Terraform terraform_remote_state Partial Configuration

Question

My team relies heavily on S3 remote state from within Terraform. We use the -backend-config feature of the CLI to specify the S3 configuration when initializin

Answer 1

The backend block is rather special because it gets processed so early in Terraform's workflow, and thus it doesn't have access to normal Terraform features such as variables. That's why it has its own special mechanism for configuring it.

The terraform_remote_state data source, on the other hand, is just a regular data source and so any normal interpolation strategy can be used with it. To pass settings from the CLI, for example, you could use variables:

variable "dns_state_region" {
}

variable "dns_state_key" {
}

data "terraform_remote_state" "dns" {
  backend = "s3"
  config {
    region = "${var.dns_state_region}"
    key    = "${var.dns_state_key}"
  }
}

You can then pass these to the terraform plan command:

$ terraform plan \
    -var="dns_state_region=us-west-1" \
    -var="dns_state_key=configurations/production/dns/terraform.tfstate"