A reader commented in private that the article about the technology SPOF was too abstract and lacked a few simple illustrations. The opposite would have been surprising ^^ The subject seems universal, which is no reason not to give a good example.
So, there I have it, example with an "all-in-one" security appliance, as is too often so often used in SMBs. It's mainly sold as a corporate firewall and serves many other uses.
The first SPOF is the hardware one. When the hardware fails, you've got a problem:
You can resolve that SPOF by adding another piece of hardware:
The second kind of SPOF is the network one. You have the backup hardware, but it's not available:
In this case, it's completely useless... You can solve this problem by making sure that the access to the redundant appliance is also redundant:
The third kind is the configuration SPOF. The backup is ready, working and available, but it's not used because clients are not configured to use it. For instance:
For this, you just have to configure the backup to be used in case of problem on the master or, if it's not possible, to setup an emergency procedure that switches from a configuration with the master to a configuration with the backup. That should look like:
Finally, and that the point in my previous post, you've got the technology SPOF, which means that both the master and the backup suffer from the same problem. This could be anything from "disk full" to "corrupted configuration file" ranging through "expired license". In this case, it's no help that you have a backup:
You just have to be sure about the list of the services you provide with that specific technology, and which of those are critical enough to require a reduced/degraded mode: