AWS multi-region Terraform Design and implementation

A locals map per region

What this code does: Declares the wanted network state per region as a map. The outer key is the region (seoul), the inner key is the VPC name (demo-seoul), and the value holds the VPC CIDR and its subnet list (name, CIDR, availability zone, and whether public IPs are auto-assigned). Adding a region or a VPC is one more entry in this map; the code that creates resources does not change.

# Declare the wanted state per region. Adding a region or a VPC is one more map entry.
locals {
  networks = {
    seoul = {
      demo-seoul = {
        cidr_block = "10.0.0.0/16"
        subnets = [
          { subnet_name = "demo-seoul-a", subnet_cidr = "10.0.1.0/24", ipv6_cidr_block = null, availability_zone = "ap-northeast-2a", autoassign_public = "false", tags = {} },
          { subnet_name = "demo-seoul-c", subnet_cidr = "10.0.2.0/24", ipv6_cidr_block = null, availability_zone = "ap-northeast-2c", autoassign_public = "false", tags = {} },
        ]
      }
    }

A module with for_each reads the map

What this code does: Runs the VPC module once for each entry in the Seoul map (local.networks.seoul). The map key (each.key) becomes the VPC name, and the CIDR and subnet list come from the value (each.value). The providers line binds the Seoul alias (aws.seoul), so everything this block creates lands in Seoul, and because the key is the resource address, reordering the map never recreates an existing VPC.

module "seoul_vpc" {
  source   = "../../modules/vpc"
  for_each = local.networks.seoul

  providers = { aws = aws.seoul }

  vpc_name            = each.key
  cidr_block          = each.value.cidr_block
  instance_tenancy    = "default"
  use_ipv6_cidr_block = false
  subnets             = each.value.subnets
}