How do I prevent dual Login of my users in my web application

how do I prevent my users for more than one simultaneous login per account. I am using开发者_如何学Python Vb.Net SQL.

Just coming from a 'web programmers' point of view, there is one really simple way:

You have to use some sort of central session system, where a Cookie on the user's browser has a hash, or some unique key that is also stored in the database. One each page request, or at set intervals, you check if that hash exists in the database.

Then, whenever you have a user log in, you delete any hashes that are tied to that user in the database, and then you create a new one for the user logging in.

What happens is if a user logs in, and you delete existing hashes from the database, then the other user will be logged out when you go to check for their hash in the database.

Not VB-specific, but: when the user logs in, keep track of the fact they are logged in. You can keep this in a cookie (watching out for the fact that a cookie can be tampered with.) You can keep track of it in a session variable. Or you could keep a boolean field in the database, "is_logged_in".

Then, when the user attempts to log in, you can check to see if they've already done so. If they're already logged in, the script might just return them to the home page (provided they used valid credentials. Otherwise a rogue user could type a username but leave the password blank, and depending on the behavior of your program, could see if a user was logged on at that specific time.)

Also, it is common that when someone is logged in, the webpage itself replaces the "Login" link with some text that says "Welcome, rascher!" and maybe links to their profile or preferences page.

Edit: Also remember to set is_logged_in (no matter how you track it) to "false" if they log out. It might also help to time this out - say, when the user closes the browser, or after "n hours" of inactivity (though that can be really annoying.) It will depend on how long people are generally logged into the system. Also note that someone might log in on their home computer, stay logged in, and then try to log in to the same place from work or their iphone. You might could look at ways of dealing with this (if the IP address is different than the current login, then log the other person out? Or something.)