The Problem of State

We will look at…Sometimes web development is just plain

weird!

Internet / World Wide WebAspects of their operationThe role of clients and servers

ASPX Page (Web Form)How it is passed between browser and serverThe structure of the pageHow it provides functionality to the browser

The problem of state

The Internet and TCP/IP

Network of networks Defence research in the 60sTCP/IP (Transmission Control Protocol /

Internet Protocol)Allows programs on computers to talk to

each other

The IP AddressUniquely identifies each machine32 bit number made up of four 8 bit numbers Visit http://209.85.227.105/

Assigned in blocks

www.dmu.ac.uk 146.227.160.79www.cse.dmu.ac.uk 146.227.57.2G677 (my server) 146.227.53.94

Name Servershttp://209.85.227.105/ not obviously www.google.com

PortsTCP/IP allows programs on machines to

communicateIP address identifies machineport number identifies programThere is no law that states a specific port must be

used for a service however there are certain ports that traditionally provide services.

80 HTTP (web pages)21 FTP (File transfers)119 NNTP (Network News Transfer Protocol)443 HTTPS (secure web pages)

The Good Old DaysUp until about 1989 the Internet existed

quite happily without the World Wide Web

File Transfer Protocol (FTP) TelnetUsenet

World Wide Web - Is not the internet!

The Web’s Client Server Model

Where is Client and Server in Visual Studio?

Server v Client Side CodeCode may be added at either end of the process

Client side code runs at browserAction Script (Flash)JavaScript VBScript

Server Side CodeASP.NET (C # VB.NET)PHPJSP

Server Side Code – Dynamic Pages

HTML Forms GET and POSTHTML allows simple form creation

HTML Form Code

Change POST to GET

http://g519-md.ad.cse.dmu.ac.uk/Request/?txtFirstName=Matthew&txtLastName=Dean&Submit1=submit

Active Server Pages (ASPX)Events & Handlers User triggered events 

Click Triggered when a user presses a button 

Selected Index Changed Activated when the user selects an item off a drop down list

  System generated 

Load Runs when the ASPX page is loaded by the server

Unload Runs when the ASPX page is unloaded from the server

Create a Similar Form in ASP

Active Server Controls

Note the tag <asp

Post Back = FalsePost back is false on the first HTTP request

The browser sends the request to the server for the page

The server runs the page load eventThe server runs page unload eventASPX controls converted to HTML and sent to

the requesting browser

What the Browser Gets…ASP & Code never makes it to the browser!

Post Back = True

The browser sends the HTTP request to the server

The server runs the page load eventThe server runs other events (in this case the

click event of the Go button)The server runs the page unload eventAll asp controls changed into suitable HTML

controls and sent back to the requesting browser

NOTE Load and Unload Events ALWAYS RUN!

The Problem of State We have seen the following points

The web follows a client server mode of operation The ASPX page is rendered on the server and sent to the browser as HTML The page is rendered in two modes

PostBack = False

(The first time the page is requested, Load – Unload events) PostBack = True

(Subsequent renderings of the page, Load – Other Events – Unload)

  The thing to note in all of this messing about is that the settings of

the page are not automatically remembered on each round trip.  The web is referred to as stateless.

So how is this problem addressed?

Use CookiesCookies are small files stored on the client

computer that allow the web page to record details of its visit to that machine.

 Cookies may be turned off by the user of the

client machine.

Not suitable for sensitive data.

Use a Query String

This is achieved by setting the HTML forms method to Get rather than Post.

 This is a good technique so long as the data isn’t a

potential security risk. This would be a very bad query string. http://www.mysite.com/login.asp?

UserName=Fred&Password=password123

Use Session Variables

Use in conjunction with IsPostBack in the Load Event of the page…

Browser Server

HTTP Request

HTML Page

Load session variables in the load event

Save session variables in the unload event

Potential Problem…Remember the load event runs every time the page is

processed and it is the first thing the server does.  If we load the messages on subsequent renderings of the

page we get the following problem… 

I click an entry in the list and press delete The load event runs re-setting the list removing my selection The delete click event fails because the list has been re-set

 To avoid this kind of problem we need to check IsPostBack

to see if it is appropriate to read data at the server.

SummaryBecause the web is stateless and processes

pages the way that it does you will at some point get very confused about state!

Remember Load event runs firstOther Events nextUnload event last

Load and Unload always run!