Mutable Generate

Deduplicate the files produced by generate blocks through content-addressable storage, with a mutable attribute to opt out.

mutable-generate - What it does

With the experiment enabled, the contents a generate block produces are stored in the CAS, and the file written at path is a read-only link to that stored copy rather than a file of its own. The link is read-only because the stored copy is shared: an edit through one path would otherwise change what every later reader of that content sees.

Because the stored copy is addressed by the hash of its contents, anything generating identical contents links to the same copy instead of writing its own. That matters most for a generate block declared in a parent configuration, which writes the same content into every unit that includes it. A provider or backend block shared across a few hundred units used to mean a few hundred copies in .terragrunt-cache, and now means one.

The experiment also adds a mutable attribute to the generate block, for cases where a shared read-only file does not work:

generate "provider" {
  path      = "provider.tf"
  if_exists = "overwrite"
  mutable   = true
  contents  = <<EOF
provider "aws" {
  region = "us-east-1"
}
EOF
}

mutable = true gives the block a writable file of its own. Use it when something rewrites the generated file in place, such as a hook that patches it before tofu/terraform runs. Terragrunt regenerates the file rather than editing it, and tofu/terraform only read it, so most generate blocks do not need it.

Setting mutable without the experiment enabled is an error, because older Terragrunt versions reject the attribute outright. The CAS is required, so --no-cas writes generated files directly and mutable has no effect.

mutable-generate - How to enable it

# Via CLI flag
terragrunt --experiment mutable-generate run --all -- apply

# Via environment variable
export TG_EXPERIMENT=mutable-generate
terragrunt run --all -- apply

mutable-generate - How to provide feedback

Track and discuss this experiment in gruntwork-io/terragrunt#6559. When reporting issues or providing feedback, please include:

• Whether anything in your pipeline writes to a generated file after Terragrunt creates it.
• The generate blocks involved, and how many units share them.
• The disk usage of .terragrunt-cache before and after enabling the experiment.

mutable-generate - Criteria for stabilization

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

• A mutable attribute on the generate block, parsed from both the block and attribute forms.
• Deduplication of non-mutable generated files through the CAS store, materialized as read-only hard links.
• Confirmation that read-only generated files break no established workflow, including hooks and IaC engines.
• A decision on whether remote_state.generate should participate, given its per-unit backend keys rarely repeat.
• Community feedback on the disk savings actually observed at scale.