While we are talking about state management we consider Session is one of most useful server side state management mechanism for a web application. In ASP.NET mainly we are having two type of state management 1. In Process and 2. Out Process . “In process” is the by default session 
storage mode for ASP.NET Web application and this is taken care by worker process in IIS. When it comes under Out Process we can use either of state server or SQL Server to persist session data. In case of In Process, session data stored in In memory of worker process. But when we are talking about “OutProc” session mode, we need to ensure that session data should be “Serialized” first . So, when we are moving session data from Web Server to Out Process Server ( State Server or SQL Server ) it can be a performance overhead based on the size of data that we are storing in Session.
ASP.NET 4.0 comes with a new option for compressing the Session data with Out Process Session mode. To enabling this functionality we need to add “compressionEnabled=”true” attribute with the SessionMode in web.config .
