How to Build Reusable IaC Modules with Terraform

Reusable Terraform modules make managing infrastructure simpler, faster, and more reliable. They save time, reduce errors, and help maintain consistency across environments. Here's what you need to know:

• Why use reusable modules? They cut costs, enforce standards, and minimise configuration drift, making them ideal for scaling cloud environments.
• Key principles for module design: Focus on single responsibility, parameterise inputs, and avoid hardcoding values. Include clear documentation for easy adoption.
• Best practices: Use version control, semantic versioning, and registries (public or private) to manage and distribute modules effectively.
• Common pitfalls to avoid: Avoid over-complicating with too many modules, hardcoding values, and neglecting documentation.

Reusable modules are essential for teams embracing DevOps, enabling faster deployments and easier management across platforms like AWS and Azure. Start small, focus on simplicity, and scale as needed.

Core Principles of Terraform Module Design

Structure and Components of a Terraform Module

A well-organised directory layout makes Terraform modules easier to find, understand, and trust [3]. The essential components of a module include:

• main.tf: Contains the primary logic.
• variables.tf: Defines input parameters.
• outputs.tf: Specifies return values.
• versions.tf: Lists provider constraints.
• README: Provides documentation.

Optional additions can further enhance a module's usability. For example, you might include nested modules in a modules/ directory, practical examples under an examples/ folder, or a LICENSE file if the module is shared publicly [1]. It's also important to provide clear descriptions for all variables and outputs, ensuring users understand each parameter's purpose and what the module returns [1].

With this structure in place, adhering to disciplined design principles ensures your modules are both reusable and effective.

Best Practices for Module Design

A well-designed module should have a clear purpose and focus solely on one task. If you struggle to summarise its function in a single sentence, it’s likely too complex and should be broken into smaller, more manageable modules [2].

Each module should be self-contained and follow the principle of single responsibility. Avoid hardcoding values that might vary across deployments by using clear and flexible parameterisation [4]. Outputs should be limited to only what consumers of the module need, and sensitive data must be flagged appropriately to maintain security [4].

When it comes to provider configurations, avoid including them within shared modules. Instead, configure providers in the root module, while using the versions.tf file within the module for declaring required providers and their constraints [3].

These principles not only improve module design but also allow for customisation to meet specific needs, such as regional requirements.

UK Team Considerations

For development teams in the UK, it’s important to adapt modules to align with regional standards. Use British English spelling in all documentation and code comments. Where relevant, default to metric measurements and ensure modules account for UK-specific regulations such as GDPR and data protection compliance [5].

Documentation should reflect UK practices, including references to UK postal codes, currency in pounds sterling (£), and business scenarios specific to UK organisations. For deployments spanning multiple regions, use variables and outputs to dynamically pass region-specific values [5].

Step-by-Step Guide to Building a Reusable Terraform Module

Setting Up the Module Directory and Files

To start, create a new directory for your Terraform module. Inside this directory, you'll need to include four key files: README.md, main.tf, variables.tf, and outputs.tf. Here's what each file does:

• main.tf: Contains the core infrastructure code.
• variables.tf: Defines input parameters with clear names, types, and descriptions.
• outputs.tf: Specifies values that other configurations might need to access.
• README.md: Provides documentation about the module's purpose and how to use it.

For larger or more complex modules, you can organise your code further by splitting it into additional files (like network.tf or instances.tf) or creating subdirectories such as examples/ or docs/. Once your structure is in place, focus on designing how users will interact with the module.

Defining Input Variables and Outputs

The key to reusability lies in well-defined inputs and outputs. Inputs act as the interface for your module, allowing users to customise its behaviour without modifying the module's source code. To create effective inputs:

• Expose only the parameters that are necessary.
• Include a description, set type constraints, and provide default values (where appropriate) for each variable.
• Use validation blocks to catch errors early in the configuration process.

Outputs, on the other hand, make it easy to retrieve important details about the infrastructure you've deployed. These could be things like IP addresses, resource IDs, or connection strings. Documenting both inputs and outputs thoroughly ensures that users understand their purpose and makes the module easier to maintain or update later.

With inputs and outputs clearly defined, you can move on to writing logic that balances flexibility with consistency.

Implementing Logic for Reusability and Flexibility

To make your module adaptable to different scenarios, write logic that is both flexible and consistent. Use for_each and count meta-arguments to dynamically create multiple resources - like subnets, security groups, or virtual machines - without duplicating code. This allows a single module to handle varying requirements efficiently.

Parameterise provider details so your module can be deployed across different platforms or regions. Use conditional expressions and feature toggles to include or exclude resources as needed. For example, you can add optional features like monitoring, backups, or high availability without hardcoding values. Even seemingly fixed settings, such as resource tags and naming conventions, should be parameterised to match organisational standards.

