Choosing Terramate Code Generation vs Manual Modules for DRY Terraform

Decision and constraints

When you need to keep Terraform configuration DRY across many stacks, you can either let Terramate generate the files from a template or maintain the code yourself with modules or Terragrunt patterns. The decision hinges on:

• Stack metadata must live in a terramate.hcl file.
• The generate block must be enabled in that file.
• Terramate CLI version 0.10 or newer is required for the generate command.

Supported options comparison

Option	Ease of use	Duplication	Learning curve
Terramate generate with HCL templating	High – one command creates files	Low – template drives all stacks	Medium – learn Terramate syntax
Manual Terraform modules	Medium – write and call modules	Medium – risk of drift if versions differ	Low – standard Terraform
Terragrunt include/exclude patterns	Medium – extra Terragrunt file	Low – patterns reuse code	Medium – learn Terragrunt

Trade‑offs

Using Terramate’s code generation reduces boilerplate and guarantees that provider/backend blocks are identical in every generated file. The trade‑off is an additional build step and a dependency on the Terramate CLI. Manual modules avoid extra tooling but can lead to version drift and repetitive module calls. Terragrunt gives pattern‑based reuse but adds another layer of configuration to maintain.

Concrete implementation and validation

1. Create a stack directory with a terramate.hcl file that defines the generation:

   # terramate.hcl
   terraform {
     # optional backend/provider config can go here
   }
   
   # Enable code generation
   generate {
     path = "generated/main.tf"
     template = "templates/main.tftpl"
   }
   

2. Place your HCL template (e.g., templates/main.tftpl) in the same directory. A simple template might look like:

   # templates/main.tftpl
   provider "aws" {
     region = "{{ .var.region }}"
   }
   
   resource "aws_instance" "example" {
     ami           = "{{ .var.ami }}"
     instance_type = "{{ .var.instance_type }}"
   }
   

3. Run the generation step (requires read/write permission on the workspace):

   terramate generate
   

   After execution, verify that generated/main.tf appears and contains the rendered template.

4. Initialize and validate the generated Terraform code:

   terraform init
   terraform plan
   

   If the plan shows the expected resources (e.g., an AWS instance), the generation succeeded.

Limitations and practical checks

• Terramate < 0.10 will error on generate; run terramate version first.
• Generated files are overwritten on each run; manual edits will be lost unless you add an ignore pattern to terramate.hcl.
• To confirm the result, compare the output of terraform plan before and after a change to the template; the plan should reflect only the intended differences.