Here are a couple of quite painless ways to do this.
Solution No. 1
Create a CGI script that works with a database and returns
(prints) an image. Then, in your web pages, you can set
up JavaScript code to reload an image (which can be very
small) every 15 seconds or so. Now, the image source
will be that CGI program. Every time the CGI is invoked,
it will log username and time to the database. Now, to
find out who is logged in,
SELECT USERNAME FROM LOGGED_IN WHERE LAST_TIME > NOW() - 15
The SQL will be db-specific, but you get the point. The
good thing about this approach is that you don't need a
separate frame. The downside is that you have to know
some JavaScript.
Solution No. 2
This one includes a Java Applet. On initialization, it
can send HTTP request and log the user in. When the user
leaves your site or closes his browser, the cleanup
procedure is invoked, which can log the user out (delete
a record from a database or a file).
The second way is not very good if someone trips over his
powercord, or Netscape crashes. You pick the way to do it.
$.02
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