What happens when one openfire server crashes

Dear All,

If i have Openfire running and handling around 4000(for example) users. and lets say all of a sudden the server crashes.

Then

1) Is it possible i can configure a backup server to kick into place and handle all my 4000 users(as long as they share a databse and share the users also?). Basically can i have a backup server, so my end-user does not experience any delay and continues to think that everything is fine.

2) If a backup server is not possible, how do I handle fault-tolerance for these 4000 users so they get a seamless experience.

Thanks

Vinu