Terragrunt: Using output from a different module fails on the plan-all action, and works on apply-all

[sa-mao]

Having this dependency declaration in module example-app:

dependency "infra_base" {
  config_path = "../stack-infra-base"
}

inputs = {
  public_subnets_ids = dependency.infra_base.outputs.public_subnets_ids
}

running terragrunt plan-all for the first time fails with this error:

example-app/terragrunt.hcl:14,53-72: Unsupported attribute; This object does not have an attribute named "public_subnets_ids".
[terragrunt] 2019/08/16 09:54:37 Unable to determine underlying exit code, so Terragrunt will exit with error code 1

If you run terragrunt apply-all the error is gone.

Is this an expected behavior?

[brikis98]

When you say "plan-all for the first time fails", by "first time," do you mean _before_ stack-infra-base has been applied?

[sa-mao]

Exactly, stack-infra-base is not applied yet.

[brikis98]

@yorinasub17 The dependency block should probably detect if a module hasn't been applied and show a clear error to indicate this.

[yorinasub17]

Opened PR to handle report better error messages: https://github.com/gruntwork-io/terragrunt/pull/840

In general, plan-all is not expected to work cleanly on a completely new infrastructure build if you have dependencies, because we don't have advanced features that stitch terraform plans together.

Internally in terraform, there is a concept of computable outputs which are then used to seed the inputs of other resources and modules, which "taint" and propagate through to predict if a resource would be created or not. In terragrunt, when you cross modules, we can't use that same terraform system to taint the plan across the modules because it is a completely different terraform module, and it is a huge overhead to implement something like that.

However, to allow you to simulate something like this, I also implemented a new feature: default_outputs. This is a static map that you can use as dummy outputs if no module has been applied, so that terraform can compute a plan or run validate. Otherwise, terragrunt can't compute the inputs so it will error out. Check out the PR and updated README for more info.

[yorinasub17]

Resolved and released in https://github.com/gruntwork-io/terragrunt/releases/tag/v0.19.23 (binaries should show up shortly). See the relevant section of the README for more info.