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docs/src/data/experiments/version-attribute.mdx

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Support for a version attribute on the terraform block that resolves a registry module from a version constraint.

version-attribute - What it does

When enabled, the terraform block accepts an optional version attribute holding a version constraint for a tfr:// registry module, such as ~> 3.3 or >= 1.0.0, < 2.0.0:

hcl
terraform {
  source  = "tfr://registry.opentofu.org/terraform-aws-modules/vpc/aws"
  version = "~> 3.3"
}

Terragrunt resolves the constraint against the registry's list-versions endpoint before the download begins, then fetches the highest published version that satisfies it. This brings the terraform block to parity with the version argument on OpenTofu and Terraform module blocks, so you no longer have to pin an exact version in the source URL.

The constraint lives only in the version attribute. By the time the module is downloaded and cached, it has been resolved to an exact version, so the --source override and the cache key still carry a concrete pin.

version-attribute - How to enable it

bash
# Via CLI flag
terragrunt --experiment version-attribute run -- plan

# Via environment variable
export TG_EXPERIMENT=version-attribute
terragrunt run -- plan

version-attribute - How to provide feedback

Track and discuss this experiment in gruntwork-io/terragrunt#1930. When reporting issues or providing feedback, please include:

  • The tfr:// source and the version constraint you set.
  • The version you expected Terragrunt to resolve, and the version it resolved.
  • Whether you rely on prereleases, and how you upgrade across newly published versions.

version-attribute - Criteria for stabilization

To transition the version-attribute feature to a stable release, the following must be addressed, at a minimum:

  • Decide and document the cache invalidation policy. A constraint resolves once and is cached by source URL, so a newly published match is only picked up after --source-update. Confirm this is the right default, or re-resolve on every run.
  • Decide and document the prerelease policy. A constraint that names a prerelease (for example >= 1.0.0-rc1) opts into prereleases, matching OpenTofu and Terraform.
  • Decide whether --source and TG_SOURCE should keep accepting exact pins only, or also accept constraints.
  • Community feedback on real-world usage.