Amazon’s Availability Zones are a fabulous new feature that allows users to assign instances to locations that are very fault-tolerant from one another yet that have very high bandwidth between each other. I wish I could have done something like that as easily when I was responsible for operations at Citrix Online and we had 5 datacenters worldwide. As I’ll explain in this post, what Amazon actually provides us is much better than just putting servers into multiple datacenters.
The most confusing thing about availability zones is the name: In the cloud, what exactly is an “availability zone”? The easiest way to think about it is that a zone equals a datacenter. If power goes out in one datacenter and the generators fail to start (naah, that never happens…) then it doesn’t affect the other datacenter. Or if there’s a fire, one datacenter may burn out or be otherwise incapacitated, but others are unaffected. In reality zones don’t necessarily correspond to datacenters. Given careful engineering, it’s possible to have multiple “rooms” in a datacenter that are highly failure isolated while technically still being part of the same datacenter (imagine football-sized fields here).
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