Refactor Module

Refactor Module

Input Parameters

Execution Steps

1. Analysis Phase

• Group resources by logical function
• Identify repeated patterns
• Map resource dependencies and cross-references
• Evaluate state migration complexity
• Measure variable propagation depth

2. Module Design

Interface Design

Define typed input contracts with validation and descriptive outputs:

variable "network_config" {
  description = "Network configuration parameters"
  type = object({
    cidr_block         = string
    availability_zones = list(string)
    enable_nat         = bool
  })

  validation {
    condition     = can(cidrhost(var.network_config.cidr_block, 0))
    error_message = "CIDR block must be valid IPv4 CIDR."
  }
}

output "vpc_id" {
  description = "ID of the created VPC"
  value       = aws_vpc.main.id
}

output "private_subnet_ids" {
  description = "List of private subnet IDs"
  value       = { for k, v in aws_subnet.private : k => v.id }
}

Encapsulation Guidelines

Include in module:

• Tightly coupled resources (e.g., VPC + subnets)
• Resources with shared lifecycle
• Configuration with clear boundaries

Keep separate:

• Cross-cutting concerns (monitoring, tagging)
• Resources with different lifecycles
• Provider-specific configurations

3. Code Transformation

Extract hardcoded values into variables, replace repeated resources with for_each, and use merge() for tag propagation.

See references/transformation-example.md for a complete monolithic→modular VPC refactoring example.

4. State Migration

Use moved blocks (Terraform 1.1+) for declarative state migration. For older versions, use terraform state mv commands.

See references/state-migration.md for examples of both approaches.

Verify migration in non-production first:

terraform plan -out=migration.tfplan    # expect no changes
terraform show migration.tfplan         # review carefully
terraform apply migration.tfplan        # apply only if clean

5. Testing

Use skill terraform-test. Organize tests in a tests/ directory:

my-module/
├── main.tf
├── variables.tf
├── outputs.tf
└── tests/
    ├── unit_test.tftest.hcl         # plan mode
    └── integration_test.tftest.hcl  # apply mode (creates real resources)

6. Documentation

Each module needs a README with: Overview, Usage example, Requirements, Inputs table, Outputs table.

See references/module-documentation-template.md for the standard template.

Refactoring Patterns

Pattern 1: Resource Grouping

Extract related resources into cohesive modules:

• Networking (VPC, Subnets, Route Tables)
• Compute (ASG, Launch Templates, Load Balancers)
• Data (RDS, ElastiCache, S3)

Pattern 2: Configuration Layering

module "vpc_base" {
  source = "./modules/vpc-base"
  # Minimal required inputs
}

module "vpc_prod" {
  source = "./modules/vpc-production"
  # Inherits from base, adds prod-specific config
}

Pattern 3: Composition

Build complex infrastructure from small, focused modules:

module "vpc" {
  source = "./modules/vpc"
}

module "security_groups" {
  source = "./modules/security-groups"
  vpc_id = module.vpc.vpc_id
}

module "application" {
  source     = "./modules/application"
  vpc_id     = module.vpc.vpc_id
  subnet_ids = module.vpc.private_subnet_ids
  sg_ids     = module.security_groups.app_sg_ids
}

Common Pitfalls

Over-Abstraction

Use specific, typed interfaces. map(map(any)) prevents validation and obscures intent.

# Prefer this
variable "database_config" {
  type = object({
    engine         = string
    instance_class = string
  })
}

Tight Coupling

Pass dependencies through the root module rather than coupling modules directly, keeping modules independently testable.

Missing State Migration Verification

Run terraform plan after migration and confirm zero changes before applying to production.

Version Control Strategy

# Pin module versions with semantic versioning
module "vpc" {
  source = "git::https://github.com/org/terraform-modules.git//vpc?ref=v1.2.0"
}

Pin exact versions in production; use version ranges in development.

Success Criteria

• Module has single, well-defined responsibility
• All variables have descriptions and types
• Validation rules prevent invalid configurations
• Outputs provide sufficient information for consumers
• Documentation includes usage examples
• Tests verify module behavior
• State migration completed without resource recreation
• No plan differences after refactoring

Related Skills

• Terraform Style Guide
• Azure Verified Modules

External References

• Terraform Module Development
• Module Best Practices

Revision History