How to generate random strings in Terraform

I’ve been working with Terraform recently for deploying the required architecture for a data pipeline in Google Cloud Platform (GCP). It’s my first project with Terraform and I decided to dive into its random_something resources, because I couldn’t have duplicate resource names. Here are some findings.

Randomize a resource name with Terraform random_string

Randomizing a resource’s name or ID is super easy with Terraform. You can use the random_string resource, and use it as a suffix or a prefix. In the following example, I’ve created a random string of three digits, and used it as the suffic of a GCP Cloud SQL database.

resource "random_string" "random_suffix" {
  length  = 3
  special = false
  upper   = false
}

resource "google_sql_database_instance" "my_database" {
  name             = "my-database-${random_string.random_suffix.result}"
  database_version = "POSTGRES_13"
  ...

However, this could lead to issues along the way. Whenever you’re rerunning the apply command, a new suffix will be generated, and your new resource name won’t match the one from the previous state. Consequently, Terraform will try to apply the new name to the resource — which isn’t even always possible.

That’s why you should set the keeper argument in your Terraform schema. The keeper argument behaves like a random seed. You could set it to a specific value, so that it only changes once that specific value changes. Another approach is to link the existence of your random_string to the ID of another resource. When the other resource is created again, so will the random_string. Even more interesting: you can use multiple values in the keeper argument!

In this example, I set the IP address of a GCP compute instance as the random seed.

resource "random_string" "random_suffix" {
  length  = 3
  special = false
  upper   = false
  keepers = {
    instance_ip = "${google_compute_address.my_instance.address}"
  }
}

resource "google_sql_database_instance" "my_database" {
  name             = "my-database-${random_string.random_suffix.result}"
  database_version = "POSTGRES_13"
  ...

The resulting name of my_database? my-database-ht5. Exciting, right? 🤷‍♀️

Randomize a resource name with Terraform random_pet

Terraform has quite a lot of random providers to create random strings. The random_id resource has quite a lot in common with random_string, but is focussed on minimizing collision (receiving the same ID twice or more).

Without doubt, the most interesting random provider is random_pet, which can be used to generate a random pet, with some adjectives — how many depends on the length argument.

resource "random_pet" "pet" {
  keepers = {
    some_id = "100"
  }
  length = 10
}

For demonstration purpose only, I set the length argument to 10, which gave me the following random pet name: “tightly-generally-moderately-mostly-usually-arguably-ultimately-badly-willing-lizard”. Extremely useful!