Let’s consider an example: a module for deploying an Azure Linux virtual machine. This module might manage resources such as azurerm_linux_virtual_machine, azurerm_network_interface, and azurerm_public_ip. By parameterising key components, users can customise the deployment to fit their specific needs while relying on the module's consistent structure.

How to Build Reusable, Composable, Battle tested Terraform Modules

Best Practices for Managing and Publishing Modules

Once you've developed reusable Terraform modules, the next step is to manage and distribute them effectively. Proper module management not only ensures consistency across your projects but also reduces maintenance efforts and helps teams collaborate efficiently while upholding security and compliance standards. These practices build on the module design principles discussed earlier, making it easier to integrate modules into your DevOps workflow.

Using Version Control for Module Repositories

Version control is a cornerstone of effective module management. Tools like Git allow you to track changes to your Terraform modules and implement a structured branching strategy. For instance, use a main branch for stable, approved code and create separate feature branches for development work [7].

Adopt Semantic Versioning (SemVer), which uses the Major.Minor.Patch format to indicate the impact of changes [6][8]. Here's how it works:

• Major: Breaking changes that require updates to dependent configurations.
• Minor: New features added without breaking existing functionality.
• Patch: Bug fixes or minor improvements.

Automating versioning through CI/CD pipelines can eliminate manual errors. Set up automated tagging and release processes that trigger whenever changes are merged into the main branch. Additionally, pin exact module versions in your Terraform configurations to avoid unexpected issues caused by breaking changes. Use the ~> operator to allow updates for compatible patch versions while staying within the same major and minor version [6].

Publishing Modules to Registries

Once your version control system is in place, you’ll need a strategy for distributing your modules. Publishing them to registries makes them easier to discover and use. You can choose between public registries like the official Terraform Registry or private options such as Terraform Cloud for internal use [9].

Private registries offer key advantages, including better security, centralised governance, and compliance controls. They allow you to customise modules for your organisation’s specific needs while maintaining access restrictions. This way, you can share proprietary configurations internally without exposing them to the public.

When publishing to any registry, follow consistent naming conventions and ensure your modules include version tags. Implement robust security practices, such as code reviews for all module changes and audit logging for registry activities [10]. If you’re using private registries, establish clear policies for module approval and publication. For example, require peer reviews before making modules available to other teams to maintain high-quality standards across your infrastructure.

Documenting Modules for Team Collaboration

Clear documentation is essential to help teams adopt and maintain your modules. Without proper documentation, even the most well-designed modules can become a source of confusion. Each module should include a detailed README.md file that explains its purpose, provides usage instructions, and lists all inputs and outputs [11].

Good documentation should answer four key questions:

• What does the module do?
• Why is it needed?
• How do you use it?
• How can it be modified? [12]

Include practical examples that demonstrate common usage scenarios, and provide a sample terraform.tfvars.example file to help users get started quickly [2]. Document all inputs with clear descriptions, type constraints, and default values, specifying which inputs are mandatory and which are optional. Similarly, describe all outputs and how they can be used by other configurations.

To ensure consistency, establish organisation-wide documentation standards. These might include required sections, formatting guidelines, and review processes [2]. Encourage collaboration by creating contribution guides that explain how team members can suggest changes or improvements to existing modules. Adopting open-source principles internally can foster a culture of knowledge sharing and teamwork.

Finally, treat documentation as an integral part of your module. Update it alongside any code changes to keep everything in sync. Well-documented modules not only reduce the need for support but also make it easier for teams to work independently while staying aligned with organisational standards.

Common Pitfalls and How to Avoid Them

When designing Terraform modules, it's easy to fall into traps that can make your infrastructure harder to manage and less efficient. These pitfalls often stem from neglecting long-term maintainability. By identifying these challenges and addressing them proactively, you can save valuable time, streamline maintenance, and boost your team's overall productivity.

Balancing Monolithic and Composable Module Design

Striking the right balance between simplicity and flexibility in module design is no small feat. Over-modularising your code can lead to unnecessary complexity, while under-modularising can result in rigid, hard-to-scale configurations.

Monolithic modules group multiple resources into a single unit. While this makes them straightforward to deploy and manage, they can be tough to customise. They're ideal for standardised environments but may falter when flexibility is required. On the other hand, composable modules offer greater flexibility and reusability but often require more expertise and can lead to sprawling configurations.

