One of the most important limitations of Microsoft Azure is the lack of an SLA for single-instance virtual machines. If a virtual machine is not part of an availability set, that instance is not covered by any kind of SLA. The reason for this is that when Microsoft needs to perform maintenance on Azure hosts, in many cases, a reboot is required. Reboot means the virtual machines on that host will be unavailable for a while. So, in order to accomplish High Availability for your application, you should have at least two instances of the application running at any point in time. As mentioned in previous chapters Microsoft is working on some sort of hot patching which enables virtual machines to remain active on hosts being patched. Details are not available at the moment of writing.

High Availability is a crucial feature that must be an integral part of an architectural design, rather than something that can be "bolted on" to an application afterwards. Designing...