Terraform Module Developer

Terraform Module Developer

The strict 8-file pattern: 4 root files (main.tf + variables.tf + outputs.tf + versions.tf) + 4 wrapper files (wrappers/*.tf) + a polished README.md with terraform-docs auto-injection. Multi-provider; works for any Terraform/OpenTofu provider.

When to use

• The user asks for a new Terraform module, IaC module, .tf resource, or a wrapper module.
• The user wants to harden, refactor, or review an existing module.
• A polyglot infra repo needs consistent module structure across services / regions / providers.
• The user wants terraform-docs wired up so READMEs stay in sync with code.
• The user wants tflint / tfsec / checkov running pre-commit and in CI.

Skip this skill for: cloud-vendor-specific business logic (use a vendor SDK skill), application code (different language skills), or non-IaC config like Helm charts (use gitops-cd-developer).

Required structure

Every module is exactly this — no fewer files, no extra files, this layout:

modules/<service>/<resource>/
├── main.tf            # resource definition, count = var.<prefix>_create ? 1 : 0
├── variables.tf       # all vars prefixed `<prefix>_*`, heredoc descriptions
├── outputs.tf         # try/element/concat pattern for every output
├── versions.tf        # required_version + required_providers (pinned)
├── README.md          # static intro + BEGIN_TF_DOCS / END_TF_DOCS markers
└── wrappers/
    ├── main.tf        # module "wrapper" { for_each = var.items, ... }
    ├── variables.tf   # `defaults` (any) + `items` (any)
    ├── outputs.tf     # output "wrapper" { value = module.wrapper }
    └── versions.tf    # same constraints as root

Total: 2 directories, 9 files (8 .tf + 1 README.md). Validators check this exactly.

Plus, repo-root tooling (one-time setup):

• .terraform-docs.yml — config that injects between the README markers.
• .tflint.hcl — linting rules + provider plugins.
• .pre-commit-config.yaml (or fragment) — wires terraform-fmt, terraform-docs, tflint, tfsec into commit hooks.

Workflow

1. Confirm the provider. Default: ask the user which provider (aws, azurerm, google, huaweicloud, oci, digitalocean, cloudflare, etc.) and pin a version. Save provider preference to memory after first run.
2. Confirm the service + resource. <service> is the namespace (e.g., vpc, compute, storage). <resource> is the specific resource type (e.g., subnet, instance, bucket). Variable prefix derived as <service>_<resource> with hyphens turned into underscores.
3. Scaffold the structure. Run bash scripts/scaffold-module.sh <service> <resource> <provider> <provider-version>. Creates the 9 files from assets/templates/terraform/. See references/module-anatomy.md for the file-by-file breakdown.
4. Fetch provider docs. Open the resource page on the Terraform Registry. Capture the full set of arguments, optional fields, dynamic blocks, and exported attributes. The heredoc descriptions in variables.tf should mirror the registry exactly.
5. Fill in main.tf. Map provider arguments to module variables. Use the count conditional. Apply references/dynamic-blocks.md for any nested blocks.
6. Fill in variables.tf. Apply references/variable-patterns.md — naming, heredoc, complex types with optional(), the <prefix>_timeouts object pattern when the resource supports timeouts.
7. Fill in outputs.tf. Apply references/output-patterns.md — try(element(concat(<resource>.this.*.<attr>, [<default>]), 0), <fallback>) for every export.
8. Fill in wrappers/. Apply references/wrapper-pattern.md — every root variable gets a corresponding try(each.value.<x>, var.defaults.<x>, <default>) line in wrappers/main.tf.
9. Wire terraform-docs. Apply references/readme-and-terraform-docs.md — confirm .terraform-docs.yml exists at repo root, README has the BEGIN_TF_DOCS / END_TF_DOCS markers, and the pre-commit hook is registered.
10. Wire security scanning + tests. Apply references/security-scanning.md (tflint + tfsec/trivy + checkov) and references/module-testing.md (terraform test framework + examples/ directory).
11. Validate. Run bash scripts/validate-module.sh <module-path> — checks file count (2 dirs / 9 files), no list(any), prefix consistency, iterator patterns, README markers. Aim for 100%.
12. Format + native validate. terraform fmt -recursive && terraform validate from the module directory.
13. Generate docs. terraform-docs -c .terraform-docs.yml <module-path> — confirm the README inputs/outputs tables fill in correctly.

Available resources

• references/module-anatomy.md — 8-file structure, four-file pattern, naming conventions, count conditional, file-by-file breakdown.
• references/variable-patterns.md — naming convention, heredoc descriptions, complex object types with optional(), <prefix>_timeouts object pattern.
• references/output-patterns.md — try(element(concat(...), 0), <fallback>) pattern with examples for string / list / map / object outputs.
• references/dynamic-blocks.md — iterator pattern (iterator = <name>_cfg), nested dynamic blocks, can(length()) guard.
• references/wrapper-pattern.md — for_each = var.items, defaults + items variables, try(each.value, var.defaults, <fallback>) chain, type-safe wrapper variables for timeouts.
• references/readme-and-terraform-docs.md — README template structure, BEGIN_TF_DOCS / END_TF_DOCS markers, .terraform-docs.yml config (mode: inject, formatter: markdown table), pre-commit hook integration, CI integration.
• references/provider-conventions.md — multi-provider notes (AWS / Azure / GCP / HuaweiCloud / OCI / DigitalOcean / Cloudflare): naming differences, region handling, tagging conventions, provider-specific gotchas.
• references/security-scanning.md — tflint config + plugins, tfsec / trivy security scanning, checkov compliance scanning, integrating all three into pre-commit and CI.
• references/module-testing.md — Terraform 1.6+ native test framework (*.tftest.hcl), examples/ directory pattern, integration tests, contract tests.
• references/anti-patterns.md — 12 named anti-patterns with the better alternative.
• assets/templates/terraform/{main,variables,outputs,versions}.tf.tmpl + README.md.tmpl — root module starting points (renamed .tmpl so IDE Terraform plugins don't try to parse the placeholder tokens).
• assets/templates/terraform/wrappers-{main,variables,outputs,versions}.tf.tmpl — wrapper module starting points.
• assets/templates/configs/{terraform-docs.yml,tflint.hcl,pre-commit-fragment.yaml,editorconfig} — repo-root tooling configs.
• assets/examples/{simple-module,dynamic-blocks,multi-block,module-test}.md — full worked examples (network / storage / firewall / test).
• scripts/scaffold-module.sh — generic, multi-provider scaffolder. Run it instead of writing 9 files by hand.
• scripts/validate-module.sh — score a module against the checklist. Run after editing.
• scripts/setup-tooling.sh — install terraform-docs, tflint, tfsec, checkov via standard package managers.

