Terraform is awesome!

As of today, Terraform 0.11 and 0.12 support only static (known, fixed, already computed) values in tfvars files. There is no way to use Terraform interpolation functions, or data-sources inside tfvars files in Terraform to update values.

While working on modules.tf (a tool which converts visual diagrams created with Cloudcraft.co into Terraform configurations), I had a need to generate code which would chain invocations of Terraform AWS modules and pass arguments between them without requiring any extra Terraform code as a glue. Terragrunt is a great fit for this, it allows to reduce amount of Terraform configurations by reusing Terraform modules and providing arguments as values in tfvars files.

Some languages I know have concepts like annotations and decorators, so at first I made a shell script which replaced values in tfvars based on annotations and was called by Terragrunt hooks. tfvars-annotations shares the same goal and it has no external dependencies (except terraform or terragrunt).

1. modules.tf and Terragrunt (recommended)
2. Terraform (not implemented yet)
3. General example - AMI ID, AWS region

1. Supported annotations:

• terragrunt_output:
  • @tfvars:terragrunt_output.vpc.vpc_id
  • @tfvars:terragrunt_output.security-group.this_security_group_id
• terraform_output
• data-sources generic

1. Type wrapping:

• to_list: Wrap original value with [] to make it it as a list

Run tfvars-annotations before terraform plan, apply, refresh.

It will process tfvars file in the current directory and set updated values.

E.g.:

$ tfvars-annotations examples/project1-terragrunt/eu-west-1/app
$ terraform plan

Put @tfvars:disable_annotations anywhere in the terraform.tfvars to not process the file.

See examples for some basics.

1. Get values from other sources:

• data sources generic
• aws_account_id or aws_region data sources

2. terragrunt_outputs from stacks:

• in any folder
• in current region

3. cache values unless stack is changed/updated
4. functions (limit(2), to_list)
5. rewrite in go (invoke like this => update_dynamic_values_in_tfvars ${get_parent_tfvars_dir()}/${path_relative_to_include()})
6. make it much faster, less verbose
7. add dry-run flag
8. Proposed syntax:

• @tfvars:terragrunt_output.security-group_5.this_security_group_id.to_list

• @tfvars:terragrunt_output.["eu-west-1/security-group_5"].this_security_group_id.to_list

• @tfvars:terragrunt_output.["global/route53-zones"].zone_id

• @tfvars:terragrunt_data.aws_region.zone_id

• @tfvars:terragrunt_data.aws_region[{current=true}].zone_id

1. Add support for maps (and lists of maps). Strange bugs with rendering comments in wrong places.

On OSX, install it with Homebrew (not enough github stars to get it to the official repo):

brew install -s HomebrewFormula/tfvars-annotations.rb

Alternatively, you can download a release suitable for your platform and unzip it. Make sure the tfvars-annotations binary is executable, and you're ready to go.

You can also install it like this:

go get github.com/antonbabenko/tfvars-annotations

Or run it from source:

go run . -debug examples/project1-terragrunt/eu-west-1/app
go run . examples/project1-terragrunt/eu-west-1/app

1. Make GitHub Release: hub release create v0.0.3. Then Github Actions will build binaries and attach them to Github release.
2. Update Homebrew version in HomebrewFormula/tfvars-annotations.rb. Install locally - brew install -s HomebrewFormula/tfvars-annotations.rb

This project is created and maintained by Anton Babenko with the help from different contributors.

This work is licensed under MIT License. See LICENSE for full details.

Copyright (c) 2019 Anton Babenko