We have to use Mirror servers for backup purposes.. which will be used only when the primary server goes down. Will the same license do?
That is fine, from a purely licensing point of view. As long as only one site is active (i.e., publicly accessible) at any one time, you’re good. A switchover / server backup is not an issue.
What is the protocol for the same?
Sorry, the protocol for what?
Please note Both the servers are going to be rsynched
Are you expecting many file uploads and such? Otherwise it’s the database you should be concerned about, not so much the physical files.
can the application server & database rest on two diff servers.
Yes, EE can use an external MySQL db server.
Suppose right now i have my site installed on different hosting provider & later if i want to move my site to different hosting provider dose it effect the expressionengine if yes then is there any solution for the same
Moving a site is certainly possible. It’s not fully automatic, though. Please take a look at this article.