restrict user to single login instance of web application

You should nob bother about this roundtrip, since you need to do this only at login, not on every request. Storing such data in the server's memory could slightly increase performance at that point, but can be a huge overhead in matter of resources. Of course, if you have your database remotely, you can make a local database with just these information (you don't need a server for that). In such complex cases session handling should not be inprocess, thus you will need an other alternative for storing sessions. This can be the same local database. And here you have a common goal: both the sessions and the single instance logon are in the same scope. Of course, you can use other appliances to store session data, I suppose those can be also convinced to get queried for already logged-on users. But be aware to set a proper session timeout, or your users will be faced with long lockouts.

Hi,
I have solved my problem by using application variable which contain list of userIds as string.I am setting this application varible to blank when the application is started in Global.asax.After this userIds append in this string for every user login.For every login, userid enter by user is cheked in this application variable and if found error msg is display other wise other validation done for successful login.When user logged out or close the application userId from that string removed.