Feature Request: Lifecycle pre-update hook for resource metadata modification

Terraform Version

Terraform v1.15.4

Use Cases

Many organizations require resource metadata such as tags or labels to reflect the last modification performed through Terraform.

Common examples include:

• modified-datetime
• last-updated
• updated-by
• change-ticket
• deployment-reference

Today Terraform can set those values during resource creation, but it has no native mechanism to update them automatically when a resource is modified.

This makes governance, auditing and operational tracking difficult, especially in large environments where thousands of resources are managed through Terraform.

The end goal is to allow users to automatically update metadata whenever Terraform detects a resource change and applies an update.

Attempted Solutions

Several approaches were evaluated:

Using timestamps

tags = {
  modified-datetime = timestamp()
}

This causes perpetual diffs because the timestamp changes on every plan.

Using external data sources

data "external" "git_date" {
  program = ["bash", "-c", "git log -1 --format=%cs"]
}

This only reflects repository state and not actual Terraform resource updates.

Using null_resource triggers

resource "null_resource" "metadata" {
  triggers = {
    modified_datetime = timestamp()
  }
}

This does not provide a way to update attributes of another resource before it is modified.

###Using lifecycle

lifecycle {
  ignore_changes = [
    tags["modified-datetime"]
  ]
}

This avoids drift but prevents Terraform from managing the value.

None of these approaches provide a generic and reliable mechanism to update resource metadata only when a resource is actually modified.

Proposal

Introduce a lifecycle hook executed immediately before a resource update operation. Example:

resource "aws_s3_bucket" "example" {
   bucket = "my-bucket"
   tags = { 
       Environment = "prod" 
    } 
  lifecycle { 
      before_update { 
            tags = merge(self.tags, { modified-datetime = timestamp() }) }
     } 
}

References

Related discussions:

• Support for resource lifecycle hooks
• Support for update-time metadata tracking
• Common requests around automatic tagging and auditing

I was unable to find a Terraform Core feature that currently addresses this requirement.