cofin/terraform

Terraform

Overview

Use this skill to keep Terraform work small-state, reviewable, and brownfield-safe.

This repo should treat Terraform as four separate concerns:

1. Place Terraform in the right part of the repo.
2. Provision new infrastructure with clear root-module boundaries.
3. Adopt existing infrastructure without taking unsafe shortcuts.
4. Operate Terraform through validated plans, CI, and policy checks.

For this repo family, default to brownfield embedding inside an existing product repo unless the user explicitly wants a dedicated infrastructure repository.

Quick Reference

| Decision | Default | Avoid | |---|---|---| | Brownfield placement | infra/terraform/ inside the existing repo | Mixing Terraform into src/ or app runtime folders | | Environment model | Separate directories and separate state per env/root | Using CLI workspaces for dev / stage / prod | | State size | One root module per deployable unit or boundary | Giant multi-service roots | | GCP backend | GCS remote state | Local state for shared environments | | Local auth | ADC | Long-lived service account keys | | CI auth | Attached service account on GCP, otherwise WIF | Downloaded JSON keys when avoidable | | Sensitive values | Treat state and plan artifacts as sensitive; prefer Secret Manager plus sensitive / ephemeral patterns when supported | Hardcoding secrets or committing plan/state artifacts | | Brownfield adoption | Export/import, review, then refactor with moved blocks | Hand-editing state to “make it fit” |

Operating Lanes

Lane 1: Repo and State Design

• Keep Terraform in a clearly scoped infrastructure area.
• Prefer infra/terraform/ for brownfield repos.
• Keep reusable modules separate from live environment roots.
• Keep each root module small enough that reviewers can understand a plan.

Lane 2: Greenfield Provisioning

• Start with a small root module and a reusable module boundary only where reuse is real.
• Pin provider and module versions.
• Check in .terraform.lock.hcl.

Lane 3: Brownfield Adoption

• Inventory what already exists.
• Export or import existing resources into Terraform.
• Normalize the generated or imported configuration.
• Refactor with moved blocks instead of destructive rename/recreate cycles.

Lane 4: Day-2 Operations

• Run fmt, validate, saved plan, and policy checks before apply.
• Prefer CI-mediated plan and apply, especially for shared environments.
• Upgrade versions intentionally and review lockfile drift.

<workflow>

Workflow

Step 1: Choose Repo Placement

Use the smallest layout that preserves clear ownership.

• Brownfield application repo: put Terraform under infra/terraform/.
• Dedicated infra repo: keep the same internal split of modules/ and live environment roots.
• Service-specific IaC: place service roots under infra/terraform/environments/<env>/<service>/.

Read references/layout.md before creating directories.

Step 2: Define Root-Module Boundaries

A root module is a state boundary. Treat it as an operational boundary too.

• Split by application, shared platform service, or blast-radius boundary.
• Prefer separate roots for shared networking, project/bootstrap, and app-service stacks.
• Keep unrelated systems out of the same state even if they deploy together.

Step 3: Establish Backend and Auth

For GCP, default to:

• Local development: ADC
• Privileged local work: service account impersonation
• CI on Google Cloud: attached service account
• CI outside Google Cloud: Workload Identity Federation
• Remote state: GCS backend per root/environment

Read references/gcp.md before writing provider or backend configuration.

If the root spans multiple projects, regions, or beta-only resources, define explicit provider aliases instead of overloading one default google provider configuration.

Step 3.5: Handle Sensitive Values Early

Treat Terraform state files, saved plan files, and plan JSON as sensitive artifacts.

• Keep state and plan artifacts out of Git.
• Prefer Secret Manager or another external secret source over plaintext secrets in .tfvars.
• Use sensitive = true for inputs and outputs that must be redacted.
• Use ephemeral = true or write-only arguments when the provider/resource supports them and the value should stay out of state and plan files entirely.

Step 4: Pick the Environment Model

Use separate directories and separate state for dev, stage, and prod.

Use Terraform CLI workspaces only when all of the following are true:

• the configuration is the same shape in every instance
• credentials and approvals are the same
• the backend is shared intentionally
• the instances are peers, not separate systems

If any of those conditions are false, do not use CLI workspaces as the primary environment model.

Step 5: Implement and Validate

Before proposing an apply, run the low-risk checks first:

1. terraform fmt
2. terraform init
3. terraform validate
4. terraform plan -out=tfplan
5. terraform show -json tfplan > tfplan.json when policy tooling or machine review is needed
6. terraform test when the module or root justifies it

Use references/testing.md for the validation pipeline.

Step 6: Brownfield Adoption Path

When a system already exists:

1. Decide the target root-module boundary before importing anything.
2. Export existing GCP resources if that accelerates discovery.
3. Add import blocks or targeted imports for the selected boundary.
4. Generate configuration when helpful, but treat it as a scaffold.
5. Refactor into the desired module shape.
6. Use moved blocks to preserve state history during renames or splits.

