I'm building a Client / Server app that has some very specific needs. There are 2 kinds of servers: the first kind provide most of the remote procedures and clients connect to these directly, while the second kind is a single server that should keep track of what users are active (clients) and how many servers of the first kind are active when a method is called.

The main thing is that the monitor should ONLY connect to the servers and not the clients directly. My first idea was to implement a simple login/logout rmi method when a client connects/ disconnects and keep track of those in a list but the main problem is when a client or server end abnormally.

For example, if a client goes offline abruptly the server should somehow be notified and update the list accordingly, while if a server goes out all of the clients connected to it should be marked as not active in the control server.

Any ideas of how to implement this functionality would be appreciated.