ASP.NET state management

ASP.NET applications are hosted in a web server and are accessed over the stateless HTTP protocol. As such, if the application uses stateful interaction, it has to implement state management on its own. ASP.NET provides various functionality for state management in ASP.NET applications. These functionality include Application state, Session state and View state variables.

Application state
Application state is a collection of user-defined variables that are shared by all invocations of an ASP.NET application. These are set and initialized when the Application_OnStart event fires on the loading of the first instance of the applications and are available till the last instance exits. Application state variables are accessed using the Applications collection, which provides a wrapper for the application state variables. Application state variables are identified by their names.

Session state
Session state is a collection of user defined session variables, which are persisted during a user session. These variables are unique to different instances of a user session, and are accessed using the Session collection. Session variables can be set to be automatically destroyed after a defined time of inactivity, even if the session does not end. At the client end, a user session is identified either by a cookie or by encoding the session ID in the URL itself.

ASP.NET supports three modes of persistence for session variables:

1.)InProc
The session variables are maintained within the ASP.NET process. However, in this mode the variables are destroyed when the ASP.NET process is recycled or shut down.

2.)StateServer
In this mode, ASP.NET runs a separate service that maintains the state variables. Because the state management happens outside the ASP.NET process, this has a negative impact on performance, but it allows multiple ASP.NET instances to share the same state server, thus allowing an ASP.NET application to be load-balanced and scaled out on multiple servers. Also, since the state management service runs independent of ASP.NET, variables can persist across ASP.NET process shutdowns.

3.)SqlServer
In this mode, the state variables are stored in a database server, accessible using SQL. Session variables can be persisted across ASP.NET process shutdowns in this mode as well.

View state
View state refers to the page-level state management mechanism, which is utilized by the HTML pages emitted by ASP.NET applications to maintain the state of the web form controls and widgets. The state of the controls are encoded and sent to the server at every form submittal in a hidden field known as __VIEWSTATE. The server sends back the variable so that when the page is re-rendered, the controls render at their last state. At the server side, the application might change the viewstate, if the processing results in updating the state of any control. The states of individual controls are decoded at the server, and are available for use in ASP.NET pages using the ViewState collection.