For those cases where users can’t miss a heartbeat.
The idea of using virtual machines to build high available clusters is not new. Some software companies claim that virtualization is the answer to your HA problems, off course that’s not true. Yes, you can reduce downtime by migrating virtual machines to another physical machine for maintenance purposes or when you think hardware is about to fail, but if an application crashes you still need to make sure another application instance takes over the service. And by the time your hardware fails, it’s usually already too late to initiate the migration.
So, for each and every application you still need to look at whether you want to have it constantly available, if you can afford the application to be down for some time, or if your users won’t mind having to relogin when one server fails.
If you need to have your application always available, you’ll need to investigate if you can run two instances of the application at the same time and what happens if you would connect to another application instance in the middle of a session.