terraform-style

Terraform style conventions

Apply these conventions to every module and root configuration; match them when editing existing code. They are load-bearing for readability and call-site ergonomics, not cosmetic preferences. For the rationale behind each rule and extended examples, see reference.md.

1. Collections: set(string) for unordered, unique items

Use set(string) (not list(string)) whenever elements must be unique and order is irrelevant — subnet IDs, instance types, CIDRs, ARNs, repository names, regions, namespaces. Uniqueness becomes intrinsic, for_each gets stable keys without toset(), and the type documents intent.

variable "private_subnet_ids" {
  description = "Private subnet IDs the cluster spans."
  type        = set(string)
}

Reserve list(...) for genuinely ordered data or where duplicates are meaningful. Terraform auto-converts a set to a list when an argument requires one.

2. Never name a resource this

• A module's single primary resource → main (aws_eks_cluster.main).
• A for_each/count resource → a singular noun for one element (aws_ecr_repository.repo).
• A secondary singleton → a purpose word (aws_iam_openid_connect_provider.irsa, aws_ecr_lifecycle_policy.retention).

The name should read well at the reference site, not echo the resource type (aws_eks_node_group.node_group is redundant; .main is not).

3. Prefer name_prefix over name

For resources whose name is unique within an account/region (IAM roles and policies, instance profiles, security groups, launch templates, target groups), use name_prefix with a trailing -. The provider appends a random suffix, so two stacks — or a create-before-destroy replacement — never collide on a hard-coded name.

resource "aws_iam_role" "node" {
  name_prefix = "${var.cluster_name}-node-" # good: stands up twice without a clash
}
# avoid: name = "${var.cluster_name}-node"

Keep the prefix ≤ 38 chars: the AWS random suffix is 26 and IAM role names cap at 64.

4. Toggle resources with for_each, not count

Gate a resource on a boolean with for_each over a one-or-zero-element set, keeping a stable address instead of a positional index:

resource "aws_iam_role" "node_windows" {
  for_each = toset(var.enable_windows_nodes ? ["true"] : [])
  # referenced as aws_iam_role.node_windows["true"].arn — not [0]
}

Reserve count for genuine cardinality (N identical resources).

5. Don't prefix outputs with the module name

Outputs read as module.<name>.<output>, so a concept prefix duplicates the module name at every call site. Name the output for the attribute it exposes:

output "endpoint" { value = aws_eks_cluster.main.endpoint } # module.cluster.endpoint
# avoid: output "cluster_endpoint"  → module.cluster.cluster_endpoint

6. Group cohesive variables into one object

Prefer one object variable over a cluster of flat scalars that are always configured together. Give every attribute optional(type, default) and the variable default = {} so module "x" {} still works and callers set only what they need:

variable "endpoint_access" {
  description = "Cluster API endpoint exposure."
  type = object({
    private      = optional(bool, true)
    public       = optional(bool, false)
    public_cidrs = optional(set(string), [])
  })
  default = {}
}

Group only what is genuinely cohesive — unrelated knobs stay flat.

7. Group cohesive outputs into one object

When several outputs are facets of one concept and would share a prefix (oidc_arn, oidc_url), fold them into a single object output named for the concept:

output "node_role" {
  description = "IAM role shared by worker nodes."
  value = {
    arn  = aws_iam_role.node.arn
    name = aws_iam_role.node.name
  }
}

A primary resource's own top-level attributes (name, arn, endpoint) stay flat.

8. null over empty-string defaults

Never default an optional string to "". Use default = null for a flat variable, or optional(string) with no second argument for an object attribute — null is the unambiguous "unset" signal. Consumers absorb the null with coalesce(var.proxy.http, "") or compact([...]).

9. Canonical file layout

Provider and required_version constraints live in terraform.tf — never versions.tf. A module consists of terraform.tf, main.tf, variables.tf, outputs.tf, README.md, plus optional domain-specific files (iam-*.tf). Every variable and output carries a description.

To scaffold a new module with this layout, use the terraform-module skill; before committing, run the terraform-validate workflow.