I wrote a multi-user application that used the following the method. When each user attempted to log in to the application the application would first check for the existence of a certain file on the server; if found, the application would not allow the user to log in but would give the message that the application was off-line for maintenance. For the users currently logged in to the application, the front-end GUI had a timer that would check for the existence of the file as well. If the timer found the file, then the timer method would notify the user that the application is going off-line in a fixed time. In this case the application would start a second timer that would force a log-out of the user after this fixed time interval -- this gives the user the ability to complete their work and disconnect from the application; otherwise, the second timer event would force the disconnect.

Now after the fixed time interval has expired, the application is available for backup and other system maintenance. Once the maintenance is completed, the file on the server is deleted and this now allows users to begin to work again. You can include in the file the time/date that the application maintenance is expected to be completed.