Active Server Page Processing and the World Wide Web

1. Active Server Page Processing and the World Wide Web The Problem of State

2. 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

3. 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

4. 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 146.227.160.79 • www.cse.dmu.ac.uk 146.227.57.2 • G677 (my server) 146.227.53.94

5. Name Servers • http://209.85.227.105/ not obviously www.google.com

6. 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)

7. 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!

8. The Web’s Client Server Model

9. Where is Client and Server in Visual Studio?

10. 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

11. Server Side Code – Dynamic Pages

12. HTML Forms GET and POST • HTML allows simple form creation

13. HTML Form Code Change POST to GET http://g519-md.ad.cse.dmu.ac.uk/Request/?txtFirstName=Matthew&txtLastName=Dean&Submit1=submit

14. 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

15. Create a Similar Form in ASP

16. Active Server Controls Note the tag <asp

17. 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

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

19. 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!

20. 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?

21. 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.

22. 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

23. Load session variables in the load event HTTP Request Browser Server HTML Page Save session variables in the unload event Use Session Variables • Use in conjunction with IsPostBack in the Load Event of the page…

24. 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.

25. 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!