Erlang lets you write applications supporting zero downtime by switching one live system to another running a different application version converting the application’s state on the fly to the new representation. Debian packages however can have only one installed version on a system which prevents using Erlang’s hot-upgrade feature easily.
Engineers at Yakaz (Jean-Sébastien Pédron) came up with a nice solution by creating separate directories for each application release and creating .deb packages for managing the transitions. I had to solve the same problem recently and found that the erlsvc Perl application they created needed a few patches to be usable with latest Erlang and other packages and with the changes it worked perfectly. Yakaz was not interested in accepting the patches and developing it further, but let me continue the maintenance. Please find the updated erlsvc application under my GitHub account and feel free to submit patches if you find something to fix in it.
I have also packaged erlsvc as an official Debian package and it is waiting in the NEW queue for being accepted. When it enters unstable you will have to make very little effort to make your applications hot-upgradeable on Debian!
update: The package has now been accepted to the archive. Thanks to the FTP Masters for their hard work!