How to "override" variables when using modules?

[bth...@elementum.com]

I've seen a few other posts on this topic but none that seem to address the issue specifically.

What I'm trying to do is use either 'terraform plan -var "foo=bar"' or set $TF_VAR_foo to "override" variables, which seems straightforward. However, with a simple setup involving modules this doesn't work at all.

Given these three files:

modules/foo/variables.tf

variable "region" {

  default = "us-east-1"

}

variable "itype" {

  default = "t2.micro"

}

modules/foo/example.tf

provider "aws" {

  region     = "${var.region}"

}

resource "aws_instance" "foo" {

  ami           = "ami-b374d5a6"

  instance_type = "${var.itype}"

  subnet_id     = "subnet-3ca6c952"

}

service/main.tf

module "example" {

  source = "../modules/foo"

  itype  = "c3.large"

}

Running "terraform plan" works as expected - the resource uses instance_type=c3.large. Also removing the "itype" line from main.tf also works as expected, setting instance_type to t2.micro, the variable's declared default.

What does not work as expected is trying to run, for example, 'terraform plan -var "itype=i2.xlarge"', nor setting TF_VAR_itype=i2.xlarge. The 'override' value is ignored. In fact, I can find no way to use modules and get envars or -var settings work at all - no matter what I do they're ignored. I don't understand why, and as a second issue, I can't figure out how to work around it. 

Has anyone gotten this to work successfully?

[Philip Nelson]

You just need to make the "c3.large" a variable in the root, i.e. corresponding to main.tf. passing -var only overrides variables in the root configuration. Good luck!

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[bth...@elementum.com]

Not sure I understand. When you say "only overrides variables in the root configuration" do you mean "only overrides variables declared in the root configuration"? Is there a reason why, or is it documented anywhere? https://www.terraform.io/docs/configuration/variables.html makes no mention of it or indeed any caveats at all.

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[Philip Nelson]

without a deep search, check this out: https://github.com/hashicorp/terraform/issues/2648

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[bth...@elementum.com]

Thanks, appreciate the pointer.