Here’s a quick comparison of these approaches:

The goal is to create modules that are both opinionated and adaptable. HashiCorp advises designing modules to do one thing well and to support the majority of use cases without overcomplicating things [2]. For example, Cloud Posse's terraform-null-label module focuses on consistent resource naming and tagging, offering a simple yet versatile solution [16]. Start small with focused modules and combine them as necessary. A well-designed module should focus on a single responsibility, keeping it both clear and scalable.

The next step is to address how to avoid hardcoding, ensuring your modules remain portable and adaptable.

Avoiding Hardcoded Logic and Limited Portability

Hardcoding values in your modules is a common mistake that significantly reduces their flexibility and usability. When specific resource names, regions, or configurations are hardcoded, it becomes difficult to adapt the module for different environments or teams.

For instance, hardcoding AWS regions, resource names, or instance sizes can lock your module into a specific setup. Instead, parameterise these values and provide sensible defaults. This approach ensures the module is customisable while maintaining consistency across deployments.

To make your modules more versatile, avoid tying them to a single cloud provider. Use abstractions that can work across platforms, and implement validation rules to ensure input values are appropriate [14]. Additionally, adopt a tagging strategy that remains consistent regardless of the infrastructure [15].

Another useful approach is creating wrapper modules around popular third-party modules. This allows you to integrate your organisation's standards while benefiting from the reliability of community-maintained code [13].

Finally, support your module design with thorough documentation to encourage adoption across your team.

Ensuring Complete Documentation and Flexibility

Even the most well-designed module can fail if it's poorly documented. Without clear guidance, teams may struggle to understand how to use the module effectively, which can limit its adoption and usefulness.

Documentation often gets overlooked or treated as an afterthought. Teams might skip over edge cases, leave variable descriptions vague, or fail to provide practical examples. To avoid this, ensure your documentation includes:

• A comprehensive README.md that explains the module's purpose.
• Detailed examples showing real-world usage scenarios.
• Clear descriptions of all inputs, outputs, and dependencies.

Your documentation should answer four key questions: What does the module do? Why is it needed? How do you use it? How can it be modified? Tools like terraform-docs can help automate documentation generation, keeping it consistent and reducing the effort required to maintain it [17].

Modules help keep provider versioning such as Terraform AWS provider or Terraform Azure provider consistent across your configuration. – ControlMonkey [13]

Additionally, provide examples that go beyond the basics. Real-world scenarios can help teams understand how to apply the module effectively. By documenting every input, output, and dependency, you reinforce the module's reusability and ensure that even as your infrastructure grows in complexity, it remains manageable [17].

Conclusion and Key Takeaways

Reusable Terraform modules are revolutionising infrastructure management. They simplify operations, support growth, and address the evolving challenges faced by modern DevOps teams.

Why Reusable Modules Matter

The advantages of reusable modules are both immediate and far-reaching. By eliminating repetitive provisioning and reducing redundant configurations, they significantly cut costs. Teams leveraging these modules have reported a 30% boost in development speed and a 20% drop in infrastructure errors [19]. This translates into lower operational expenses and better use of resources.

Reusable modules also enhance scalability. For instance, consider a software company deploying microservices on AWS. By creating individual modules for each microservice - handling resources like EC2 instances, RDS databases, and load balancers - they can easily scale specific components as needed. This modular approach ensures resources are allocated efficiently and operations run smoothly [19].

Terraform Modules serve as a cornerstone for managing scalable infrastructure efficiently, fostering reusability, collaboration, and adherence to best practices. – Fedir Kompaniiets, DevOps and Cloud Architecture Expert, Co-founder of Gart [18]

Another key advantage is operational efficiency. Reusable modules enforce standardisation, reducing configuration drift and minimising deployment errors. According to the State of IaC 2023 report, Terraform is the most widely used tool for managing cloud resources [18], with over 70% of software development teams integrating its modules into their workflows [19]. This consistency is particularly beneficial for UK organisations navigating strict regulatory and compliance requirements.

These benefits lay the groundwork for practical adoption and implementation.

Steps to Implement Terraform Modules

To get started, organise your infrastructure into logical, single-purpose components that align with team responsibilities and deployment environments. Focus on creating modular components that can be combined as needed rather than building one-size-fits-all solutions. Consistent naming conventions and directory structures are crucial - even for local modules - as they enhance clarity and maintainability [21].

Modules allow you to define and reuse infrastructure components across environments, eliminating configuration drift and ensuring consistency. This standardisation makes managing complex infrastructure more efficient. – Blackstack [20]

Automating module testing is another critical step to ensure stability and avoid regressions. Comprehensive documentation and inline comments further improve collaboration and make future scaling efforts more seamless [21].

For UK businesses looking to cut costs and streamline deployments, Hokstad Consulting provides tailored solutions to help implement Terraform modules effectively.

FAQs