When the service that is to be backed up is a source of data this method of creating redundant servers, though not well suited, can still be successfully applied. A backup POP3 or IMAP server could be configured such that an email explaining the current situation is delivered. Key parts of a web site can be duplicated and warning pages issued in lieu of unavailable pages.
When the ARP based solution is coupled with a TCP/IP switch then services that provide content can also be made more redundant. Finally by housing content on a NFS server backup servers can have access to content and serve it accordingly.
The redundant servers created can be used in a variety of situations. First and foremost their activation can be automated such that the backup servers are called into service in emergency situations. Automation is particularly attractive here as such situations typically occur around 2am. Additionally, the redundancy can be used to prevent disruption to users when system maintenance and hardware upgrades are being undertaken.
We can see that using simple utilities coupled with the power of Linux redundant servers are easy to realise even for small organisation. This redundancy can be used to provide a more constant and stable level of service to users. This increases their satisfaction while reducing your support burden.
While it is obvious that the solutions presented are targeted towards low end applications there is no reason why these concepts could not be scaled up. What is important to realise is that the power of Linux enables us to create solutions that suit our needs rather than modifying our needs to fit with the solutions available.