Basic Structural Concepts of .NET

1. Basic Structural Concepts of .NET Browser – Server Interaction Christopher M. Pascucci

2. Introduction • Web applications are a type of client/server application, which means that the functions of the applications are split between the client computer and server computer. • Client and servers are connected together via the Internet or other network and they communication using HTTP (Hypertext Transfer Protocol) • Web server stores the web applications that are accessed by the clients. • They can be configured to process ASPX, JSPX, PHP, etc… • Example web servers: Microsoft IIS & Apache • Most web applications use data stored in a database so it needs to implement a connection to a DBMS like SQLserver or Oracle.

3. Processing Static Web Pages • Static web pages are simple HTML pages that are stored on a web server. These pages don’t change in response to user input and the content is always the same. • A web browser requests a page from a web server by sending the server an HTTP request. • This request is initiated by typing in a URL, clicking a link, or begin redirected. • Includes the name of the HTML file being requested, the IP address of both the web browser and web server, etc… • A web server replies to a request with an HTTP response. • The response includes the HTML doc being requested.

4. Processing Dynamic Web Pages • Dynamic web pages are HTML pages that can change in some way each time the page is displayed. They are a web form with controls that allow the user to interact with the application. • Instead of being stored in the form of HTML, they are generated dynamically by the web application. • A web server receives a request for a dynamic page, it looks up the file extension of the requested file in a list to find out which application server should process this request. • If the extension is ASPX, the request is passed on to ASP.NET for processing. • If the extension is JSPX, the request is passed on to Tomcat for processing. • The application server receives the request and it runs the web form. The web form then generates an HTML document and returns it to the web server. • The web server then sends the HTML document back to the browser.

5. Dynamic Pages • After a dynamic page is displayed on the browser, the user can interact with it using its controls. • Some of the controls let the user start an HTTP request that post the page back to the server, which is called a postback. • The page is then processed over again using the data the user entered. • The entire process that starts with a browser requesting a page and ends with server returning the page is called a roundtrip. • There are two ways to a page can cause a postback to the server. • By using a submit button. <input type=”submit” name=“btnSub”> • Through script. • <script lang=“vbscript”> • Sub checkForm() • formName.submit() • End Sub • </script>

6. ASPX Browser – Server Interaction

7. Basic Structural Concepts of .NET Managing State & Scope Christopher M. Pascucci

8. What is State • State refers to data maintained by an application for a single user. • An application must maintain a separate state for each user. • HTTP is a stateless protocol, which means it doesn’t maintain state between roundtrips. • Since HTTP is stateless the web applications must handle this.

9. How ASP.NET Maintains State • ViewState (Page) • Request • Application • Session • Cookies • Cache

10. How ASP.NET Maintains State • ViewState (Page) • Maintains the values of form control properties. • ViewState is implemented by default. • When a webform (aspx) is loaded, it can re-post to itself by means of a form submit. • A special state Boolean variable named IsPostBack is set to False when the page is initially loaded, and to True after its first postback. • Page state is maintained across postbacks to a single page by a single user. • Page state is sometimes called ViewState, because there is a ViewState object that contains property values for controls in the webformas well as the user created attribute/value collection; however, only simple data types and serializable objects can be stored in the ViewState object. • ViewStateis maintained across postbacks. • It stores the information (encrypted) in a hidden field in the form. • <input type="hidden" name="__VIEWSTATE" value="dDwtMzQzNDc3MjYyO3Q8O2w8aTwyPjs+O2w8dDw7bDxpPDY+O2k8Nz47PjtsPHQ8cDxwPGw8VGV4dDs+O2w8SEVMTE8gTVIuIEEgQTs+Pjs+Ozs+O3Q8cDxwPGw8VGV4dDs+O2w8TWVzc2FnZTogYTs+Pjs+Ozs+Oz4+Oz4+O2w8b3B0TXI7b3B0TXM7b3B0TXM7b3B0RHI7b3B0RHI7Y2hrVXBwZXI7Pj4YjhxhXxf0Ix5rYidfnJsm+x6wrw==" />

11. How ASP.NET Maintains State • Request • A Request object is created whenever a URL request is made from the browser to a web server. • Request state is maintained during the life of a single request object. • The program can access data from web form controls on the server-side by simply using Request(“FormControlName”) in the code. • Application • Application state is shared among all users (sessions) of an application. • It can be used to maintain values that apply to all users and you can add your own data/values to the application state object. • The Application is all pages contained in a given web site. The application starts when the first user sends a URL to the web server on which the web is located. It ends when the last client (user) of the web ends its session or when the application is stopped/restarted.

12. How ASP.NET Maintains State • Session • Session state is unique to each user of an application but maintained throughout their use of the application.  • A Session is started when a user sends a URL to activate a given web site or Application. • The session can end in one of 3 ways: • If there is no request sent for a time called the Session Inactive Time. There is a default value set within the web server for this time, but it can be overridden by the session.Timeout property for a particular session object. • By the program using the session.Abandonmethod • If the client is terminated by closing the browser window, but Session Inactive Time will still have to elapse.

13. How ASP.NET Maintains State • Cookies • Cookie state is maintained in the client browser and can persist across page, session and application state. • It can even persist after the application terminates, for any predetermined time period. Its lifetime can be programmatically controlled. • Cache • Cache state has application scope but variable lifetime determined by the following events: • Specific files or database tables being updated • Another Cache attribute changes value • Time expiration