Terraform is a popular tool for managing infrastructure as code, but writing reusable and maintainable modules can be challenging. In this article, you will learn some of the best practices for writing Terraform modules that follow the principles of modularity, readability, reusability, and testability.
One of the first steps to write a good Terraform module is to define its inputs and outputs. Inputs are the variables that the module expects from the caller, such as resource names, configurations, or credentials. Outputs are the values that the module returns to the caller, such as resource IDs, attributes, or references. By defining inputs and outputs, you can document the module's interface, validate the input values, and expose the output values for further use.
Start every module with a?main.tf?file, where resources are located by default. In every module, include a?README.md?file in Markdown format. In the?README.md?file, include basic documentation about the module. Place examples in an?examples/?folder, with a separate subdirectory for each example. For each example, include a detailed?README.md?file.
Applying consistent and descriptive naming conventions to the components of a Terraform module, such as files, folders, resources, variables, and outputs, is an important best practice. This can help to organize the module's structure, improve the code's readability, and prevent naming conflicts. Some suggested conventions are using lowercase letters, numbers, and underscores for file and folder names; snake_case for resource, variable, and output names; and descriptive and specific names that accurately reflect the module's purpose and functionality. Additionally, it is recommended to prefix output names with the module's name to avoid ambiguity.
DRY stands for Don't Repeat Yourself, and it is a principle that aims to reduce duplication and redundancy in code. Applying this principle to Terraform modules can help you create more reusable, maintainable, and scalable code. You can use variables and locals to store and reuse common values, data sources and dynamic blocks to fetch and generate resource attributes, for_each and count to iterate over collections and create multiple resources, and modules to encapsulate and reuse common functionality. Following the DRY principle can help you make the most of your Terraform modules.
Documentation is a critical component of writing Terraform modules, as it helps to explain the module's purpose, usage, and behavior to users and maintainers. Additionally, documentation can help to prevent errors, bugs, and misunderstandings. When writing documentation for Terraform modules, you should use comments to explain the code logic and rationale. README files should provide an overview of the module's features, requirements, inputs, outputs, and examples. Variable and output descriptions should document the expected types, values, and formats. Finally, using code formatting tools and linters will ensure consistent and clean code style.
Testing is an essential step to guarantee the quality and dependability of the Terraform module. Testing can verify the module's functionality, performance, and compatibility with different environments and scenarios, as well as help to identify and fix errors, bugs, and vulnerabilities. To test a Terraform module, you can use the terraform plan and terraform apply commands to review and execute the module's code, the terraform validate and terraform fmt commands to check the syntax and format of the module's code, the terraform output and terraform show commands to inspect the module's output and state, or the terraform destroy command to clean up the module's resources. Additionally, you can use testing frameworks and tools such as Terratest, InSpec, or Kitchen-Terraform to automate and extend the testing process.