Asp.net Session State Revisited
- by karan@dotnet
Every now and then I see doubts and queries which I believe is the most discussed topic in the .net environment - Asp.net Sessions. So what really are they, why are they needed and what does browser and .net do with it. These and some of the other questions I hope to answer with this post.
Because of the stateless nature of the HTTP protocol there is always a need of state management in a web application. There are many other ways to store data but I feel Session state is amongst the most powerful one. The ASP.NET session state is a technology that lets you store server-side, user-specific data. Our web applications can then use data to process request from the user for which the session state was instantiated.
So when does a session is first created? When we start a asp.net application a non-expiring cookie is created and its called as ASP.NET_SessionId. Basically there are two methods for this depending upon how you configure this setting in your config file. The session ID can be a part of cookie as discussed above(called as ASP.NET_SessionId) or it is embedded in the browser’s URL. For the latter part we have to set cookie-less session in our web.config file. These Session ID’s are 120-bit random number that is represented by 20-character string. The cookie will be alive until you close your browser. If you browse from one app to another within the same domain, then both the apps will use the same session ID to track the session state. Why reuse? so that you don’t have to create a new session ID for each request. One can abandon one particular Session by calling Session.Abandon() which will stop the page processing and clear out the session data. A subsequent page request causes a brand new session object to be instantiated. So what happened to my cookie? Well the session cookie is still there even when one Session.Abandon() is called and another session object is created.
The Session.Abandon() lets you clear out your session state without waiting for session timeout. By default, this time-out is a 20-minute sliding expiration. This expiration is refreshed every time that the user makes a request to the Web site and presents the session ID cookie. The Abandon method sets a flag in the session state object that indicates that the session state should be abandoned. If your app does not have global.asax then your session cookie will be killed at the end of each page request. So you need to have a global.asax file and Session_Start() handler to make sure that the session cookie will remain intact once its issued after the first page hit. The runtime invokes global.asax’s Session_OnEnd() when you call Session.Abandon() or the session times out. The session manager stores session data in HttpCache with sliding expiration where this timeout can be configured in the <sessionState> of web.config file. When the timeout is up the HttpCache will remove the session state object.
Sometimes we want particular pages not to time out as compared to other pages in our applications. We can handle this in two ways. First, we can set a timer or may be a JavaScript function that refreshes the page after fixed intervals of time. The only thing being the page being cached locally and then the request is not made to the server so to prevent that you can add this to your page:
<%@ OutputCache Location="None" VaryByParam="None" %>
Second approach is to move your page into its own folder and then add a web.config to that folder to control the timeout.
Also not all pages in your application will need access to session state. For those pages that do not, you can indicate that session state is not needed and prevent session data from being fetched from the store in requests to these pages. You can disable the session state at page level like this:<%@ Page EnableSessionState="False" %>tbc…
