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An isolated data center within a cloud region, your unit of fault tolerance
Quick Definition
An availability zone is an isolated data center location within a cloud region, with independent power, cooling, and networking. Distributing workloads across multiple zones improves fault tolerance and high availability, though cross-zone data transfer can add to networking costs.
Cloud regions are made up of availability zones: physically separate data centers with their own power, cooling, and networking. If one zone fails, the others keep running. Spreading your application across zones is the standard way to survive hardware and facility failures.
The reliability benefit is real, and so is a hidden cost: data moving between zones is usually billed. Architectures that constantly shuttle traffic across zones can incur surprising data transfer costs, so high-availability design and cost design should be considered together.
Example. A service runs in three zones, but its database lives in one. Every read from the other two zones crosses a zone boundary and is billed. Adding read replicas in each zone improves speed and removes most of the cross-zone charge.
When you review reliability architecture, put cross-zone traffic on the checklist. The Cloud Cost Management guide covers networking costs that hide in resilient designs.
Two is the minimum for fault tolerance and three is the common standard, since it keeps a majority available if one zone fails.
Usually not. Most providers charge for data moving between zones, which is why zone-aware design matters for cost.
A region is a geographic area, such as Mumbai or Virginia. Each region contains multiple isolated zones.