Amazon’s global infrastructure is broken up into Regions. Each Region supports the services for a given geography. Within each Region, Amazon builds isolated and redundant islands of infrastructure called Availability Zones (AZ). When VMware deploys a vSphere Cluster as part of the VMware Cloud on AWS managed service; all hosts for a given cluster are placed into a single AZ.
To protect against AZ failure, customers have the option to deploy a Stretched Cluster. When selected, a vSAN Stretched Cluster is created across three AZ’s, creating a vSphere Cluster that can survive the loss of entire Availability Zone. To protect against split-brain scenarios and help measure site health, a managed vSAN Witness is also created in a third AZ. The third AZ is picked at random from the remaining Availability Zones.
With a copy of the data in each AZ, vSphere HA is empowered to recover from any failure using a simple restart. In summary, Stretched Clusters simplify the cloud by providing the same trusted controls and capabilities in addition to the scale and flexibility of the AWS infrastructure. For more information about VMware Cloud on AWS Stretched Clusters, please read this article.