Setting Up A High-Availability NFS Server

I will describe how to set up a high-availability NFS server that can be used as storage solution for other high-availability services like, for example, a cluster of web servers that are being loadbalanced.
In fact, I will create two NFS servers that mirror their data to each other in realtime using DRBD and that monitor each other using heartbeat, and if one NFS server fails, the other takes over silently. To the outside (e.g. the web server nodes) these two NFS servers will appear as a single NFS server.