Platform & DevOps·Running it: capacity, failure and recovery
one datacentre lost power and a third of your instances went with it, which is the unit this is designed around.
Availability zone
Also calledAZ, zone, failure zone
An isolated datacentre or group of them within a region, with independent power, cooling and network, connected to its siblings over a low-latency link. It is the smallest unit of correlated failure a cloud offers, which makes it the unit you spread replicas across. Because the link between zones is fast but not free, spreading across them costs latency in single digits of milliseconds and real money in cross-zone data transfer.