Terraform Review

Terraform Review

Runs an 8-category review checklist on Terraform changes. Produces an Approved / Needs Changes / Blocked verdict.

When to Use

• Reviewing a Terraform pull request
• Pre-merge validation of infrastructure changes
• Self-review before opening a PR
• Auditing existing Terraform code

Process

1. Structure

• Files follow standard layout: live/, modules/, envs/
• One resource per file in modules
• terraform.tf has backend + provider config
• variables.tf, outputs.tf, locals.tf are separate files
• No .terraform/ or *.tfstate* in the PR

# CORRECT structure
terraform/
  live/terraform.tf        # backend + provider
  live/variables.tf        # inputs
  live/locals.tf           # computed values
  live/outputs.tf          # exports
  modules/{service}/       # one resource per file
  envs/{env}/              # per-environment config

# WRONG — everything in one file
terraform/main.tf          # 500 lines of mixed resources

2. State Safety

• No terraform state commands in automation
• State stored in S3 with DynamoDB locking
• prevent_destroy on critical resources (RDS, S3 with data)
• No resources removed without terraform state rm plan documented
• create_before_destroy on security groups and launch configs
• Import blocks used for adopting existing resources (not terraform import CLI)

# CORRECT — protect critical resources
resource "aws_db_instance" "main" {
  lifecycle {
    prevent_destroy = true
  }
}

# CORRECT — zero-downtime SG updates
resource "aws_security_group" "app" {
  name_prefix = "${local.name_prefix}-app-"
  lifecycle {
    create_before_destroy = true
  }
}

3. Security

• No secrets in .tf or .tfvars committed to git
• Sensitive variables marked sensitive = true
• S3 buckets block public access
• RDS/Redis in private subnets only
• Security groups follow least privilege (no 0.0.0.0/0 ingress on non-ALB)
• Encryption enabled (S3 SSE, RDS encryption, Redis transit + at-rest)
• IAM policies use least privilege, no * actions on * resources

# WRONG — overly permissive
resource "aws_security_group_rule" "bad" {
  cidr_blocks = ["0.0.0.0/0"]
  from_port   = 0
  to_port     = 65535
}

# CORRECT — scoped to specific source
resource "aws_security_group_rule" "good" {
  source_security_group_id = var.alb_security_group_id
  from_port                = 8080
  to_port                  = 8080
}

4. Naming

• Resources use local.name_prefix (pattern: {project}-{service}-{env})
• Consistent naming across all resources in the module
• Tags include: Project, Service, Environment, ManagedBy
• No hardcoded names or account IDs

# CORRECT
locals {
  name_prefix = "${var.project}-${var.service}-${var.env}"
}

# WRONG
resource "aws_s3_bucket" "assets" {
  bucket = "my-bucket-prod"  # hardcoded
}

5. Modules

• No provider blocks inside modules
• Module source uses version pinning (?ref=vX.Y.Z)
• No circular module dependencies
• Module outputs only expose what consumers need
• Modules have versions.tf with required provider versions

# CORRECT — pinned version
module "rds" {
  source = "git::https://github.com/{project}/terraform-modules.git//rds?ref=v1.2.0"
}

# WRONG — no version pin
module "rds" {
  source = "git::https://github.com/{project}/terraform-modules.git//rds"
}

6. Variables

• All variables have description and type
• Sensitive variables marked sensitive = true
• Validation blocks on critical inputs (CIDR, names, enums)
• No unused variables
• Defaults are sensible for dev, overridden per env

# CORRECT
variable "instance_class" {
  description = "RDS instance class"
  type        = string
  default     = "db.t3.micro"

  validation {
    condition     = can(regex("^db\\.", var.instance_class))
    error_message = "Must be a valid RDS instance class."
  }
}

# WRONG — no description, no type
variable "instance_class" {}

7. Providers

• Provider versions pinned with ~> (pessimistic constraint)
• required_version for Terraform itself
• Provider config only in live/terraform.tf, never in modules
• Default tags configured at provider level

8. CI/CD

• terraform fmt -check runs in CI
• terraform validate runs in CI
• Plan output posted as PR comment
• Apply only runs on merge to main
• State locking prevents concurrent applies
• Secrets injected via CI environment, not committed

Interaction Style

• Reads all changed .tf files in the PR
• Checks every category — does not skip sections
• Flags blocking issues (security, state safety) separately from suggestions
• Shows exact file and line for each finding

Rules

• Blocking issues: secrets in code, no state locking, 0.0.0.0/0 ingress, missing encryption
• Needs Changes: missing descriptions, no version pin, naming inconsistency
• Suggestions: code style, optional validations, documentation

Output

Produces a structured review:

## Terraform Review: {PR title}

### Verdict: Approved | Needs Changes | Blocked

### Findings

#### Blocked (if any)
- [ ] **[Security]** Secrets found in terraform.tfvars — file:line

#### Needs Changes (if any)
- [ ] **[Naming]** Hardcoded bucket name in s3.tf:12
- [ ] **[Modules]** Missing version pin on RDS module

#### Suggestions (if any)
- **[Variables]** Consider adding validation on `instance_class`

### Checklist Summary
| Category  | Status |
|-----------|--------|
| Structure | Pass   |
| State     | Pass   |
| Security  | Fail   |
| Naming    | Warn   |
| Modules   | Warn   |
| Variables | Pass   |
| Providers | Pass   |
| CI/CD     | Pass   |