1_always_random_for_each.tf

variable "subnets" {
  type    = list(any)
  default = ["subnet-a", "subnet-b", "subnet-c"]
}

variable "instances" {
  default = {
    TEST1 = {
      sg = ["example1"]
    }
    TEST2 = {
      sg = ["example2"]
    }
    TEST3 = {
      sg = ["example3"]
    }
  }
}

resource "random_integer" "subnet_picker" {
  for_each = var.instances
  keepers = {
    uuid = uuid()
  }
  min = 0
  max = length(var.subnets) - 1
}

resource "null_resource" "echo" {
  for_each = var.instances
  triggers = {
    random_int = random_integer.subnet_picker[each.key].result
  }
  provisioner "local-exec" {
    command = "echo 'Instance - ${each.key} - subnet ${var.subnets[random_integer.subnet_picker[each.key].result]}'"
  }
}

2_always_random.tf

variable "subnets" {
  type    = list(any)
  default = ["subnet-a", "subnet-b", "subnet-c"]
}

resource "random_integer" "subnet_picker" {
  keepers = {
    uuid = uuid()
  }
  min = 0
  max = length(var.subnets) - 1
}

resource "null_resource" "echo" {
  triggers = {
    random_int = random_integer.subnet_picker.result
  }
  provisioner "local-exec" {
    command = "echo ${var.subnets[random_integer.subnet_picker.result]}"
  }
}