terraform variable precedence

[matt-smith]

When setting the same variable (for example "example_var") in both VarFiles and Vars terraformOptions, ordering is not honoured and therefore the variable precedence is incorrect. The value of the variable in the VarFile will always be applied despite the ordering inside of the test.

For example:

...
	terraformOptions := &terraform.Options{
		// The path to where our Terraform code is located
		TerraformDir: "../examples/terraform-basic-example",

		// Variables to pass to our Terraform code using -var-file options
		VarFiles: []string{"varfile.tfvars"},

		// Variables to pass to our Terraform code using -var options
		Vars: map[string]interface{}{
			"example_var": "test123",
		},

		// Disable colors in Terraform commands so its easier to parse stdout/stderr
		NoColor: true,
	}
...

and varfile.tfvars contained

...
example_var = "test456"
...

example_var would always be set to test456

looks like https://github.com/gruntwork-io/terratest/blob/master/modules/terraform/format.go#L14-L15 is causing this behaviour.

[brikis98]

Are you expecting -var-file to go first always?

The docs say:

Any -var and -var-file options on the command line, in the order they are provided.

"The order they are provided" doesn't really make much sense in a terraform.Options struct. There's no way in Go to know what order you provided those variables.

[matt-smith]

Hey @brikis98, thanks for your response and explanation.

I think it's more likely that a user would want one or two variables passed in as -var's to take precedence over a larger -var-file perhaps. What do you think?

I've opened a small PR PR-256 with my changes.

[brikis98]

Having -var take precedence over -var-file probably makes sense. That said, in most use cases, we only use one of -var or -var-file, not both, as keeping things in one place makes it easier to reason about the code.

[sebastianczech]

As far as I was checking, #1217 resolves that issue. If you define SetVarsAfterVarFiles: true, then values passed in Vars take precedence over values from VarFiles e.g.:

terraformOptions := terraform.WithDefaultRetryableErrors(t, &terraform.Options{
	TerraformDir: ".",
	VarFiles:     []string{"example.tfvars"},
	Vars: map[string]interface{}{
		"example": "value_passed_from_test",
	},
	SetVarsAfterVarFiles: true,
})

[matt-smith]

perfect thanks @sebastianczech , I'll close this issue in that case.