Linux Journal has an interesting article series on building redundant systems out of Raspberry Pis. Even though an individual Raspberry Pi is not that redundant, two Pis configured as redundant systems are.
Two Pi R articled describes how to set up two Raspberry Pis as a fault-tolerant file server using the GlusterFS clustered filesystem. It allows to have redundant, fault-tolerant storage shared across two Raspberry Pis, we can use that as a foundation to build other fault-tolerant services. When you see “Two Pi R”, you probably think the R stands for the radius for a circle, but to the article writer it stands for redundancy. GlusterFS is a userspace clustered filesystem that was chose for this project because is simple to configure shared network filesystem.
Now that we have redundant, fault-tolerant storage shared across two Raspberry Pis, we can use that as a foundation to build other fault-tolerant services. Two Pi R 2: Web Servers article describe how to set up a simple Web server cluster on top of the Raspberry Pi foundation we already have. The article uses nginx instead of Apache HTTP server, because nginx has the right blend of features, speed and low resource consumption that make it ideal for this site. Nginx is available in the default Raspbian package repository
This was interesting reading because I have earlier experience in building redundant networking systems with some other tools (for example using UCARP to allow couple of hosts to share common virtual IP addresses in order to provide automatic failover). In a good redundant systems the redundancy is available on several system layers.