Terraform Structure: Infrastructure As Code Scalability

A working Terraform script is just the start. Organising your IaC for scalability, reusability, and team collaboration requires deliberate file structure and module design from day one.

Terraform is a powerful tool that helps your company automate, scale and manage your IT infrastructure through a few lines of code. Keeping code readable and organised for future team members — or even yourself after a few weeks — can become tricky if proper planning is not followed in the beginning. A poorly structured Terraform configuration can lead to deployment failures, inconsistencies, and maintenance nightmares down the line.

In this guide, we share some of the useful file structures used when beginning a Terraform journey and the advanced techniques learned along the way. The first goal is to get your script functioning; the next is to make it scalable.

Breaking Down Monolithic Infrastructure

When beginning a new Terraform project, it is common to start defining resources in a single file. However, when complexities are introduced, this single file structure becomes unmanageable and harder to scale.

Problems with Monolithic Structure

• Readability: A single, large file can be overwhelming and difficult to navigate, especially for new team members or when revisiting code after a long period.
• Maintainability: As your infrastructure grows, making changes to a single file becomes increasingly risky — a small change can have unintended consequences in other parts of the code.
• Reusability: It's difficult to reuse code from a monolithic structure in other projects or modules.
• Testability: Testing a monolithic structure can be challenging and time-consuming.

Organising by Resource Type

Before diving into more advanced configurations like modularisation, one of the first beneficial things to do is to organise your Terraform code by resource type. This enhances readability and maintainability while promoting a more granular understanding of your infrastructure.

Common File Structure

The most common files used in Terraform configuration are main.tf, providers.tf, outputs.tf and variables.tf. Splitting your resource configuration makes it easier to manage — all resources configured in the root directory can communicate and be referenced as if they were in the same file.

• networks.tf: Contains definitions for VPCs, subnets, route tables, internet gateways, and other networking resources.
• main.tf: Defines the core infrastructure resources.
• variables.tf: Declares input variables that can be customised when using the module.
• outputs.tf: Defines output values that can be used by the calling module or exported as outputs of the entire configuration.

Terraform Modules — Reusable Building Blocks

Terraform modules are powerful tools for organising and reusing infrastructure components. By encapsulating related resources into modules, you can promote code reusability, improve maintainability, and accelerate development.

In essence, modules function as containers for multiple resources that are used together, creating an abstraction layer that encapsulates complex infrastructure components. This parameterisation allows you to instantiate the same module with different configurations across various environments. A single Amazon RDS module could be used to deploy both production and staging databases, with environment-specific configurations passed through variables.

Module Structure

A Terraform module typically consists of:

• main.tf: Defines the core infrastructure resources within the module — the heart of the module, containing the Terraform code that provisions and manages the resources.
• variables.tf: Declares the input variables that can be customised when using the module. These variables act as parameters, allowing you to configure the module's behavior.
• outputs.tf: Defines the output values that the module exposes. These outputs can be used by other modules or to access information about the provisioned infrastructure.

Using Modules

To use a module, you use the module block in your Terraform configuration. The source argument specifies the location of the module. In a module block, the cidr_block and environment arguments provide values for the module's input variables, while a subsequent module block uses the vpc_id output from the first module to create a subnet — demonstrating how modules can interact and build upon each other.

Benefits of Using Modules

• Maintainability: Modules promote blast radius containment. Changes within a module are less likely to have unintended consequences on other parts of your infrastructure, simplifying debugging and reducing the risk of errors.
• Scalability: Modules facilitate the creation of complex infrastructure patterns through composition. You can combine smaller modules to create larger, more complex ones — an application stack module composed of networking, compute, and storage modules all working through well-defined interfaces.
• Collaboration: Modules act as technical contracts between teams. Well-defined module interfaces allow different teams to work independently — platform teams maintaining core modules while application teams consume them.

Best Practices

• Define Minimum Requirements: Before creating a module, carefully consider the minimum resources and configurations it should include. Avoid grouping unnecessary resources.
• Document Thoroughly: Clear documentation is crucial for module usability. Document the module's purpose, input variables, outputs, and any dependencies. Tools like terraform-docs can help automate this process.
• Test Rigorously: Implement comprehensive tests to ensure your modules function as expected. Terraform's built-in testing capabilities, along with tools like Terratest, can help you achieve thorough test coverage.

Testing and Documentation

Documentation and testing in Terraform modules are complementary practices that ensure both the reliability and usability of your infrastructure code. Think of documentation as a detailed blueprint of your module's capabilities, while testing validates that the blueprint accurately reflects reality.

Terraform's built-in validation blocks allow you to implement precise rules for your input variables, ensuring that values meet specific criteria before Terraform attempts to create any infrastructure. Tools like terraform-docs automatically generate standardised documentation by analysing your Terraform files — capturing not just the structure of your variables and outputs but also including validation rules in the generated documentation.

Conclusion

When starting a new Terraform project, use these steps to make your development experience smooth and easy. The cloud is known for its rapid scaling, so being able to provision and handle this scaling starts with good file structure and effective modularisation practices. Teams can build scalable, maintainable, and secure infrastructure while enforcing standardisation across their organisation.