05. Server-Based Services

Server bases services are those that are "instance" based. Services like RDS and ElastiCache are instanced based in that you can run one instance, but you will not have any fault tolerance. In order to gain high availability, you need to tell the service to provision a second instance for the primary instance to failover to, should there be an issue with it. This model is similar to traditional data center environments. A good way to tell if a service is a server/instance based service is if the service is a pre-existing product that AWS has create a service with (MongoDB, Redis, MySQL, Postgres).

To get Multi-AZ availability, you need to configure a Subnet Group within the service. A subnet is attached to an AZ, and creating a grouping of subnets within the service tells the service where it can place the primary and standby instances of a service.