[Design Guide] Terraform Module Design Will Always Collapse Without Principles

When using Terraform without module design principles, code becomes tangled and complex, collaboration between team members becomes difficult, and modifying a module can unintentionally break other environments.

Terraform has become the standard for IaC (Infrastructure as Code) thanks to its Cloud-Agnostic nature that is not tied to any specific cloud, its use of HCL, a Declarative language that describes state, and its massive community ecosystem.

The 3 design principles for good modules recommended by HashiCorp are as follows.

• Encapsulation: Only resources that are always deployed together should be grouped into a single module. For example, VNet, subnets, and NSG are appropriately configured as a single module.
• Privileges: Resources managed by different entities should be separated. Resources for the network team and development team should be separated to structurally prevent privilege misuse.
• Volatility: Long-lived infrastructure and frequently changing infrastructure should be separated. Low-volatility resources like networks or DBs and high-volatility resources like application servers should be separated to minimize change risk.

The standard module structure has main.tf, variables.tf, outputs.tf, and README.md as basic required elements, and as needed, a modules/ directory for nested modules and an examples/ directory containing usage examples are added for management.