Top gotchas (always inline — do not skip)

• 8 files, 2 directories. No fewer, no extras. A module without wrappers/ is incomplete. A module with extra .tf files (e.g., data.tf, locals.tf) is the wrong shape — fold them into main.tf.
• Variable prefix is <service>_<resource>_*. Always. vpc_subnet_name, not name. Prefix-less variables fight every consumer that imports more than one of your modules.
• Dynamic blocks REQUIRE iterator = <name>_cfg. Without the iterator, the block name shadows the variable. With it, you get a clean <name>_cfg.value.field reference.
• Output pattern is non-negotiable: try(element(concat(<resource>.this.*.<attr>, [<default>]), 0), <fallback>). Handles count = 0, null attrs, and missing fields in one expression.
• list(any) is banned. Always use list(object({...})) with optional() for optional fields. list(any) loses type safety and breaks consumer-side validation.
• Timeouts use ONE object variable, not three. <prefix>_timeouts of type object({create=optional, update=optional, delete=optional}). Never <prefix>_timeout_create / _update / _delete as separate vars.
• Wrapper variables: defaults + items only, both strictly typed. defaults is object({...}), items is map(object({...})). Every field is optional(<type>) and mirrors a root variable — same name, same type, same nested shape. type = any is the old shortcut; typed wrappers are the contract because misspelled fields and wrong types fail at plan time instead of silently being ignored.
• README has BEGIN_TF_DOCS / END_TF_DOCS markers. terraform-docs runs in mode: inject and replaces only the content between markers. The static portion (Usage, Examples, Notes, Provider Documentation link) lives outside the markers.
• .terraform-docs.yml lives at repo root. Not per-module. It applies to every module via terraform-docs -c .terraform-docs.yml <path>.
• Pin every required_providers version. Lower bound minimum (>= 1.91.0). Floating refs (~> 1, latest) break reproducibility across team members and CI.
• enterprise_project_id and similar provider-specific fields are always optional. Don't make them required even when the provider treats them as semi-required — different organizations have different setups.
• Module-level region defaults to null, not a hardcoded value. Let the provider-level region win when the module caller doesn't override. Hardcoded defaults ("us-east-1", "westus2", or any other concrete region) leak organizational choices into reusable modules.

What you DO

1. Always start from scripts/scaffold-module.sh. Never hand-write the 9 files.
2. Use the variable prefix <service>_<resource>_* consistently.
3. Use count = var.<prefix>_create ? 1 : 0 on the resource block.
4. Write heredoc descriptions on every variable that mirror the provider registry.
5. Use list(object({...})) with optional() for any list-of-records variable.
6. Use the <prefix>_timeouts object variable (not three separate vars) when the resource supports timeouts.
7. Use the iterator pattern (iterator = <name>_cfg) on every dynamic block.
8. Use the try(element(concat(...), 0), <fallback>) pattern for every output.
9. Write a wrappers/ directory with typed defaults (object({...})) and items (map(object({...}))), mirroring every root variable as optional(<type>). Thread per-resource values via the try(each.value, var.defaults, <fallback>) chain in wrappers/main.tf.
10. Place BEGIN_TF_DOCS / END_TF_DOCS markers in every README between Usage and Notes.
11. Wire terraform-docs as a pre-commit hook (and a CI check).
12. Wire tflint + tfsec/trivy + checkov as pre-commit + CI.
13. Add an examples/ subdirectory with a working basic example, then tests/*.tftest.hcl for the native test framework.
14. Run scripts/validate-module.sh after every edit; aim for 100%.
15. Run terraform fmt -recursive && terraform validate before declaring done.

What you do NOT do

• Skip the wrappers/ directory.
• Use list(any) or map(any) anywhere — the wrapper's defaults and items are also strictly typed (object({...}) / map(object({...}))).
• Hand-write per-resource wrapper variables (only defaults + items).
• Hardcode regions, project IDs, or organizational defaults.
• Use ~> or unpinned floating refs for provider versions.
• Place data.tf / locals.tf as separate files (fold into main.tf).
• Forget the iterator = <name>_cfg on a dynamic block.
• Use for_each = var.x != null ? var.x : [] (use can(length(var.x)) ? var.x : []).
• Write the inputs / outputs / providers tables manually in README — let terraform-docs inject them.
• Commit a module without running terraform fmt.
• Create a module without an examples/ working example.
• Mix count and for_each on the same resource — pick one.
• Reference provider-specific resources without including the provider in required_providers.
• Make tags optional with no merge() for the Name tag (every module merges a Name tag from <prefix>_name).

Source: MKAbuMattar/skills — distributed by TomeVault.