How can I keep a "local" tf module next to/close to the terragrunt.hcl?

[pieterbreed]

Hi, I'm fairly new to terragrunt, so apologies if I'm asking something with an obvious answer.

I'm confused by this section in the example tutorial:

stage
├── terragrunt.hcl
├── frontend-app
│   ├── main.tf
│   └── terragrunt.hcl
└── mysql
    ├── main.tf
    └── terragrunt.hcl

This seems to hint at a capability what I want, but I can't get it to work.

I have some terraform modules that are fairly generic and shared over some terragrunt directories. I have them on the same filesystem (not in git). It looks more or less like this:

├── terraform
│   └── modules
│       ├── application-cache
│       │   ├── main.tf
│       ├── centralized-alarms
│       │   ├── main.tf
...
└── terragrunt
    ├── prodapp
    │   ├── account.yaml
    │   ├── appcache
    │   │   ├── local
    │   │   │   ├── main.tf
    │   │   └── terragrunt.hcl

For one of terragrunt.hcl files, I need a little bit of local customization in how a shared module (application-cache) is called. Specifically, one of the parameters that I just provide manually in the testing environment, instead needs to be read from a cloudformation output. So I have a little bit of local, environment-specific customization around invoking the shared/generic module.

My problem is I cannot figure out how to achieve this. I don't want to create another module in the "shared" modules directory which is specifically only serves as an "entry point" module for one specific environment. I'd rather keep the terraform code "close" to the terragrunt.hcl it applies to.

I've tried to have the main.tf in the same folder as the terragrunt.hcl file like the example tutorial, which seems to imply that I must not have a terraform { source ... } block in terragrunt.hcl. But then the locals are not passed correctly to the variables I declare in the main.tf using the input block. When I run terragrunt apply it asks for the value of a variable that is declared in the main.tf and specified in the terragrunt.hcl file's inputs.

I've also tried having a ./local directory with ./local/main.tf, attempting to treat it as a terraform module. In the terragrunt.hcl file I then specify terraform { source = ".//local" } bit this fails with paths to the shared terraform modules not being resolved correctly.

| Error: Unreadable module directory
│ 
│ Unable to evaluate directory symlink: lstat ../../../../terraform: no such
│ file or directory

Really my question is, if I have shared terraform modules, and my environments have slight dissimilarities in how those shared modules are invoked, how must I structure the terragrunt repository to accomplish this?

---

Tracked in ticket #109257

[denis256]

Hi,
AFAIK, it depends on how big are dissimilarities:

• can be used dependency block and outputs passed to application-cache - but it will require to extract terraform code as separated module and reference it in Terragrunt
• can be used generate block to put custom code in the module directory before the invocation of Terraform

References:
https://terragrunt.gruntwork.io/docs/reference/config-blocks-and-attributes/#dependency
https://terragrunt.gruntwork.io/docs/reference/config-blocks-and-attributes/#generate

I also created some examples using generate and dependency in https://github.com/denis256/terragrunt-tests/tree/master/discussion-559

I hope this helps

[pieterbreed]

Thank you for the suggestions. I'll have to read the docs with attention and think carefully if they provide the capability that I need.

[ivellios]

I am having a similar issue, thus I won't create a new discussion and put it here:

In my child env/stage/vpc/terragrunt.hcl I have:

terraform {
  source = "${path_relative_from_include()}/modules//vpc"  # also tried: source = "../../../modules/vpc" with the same effect
}

In the root directory I also have terragrunt.hcl file, so it is on the same level as modules directory. When running terragrunt I see the modules showing up in the .terragrunt-cache. But still, it looks as if the module was not being run and inputs passed into it. I get the message that there's no change in the infrastructure.

But if I remove the above code and put env/stage/vpc/main.tf in the same child directory with terraform module and source param pointing to the local module, it is being used.

Terraform main.tf file:

module "vpc" {
  source = "../../../modules/vpc"
  ...

The simplified files structure:

| - env - stage - vpc
|                  | - terragrunt.hcl
|                  | - main.tf
|                  | - ...
| - modules - vpc
|              | - main.tf
| - terragrunt.hcl

How can I get rid of this main.tf file and use the module directly from the terragrunt.hcl? Am I missing something here?

Disclaimer: I am new to terragrunt, sorry!

[ivellios]

Looks like it wasn't working because of the privder/backend/versions.tf files present in the env-stage-vpc directory. I removed them and moved the module to another directory and it worked like a charm. Also no files were visibly generated by terragrunt (which is quite unexpected, but also makes it clearer to use)