CVSup

Though usually deployed to keep source code trees in sync by communicating diffs, this approach could be applied Web content sources kept under CVS as well.

All of these approaches can be made secure by running them over ssh or ssl connections and they're free. Windows NT comes with a Directory Replication service, but, surprisingly, it is often reported to not work.

Another plan of attack often used is having no content file systems on the Web servers but instead having the content NFS mounted. While elegant in its simplicity, this approach invites the troubles one finds with very busy NFS servers. Work arounds include running something like cachefs, if your OS supports it, to cache NFS content on local disks and also using dedicated and specifically engineered RAID based servers such as Network Appliance devices.

Going with NFS may warrant equipping the servers with additional network interfaces and running all of the NFS network traffic over a private network. Finally, NFS brings back the specter of a single point of failure on systems that otherwise provide a good deal of redundancy. So plan on buying at least two Network Appliances! Ugh!