Teaching Case — "Highly Available" in One Zone

Three application servers and a database, every one of them marked highly available, all of them inside a single availability zone. The count is the trap: `count 3` looks like redundancy and is not, because three instances in one zone is one zone. The engine says exactly that, in those words.

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title "Payments platform — availability review"
provider aws
note "Deliberately wrong. Everything claims redundancy from inside one zone."

internet {
  user customers "Customers"
}

cloud prod "Production" {
  region ap-southeast-2 {
    network vpc "VPC" cidr 10.0.0.0/16 {
      alb lb "Load balancer"

      zone ap-southeast-2a {
        subnet app "Application" private cidr 10.0.11.0/24 {
          ec2 api "Payments API" count 3 ha tier api
        }
        subnet data "Data" isolated cidr 10.0.21.0/24 {
          rds ledger "Ledger" multi-az
          elasticache sessions "Session cache" ha
        }
      }
    }
  }
}

customers -> lb : https 443
lb -> api : http 8080
api -> ledger : postgres 5432
api -> sessions : redis 6379