What problem does the keepers map for the random provider solve?

user_438319 0
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I am trying to understand the use case for keepers feature of the terraform random provider. I read the docs but it's not clicking-in for me. What is a concrete example, situation where keeper map would be used and why. Example form the docs reproduced below. 

resource "random_id" "server" {
  keepers = {
    # Generate a new id each time we switch to a new AMI id
    ami_id = "${var.ami_id}"
  }

  byte_length = 8
}

resource "aws_instance" "server" {
  tags = {
    Name = "web-server ${random_id.server.hex}"
  }

  # Read the AMI id "through" the random_id resource to ensure that
  # both will change together.
  ami = "${random_id.server.keepers.ami_id}"

  # ... (other aws_instance arguments) ...
}
terraform
1 Answers

The keepers are seeds for the random string that is generated. They contain data that you can use to ensure, essentially, that your random string is deterministic - until something happens that means it should change.

If you had a random string without any keepers, and you were using it in your server's Name tag as in this example, then Terraform would generate a plan to change the Name (containing a new random ID) every time you ran terraform plan/terraform apply.

This is not desirable, because while you might want randomness when you first create the server, you probably don't want so much randomness that it constantly changes. That is, once you apply your plan, your infrastructure should remain stable and subsequent plans should generate no changes as long as everything else remains the same.

When it comes time to make changes to this server - such as, in this case, changing the image it's built from - you may well want the server name to automatically change to a new random value to represent that this is no longer the same server as before. Using the AMI ID in the keepers for the random ID therefore means that when your AMI ID changes, a new random ID will be generated for the server's Name as well.

user_19821360
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