Transcript Powerpoint
The Problem of State We will look at… Sometimes web development is just plain weird! Internet / World Wide Web Aspects of their operation The role of clients and servers ASPX Page (Web Form) How it is passed between browser and server The structure of the page How it provides functionality to the browser The problem of state The Internet and TCP/IP Network of networks Defence research in the 60s TCP/IP (Transmission Control Protocol / Internet Protocol) Allows programs on computers to talk to each other The IP Address Uniquely identifies each machine 32 bit number made up of four 8 bit numbers Visit http://209.85.227.105/ Assigned in blocks www.dmu.ac.uk www.cse.dmu.ac.uk G677 (my server) 146.227.160.79 146.227.57.2 146.227.53.94 Name Servers http://209.85.227.105/ not obviously www.google.com Ports TCP/IP allows programs on machines to communicate IP address identifies machine port number identifies program There 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 Days Up until about 1989 the Internet existed quite happily without the World Wide Web File Transfer Protocol (FTP) Telnet Usenet 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 Code Code may be added at either end of the process Client side code runs at browser Action Script (Flash) JavaScript VBScript Server Side Code ASP.NET (C # VB.NET) PHP JSP Server Side Code – Dynamic Pages HTML Forms GET and POST HTML 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 = False Post 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 event The server runs page unload event ASPX 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 event The server runs other events (in this case the click event of the Go button) The server runs the page unload event All 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 Cookies Cookies 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=passwo rd123 Use Session Variables HTTP Request Load session variables in the load event Browser Server HTML Page Save session variables in the unload event Use in conjunction with IsPostBack in the Load Event of the page… 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. Summary Because 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 first Other Events next Unload event last Load and Unload always run!