTP Response body contains the HTML code (together with any script that is to be executed by the browser), ready for the browser to use. Additional HTTP Requests are used to retrieve any other resource, such as images, dictated by the HTML code returned after the first request.

Where ASP.NET Fits in with the .NET Framework

In the last chapter, we encountered some of the major concepts involved in the .NET Framework. In this chapter, we've already gained a better understanding of how a browser sends a web page request, and how the web server sends the page back to the browser. What we're going to do now is to tie the two together, as this will help us understand what is happening when we use forms and server-side controls.

 

Let's sum up the five step process for delivering a web page:

 


 

1.       The client requests a web page.

2.       The web server needs to locate the page that was requested; and if it's an ASP.NET page then this code will need to be processed in order to generate the HTML that is returned to the browser.

3.       If the name of the web page is suffixed with .aspx, the server sends it to the aspnet_isapi.dll (which is attached to the web server) for processing. The aspnet_isapi.dll doesn't actually do much itself, it just forwards the ASP.NET code to the Common Language Runtime. We looked at what role this performs in the last chapter, and here we'll just treat it as a black box. If the ASP.NET code hasn't been compiled before, it is compiled and then executed, and pure HTML comes out at the other end. In this way the HTML is created dynamically.

4.       The HTML stream is returned to the browser.

5.       The browser displays the web page.

 

There are a lot of advantages to generating a page dynamically: you can return information to the user based on their responses in a form, you can customize web pages for a particular browser, you can personalize information (and utilize a particular profile for each individual), and much more beyond the static text and graphics that pure HTML returns. This is down to the fact that the code we write is interpreted at the time it is requested.

 

Now we have a basic understanding of how the web works, it's time to get stuck into forms. We'll begin by looking at HTML forms, as they are often much misunderstood. Also, once you know about HTML forms, the ASP.NET server controls begin to look familiar, as the HTML form controls perform many of the same functions as their server-side counterparts.

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© Copyright 2002 Wrox Press This chapter is written by David Sussman, et al and taken from "Beginning ASP.NET with C#" published by Wrox Press Limited in June 2002; ISBN 1861007345; copyright Wrox Press Limited 2002; all rights reserved.

No part of these chapters may be reproduced, stored in a retrieval system or transmitted in any form or by any means -- electronic, electrostatic, mechanical, photocopying, recording or otherwise -- without the prior written permission of the publisher, except in the case of brief quotations embodied in critical articles or reviews.











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