EC2 instance: known Terraform limitations

Some things people ask the terraform-aws-modules/ec2-instance/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 wants to replace my EC2 instance because the AMI changed

The configuration resolves the AMI through a data source with most_recent = true. When AWS publishes a new image, the data source returns a different id and Terraform plans to replace a running instance - usually at the least convenient moment.

Why Terraform cannot fix this

Terraform's model is that it owns every attribute it can see. When a controller, an autoscaler, a scanner, or a deployment pipeline writes to one of those attributes, Terraform reads the new value as drift and plans to put its own value back. Both systems are behaving correctly; they simply disagree about who owns the field.

The only mechanism Terraform offers for splitting ownership is ignore_changes, and that is a lifecycle argument, which brings the literal-value restriction above with it. A module cannot accept the list of attributes to ignore as an input, so a module consumer has no way to declare the split.

The native workaround

Pin the AMI id instead of resolving the newest one. An explicit id, or an SSM parameter you promote deliberately, removes the surprise entirely and is the right answer for anything long-lived.

Where the lookup has to stay dynamic, the native fix is ignore_changes = [ami] on the instance, which is a lifecycle argument. 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_ignore_ami_changes 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_instance" "this" {
   # ... the module's own configuration, unchanged ...
+
+  lifecycle {
+    ignore_changes = [ami, image_id]
+  }
 }

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

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 "ec2_instance" {
  source = "https://cis.compliance.tf/terraform-aws-modules/ec2-instance/aws?add_rules=lifecycle_ignore_ami_changes"

  # ... 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_ignore_ami_changes - 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
• EC2 instance module reference - inputs, outputs, and the controls applied to it