AWS guarantees that if two or more instances are deployed in different availability zones (AZs), at least one of those instances will have external connectivity. The SLA does not guarantee that the application running inside the instance will be available. To address availability of the application, users will need to take additional steps to monitor the application and recover it should a failure occur.
In this eBook, the author describes in great detail what is needed to prepare the EC2 infrastructure to support business-critical applications clustered with Windows Server Failover Clustering (WSFC). In order to qualify for the 99.99 percent availability, instances must be deployed across multiple AZs. Readers will learn how to design EC2 infrastructure to support cross-AZ failover clustering.