How does Terraform know that randomly named resources already exist? - azure

Question

Consider the following Terraform template:
terraform {
required_providers {
azurecaf = {
source = "aztfmod/azurecaf"
version = "1.2.23"
}
}
}
provider "azurerm" {
features {}
}
resource "azurecaf_name" "rg_name" {
name = var.appname
resource_type = "azurerm_resource_group"
prefixes = ["dev"]
suffixes = ["y", "z"]
random_length = 5 // <------ random part in name generation
clean_input = false
}
resource "azurerm_resource_group" "example" {
name = azurecaf_name.rg_name.result
location = var.resource_group_location
}
I applied this template, than ran terraform plan. Terraform plan tells me
No changes. Your infrastructure matches the configuration.
How does that work as azurecaf_name.rg_name contains random characters? I would have expected it to create a new resource group with a new (random) name. I know that Terraform keeps a state, but doesn't it execute the template every time (=new random name) and then checks if that matches state and real resources in cloud?

Answer 1

The random characters are stored in the state file along with the other attributes of the resource like random_length, prefixes, suffixes. When you rerun terraform it will first check have any of the attributes in your configuration changed from whats in the state file. In your case all the values like random_length are the same. So terraform knows it doesnt need to regenerate anything like the random attributes since your configuration has not changed. Terraform will then check the state of the resource in the remote provider using the resource ID generated by the remote provider. If it doesnt exist terraform will create it with the same data. if it does exist but its attributes dont match then it will update the resource. if it checks its values there match whats in the state file. If they are the same then terraform knows no changes are needed.
We can see this in the random_pet resource
resource "random_pet" "foo" {
length = 3
prefix = "foo"
}
State file
"resources": [
{
"mode": "managed",
"type": "random_pet",
"name": "foo",
"provider": "provider[\"registry.terraform.io/hashicorp/random\"]",
"instances": [
{
"schema_version": 0,
"attributes": {
"id": "foo-evidently-secure-killdeer",
"keepers": null,
"length": 3,
"prefix": "foo",
"separator": "-"
},
"sensitive_attributes": []
}
]
}
],
Normally resources that have a form of randomness