[martius]

From https://cloud.google.com/docs/geography-and-regions#regions_...

> Regions are independent geographic areas that consist of zones. Zones and regions are logical abstractions of underlying physical resources provided in one or more physical data centers. > (...) > A zone is a deployment area for Google Cloud resources within a region. Zones should be considered a single failure domain within a region. To deploy fault-tolerant applications with high availability and help protect against unexpected failures, deploy your applications across multiple zones in a region.

You should use "region" and "zone" as abstract concepts with shared properties like network topology, local peering, costs, and availability. AFAIK no cloud provider discusses (nor provides guarantees) against specific threats or correlated failures.

There is no guarantee that a given risk will not impact multiple zones, but this risk is lowered by the implementation of various safeguards (for example, rollouts are not happening in multiple regions at the same time).

Google doesn't say "put your VMs in more than one zone because you can be sure we won't have all zones in a region down at the same time", but rather "by putting your VMs in multiple zones in the same region, you can target better SLOs that the SLOs in one zone".

Note that it's different from the concept of "availability zone" of AWS which explicitly says that AZs are physically separated:

> AZs are physically separated by a meaningful distance, many kilometers, from any other AZ, although all are within 100 km (60 miles) of each other.

https://aws.amazon.com/about-aws/global-infrastructure/regio...