>When the user logs in, create a text file on a shared location. Write the file name to the user table in the database. When the a user logs in, loop through the listed files, attempting to open them. If you can open a file, that user exited the application. If you can't open the text file, the user is still in the system.
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>>What are the alternatives?

I can use my .dbf lock method if there is a shared location.