06. Redundancy In AWS
ND0063 C1 L2 05 Redundancy In AWS Services
Subnet Groups are key to creating Multi-AZ redundancy in server-based services. Subnet Groups define the different availability zones that your service will run in, and having multiple instances allow for fast failover if a single AZ were to go down.
Multi-Region redundancy is more tricky. Depending on the service, it is harder, or not possible to run a service with failover between regions.