A Terraform state store can preserve every byte of data and still block a deployment if it is inaccessible. Uptime measures how often a system is up and accessible. For state storage, that makes availability an operational requirement: teams need access during routine changes and, especially, during emergencies. section 6.2.3
Uptime is commonly expressed in “nines”: 99.99% is four nines, and 99.999% is five nines. For critical Terraform state storage, four nines is a suggested minimum. When choosing a provider, check both its outage history and its service-level agreement; a promise without a transparent record is weak evidence of availability. section 6.2.3
Application uptime also depends on where resources run. If an entire application occupies one availability zone, an outage there takes down the application. Spreading the network across locations reduces dependence on that single failure domain, but costs more. A network module can therefore accept an availability-zone count, allowing a cheaper single-location development environment and a multi-location production environment to use the same module. section 10.2.2 section 10.2.4
Listing 10.9 Network module example usage
module "network" {
source = "./top_level" #1
enable_isolated_subnet = true #2
availability_zones = 3 #3
cidr_block = "192.168.0.0/16" #4
}
This module call sets availability_zones to 3 alongside the network CIDR block and isolated-subnet option. Changing the zone count selects the intended topology without switching modules; the module’s validation permits one through four zones. section 10.2.4
