Is there anyone got an idea for only one active server serving clients while have at least one server to back up when the server is going down. The active server must serve all clients and the clients must not split between the servers. When the active server goes down, the backup server becomes the active server and takes over all the clients.

My question is how can we create more than one server as a backup server? How to let the new clients know when the active server goes down and let them go to the active server? Your help is much appreciated.