RDS Aurora: known Terraform limitations

Some things people ask the terraform-aws-modules/rds-aurora/aws module for cannot be implemented by any module, in any registry. They are limits of Terraform itself. This page collects the recurring ones for this module, says plainly what causes each, gives the native workaround in full, and - where one exists - shows the Operational Rule that removes the need for a fork.

terraform destroy deleted an Aurora cluster

An Aurora cluster is destroyed by the same paths as any other resource, and the cluster carries the data rather than the instances in it. prevent_destroy cannot be passed into this module either.

Why Terraform cannot fix this

lifecycle is a meta-argument block, and Terraform evaluates it before it evaluates the rest of the configuration. Its arguments therefore cannot reference a variable, a local, or anything else that is computed. That is the whole reason no module can expose prevent_destroy, ignore_changes, or create_before_destroy as an input: there is no expression a module author could put there that Terraform would accept.

This is a property of Terraform, not of this module. The request to lift the restriction has been open since 2015, it is one of the most-supported requests in the tracker, and OpenTofu carries the same request. Until one of them ships a way to set a meta-argument from outside the resource block, every module in every registry has the same limitation.

The native workaround

Set deletion_protection = true on the cluster, which this module exposes and AWS enforces, and keep automated backups and a final snapshot configured so a deletion is recoverable rather than final. For a hard stop at plan time, the lifecycle block has to be on the cluster resource. Fork this module and add the lifecycle block to the resource yourself. That works, and it is the honest answer - it is what module maintainers do when they need it. The cost is ongoing rather than one-off: the fork has to be re-synced with every upstream release, and each sync needs a review to confirm the block still lands on the resource it was meant for. A wrapper module does not avoid this, because the lifecycle block still has to sit inside the resource block, which is inside the module you did not write.

The rule that answers it

lifecycle_prevent_destroy_data adds the block for you, server-side, while the module is being downloaded. What arrives in .terraform/modules/ is ordinary HCL with this change already in it:

 resource "aws_rds_cluster" "this" {
   # ... the module's own configuration, unchanged ...
+
+  lifecycle {
+    prevent_destroy = true
+  }
 }

Nothing else about the module changes: same inputs, same outputs, same version. The full definition is published at lifecycle_prevent_destroy_data.

Using the maintained alternative

compliance.tf serves this module from a registry that applies the rules above during terraform init. The change is the source line plus the rules you want; the inputs and outputs are the upstream module's.

module "rds_aurora" {
  source = "https://cis.compliance.tf/terraform-aws-modules/rds-aurora/aws?add_rules=lifecycle_prevent_destroy_data"

  # ... the same inputs you pass today ...
}

On a framework host you name the rules you want in ?add_rules=, on top of whatever Baseline your organization has Enforced; on an organization host that organization's configuration decides. A module compliance.tf builds for you carries a compliancetf-manifest.json naming every rule the build received, each with an outcome recording what it actually did, so the change is auditable rather than implicit.

Before changing a source line, run it: the Rules Playground opens on this module with these rules selected and shows the diff the registry would serve, without an account. The Playground takes its list as ?rules= because it previews rules on their own rather than against your Baseline; your source line above keeps ?add_rules=, which adds to that Baseline instead of replacing it.

Related

• Operational Rules - what rules are, and how they differ from compliance controls
• Operational Rule Definitions - the HCL of every selectable rule
• lifecycle_prevent_destroy_data - the rule page, with limits and failure modes
• Stop paying the Terraform fork tax - why forking a module to add a lifecycle block costs more than it looks
• RDS Aurora module reference - inputs, outputs, and the controls applied to it