On a server, a process monitors the files in a Unix file system.
If a client sends the file name to be monitored, the server has to send the report to the client whether
See also:
http://benkibbey.wordpress.com/bubblegum/
Have a look at inotify at http://en.wikipedia.org/wiki/Inotify - linux only I'm afraid.
BSD has kqueue - see http://en.wikipedia.org/wiki/Kqueue.