Use references/brownfield.md for the exact flow.

Step 7: Connect Service-Specific Patterns

After the Terraform structure is sound, pull in service-specific references:

• Cloud Run examples: ../cloud-run/references/terraform.md
• GKE examples: ../gke/references/terraform.md

</workflow> <guardrails>

Guardrails

• Do not use CLI workspaces for system decomposition or for environments with separate credentials or approvals.
• Do not put unrelated services in one state just because a single PR touches them.
• Do not hand-edit Terraform state unless it is a documented break-glass recovery.
• Do not normalize brownfield infrastructure by deleting and recreating it when import and refactor paths exist.
• Do not treat exported or generated HCL as production-ready until it is cleaned up and reviewed.
• Do not rely on service account keys by default on GCP when ADC, impersonation, or Workload Identity Federation are available.
• Do not treat remote state as non-sensitive. State files and saved plans can contain secrets, tokens, and generated credentials.
• Do not skip .terraform.lock.hcl in root modules that will be shared or reviewed.
• Do not hide destructive changes inside apply-only workflows. Save and review the plan first.
• Do not introduce Terragrunt as the default architecture unless the repo already standardized on it. Plain Terraform is the baseline.
• Do not let helper scripts become hidden infrastructure dependencies. Prefer provider resources and documented modules first.
• Do not leave stateful resources without lifecycle protection. Use prevent_destroy and provider-specific deletion protection where the platform supports them.

</guardrails> <validation>

Validation Checkpoint

Before claiming a Terraform change is ready, verify:

• [ ] the repo layout keeps Terraform out of application runtime folders
• [ ] each root module has a clear ownership and state boundary
• [ ] environment separation uses directories/state, or workspace use is explicitly justified
• [ ] backend and authentication strategy are documented
• [ ] sensitive inputs, state, and plan artifacts are handled as secrets
• [ ] provider aliases or google-beta configuration are explicit when multi-project, multi-region, or beta-only resources are involved
• [ ] provider and module versions are pinned intentionally
• [ ] .terraform.lock.hcl is committed when the root is meant to be versioned
• [ ] terraform fmt, terraform validate, and a saved terraform plan -out=... were run
• [ ] stateful resources use lifecycle protection or an explicitly documented exception
• [ ] brownfield imports, generated config, and moved blocks are documented when applicable
• [ ] policy validation and module tests were considered for shared or sensitive infrastructure

</validation> <example>

Example

Brownfield application repo layout:

repo/
├── src/
├── tests/
├── .agents/
└── infra/
    └── terraform/
        ├── modules/
        │   ├── project-services/
        │   ├── network/
        │   └── cloud-run-service/
        └── environments/
            ├── dev/
            │   ├── shared-network/
            │   └── api-service/
            ├── stage/
            │   ├── shared-network/
            │   └── api-service/
            └── prod/
                ├── shared-network/
                └── api-service/

Minimal root files:

api-service/
├── backend.tf
├── main.tf
├── providers.tf
├── terraform.tf
├── terraform.tfvars
├── variables.tf
├── outputs.tf
└── README.md

GCP root skeleton:

terraform {
  required_version = ">= 1.10.0"

  required_providers {
    google = {
      source  = "hashicorp/google"
      version = "~> 6.0" # Pin to a current minor series intentionally.
    }
  }

  backend "gcs" {}
}

provider "google" {
  project = var.project_id
  region  = var.region
}

module "service" {
  source     = "../../modules/cloud-run-service"
  project_id = var.project_id
  region     = var.region
  name       = var.name
}

</example>

---

References Index

• Layout and Workspaces
  • Brownfield repo placement, root-module boundaries, directory conventions, and when not to use CLI workspaces.
• GCP Terraform Patterns
  • ADC, impersonation, WIF, GCS backends, API enablement, module pinning, and policy validation on Google Cloud.
• Brownfield Adoption
  • Export, import blocks, generated configuration, and refactoring with moved blocks.
• Testing and Delivery
  • fmt, validate, saved plans, terraform test, CI, and policy checks.

Official References

• https://docs.cloud.google.com/docs/terraform/best-practices/general-style-structure
• https://docs.cloud.google.com/docs/terraform/best-practices/root-modules
• https://docs.cloud.google.com/docs/terraform/best-practices/operations
• https://docs.cloud.google.com/docs/terraform/best-practices/testing
• https://docs.cloud.google.com/docs/terraform/authentication
• https://docs.cloud.google.com/docs/terraform/resource-management/export
• https://docs.cloud.google.com/docs/terraform/resource-management/import
• https://docs.cloud.google.com/docs/terraform/policy-validation/quickstart
• https://developer.hashicorp.com/terraform/language/style
• https://developer.hashicorp.com/terraform/language/state/workspaces
• https://developer.hashicorp.com/terraform/language/import
• https://developer.hashicorp.com/terraform/language/modules/develop/refactoring