Terraform Module Design Patterns

Well-designed Terraform modules are the foundation of scalable infrastructure as code. Here are the patterns I use across enterprise deployments.

Module Composition

Favor small, focused modules over monolithic ones:

modules/ ├── vpc/ # Networking foundation ├── eks/ # Kubernetes cluster ├── rds/ # Database layer └── monitoring/ # Observability stack

Each module should do one thing well and expose clear interfaces.

Variable Design

Use objects for related configuration:

variable "vpc_config" { description = "VPC configuration" type = object({ cidr_block = string availability_zones = list(string) enable_nat_gateway = bool single_nat_gateway = bool }) }

Output Strategy

Export everything consumers might need:

output "vpc_id" { description = "The ID of the VPC" value = aws_vpc.main.id } output "private_subnet_ids" { description = "List of private subnet IDs" value = aws_subnet.private[*].id } output "nat_gateway_ips" { description = "NAT Gateway public IPs" value = aws_nat_gateway.main[*].public_ip }

State Management

Use remote state with locking:

terraform { backend "s3" { bucket = "terraform-state-prod" key = "networking/terraform.tfstate" region = "ap-southeast-1" encrypt = true dynamodb_table = "terraform-locks" } }

Testing Modules

Use Terratest for module validation:

func TestVPCModule(t *testing.T) { terraformOptions := &terraform.Options{ TerraformDir: "../modules/vpc", Vars: map[string]interface{}{ "cidr_block": "10.0.0.0/16", }, } defer terraform.Destroy(t, terraformOptions) terraform.InitAndApply(t, terraformOptions) vpcId := terraform.Output(t, terraformOptions, "vpc_id") assert.NotEmpty(t, vpcId) }

Conclusion

Invest time in module design upfront. Good modules reduce duplication, enforce standards, and make infrastructure changes predictable and reviewable.