Networks can be configured to be so incredibly redundant now - for reasonable prices - that there is no excuse for a data center not to achieve five nines (99.999%) of availability.

But what about the servers and applications? Why spend so much time up front configuring the network to make sure it doesn't fail, and then deploy an application to a single server?

Sure, there are ways to make sure individual servers have some redundancy to minimize failures -- things like RAID1, RAID5, or RAID10 (redundant array of inexpensive disks) which will protect against a disk drive failure (and I highly recommend this type of configuration for all production servers - and preferably the use of hardware RAID vs. software RAID). But what happens if a file gets corrupt on the RAID array? Or a recent configuration change brings the application down? Or a newly released patch conflicts with other settings and causes problems? Well, in these situations the server will go down and the application(s) hosted on that server will be offline.

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