NFS server fail-over - how do you do it?

[adp]

I am running a FreeBSD 4.9-REL NFS server. Once every several hours our main
NFS server replicates everything to a backup FreeBSD NFS server. We are okay
with the gap in time between replication. What we aren't sure about is how
to automate the fail-over between the primary to the secondary NFS server.
This is for a web cluster. Each client mounts several directories from the
NFS server.

Let's say that our primary NFS server dies and just goes away. What then?
Are you periodically doing a mount or a file look-up of a mounted filesystem
to check if your NFS server died? If so are you just unmounting and
remounting everything using the backup NFS server?

Just curious how this problem is being solved.