Mastering Terraform: Creating Composite Modules

As you advance in your Infrastructure as Code (IaC) journey with Terraform, understanding how to build reusable and maintainable infrastructure components becomes paramount. Composite modules represent a powerful strategy for achieving this, allowing you to combine multiple existing modules into a higher-level, more abstract unit of infrastructure.

What are Composite Modules?

Composite modules are essentially Terraform modules that encapsulate and orchestrate other, more granular Terraform modules. Think of them as building blocks that assemble pre-defined infrastructure patterns. Instead of deploying individual resources or simple modules, you deploy a composite module that, in turn, provisions a complete, functional piece of infrastructure, such as a web application stack or a secure network segment.

Composite modules abstract complexity by orchestrating smaller, focused modules.

They allow you to define and deploy complex infrastructure patterns as a single, manageable unit, promoting consistency and reducing duplication.

By composing existing modules, you create a higher level of abstraction. This means your root Terraform configuration can be simpler, focusing on deploying these high-level composite modules rather than managing the intricate dependencies and configurations of many individual modules. This approach significantly enhances reusability, maintainability, and the overall scalability of your IaC practices.

Benefits of Using Composite Modules

Adopting composite modules offers several key advantages for your infrastructure management:

Benefit	Description
Abstraction	Hides the complexity of underlying modules, simplifying root configurations.
Reusability	Enables the deployment of entire infrastructure patterns across different environments or projects.
Consistency	Ensures that complex infrastructure stacks are deployed with standardized configurations.
Maintainability	Centralizes the management of related infrastructure components, making updates and troubleshooting easier.
Team Collaboration	Provides well-defined infrastructure building blocks that teams can easily consume and integrate.

Designing and Implementing Composite Modules

Creating effective composite modules involves thoughtful design. You'll typically define a new module that calls other modules, passing inputs and outputs as needed. This often involves creating a

code

main.tf

file within your composite module that references the source modules.

Consider a composite module for a 'Web Application Stack'. This module might internally call a 'VPC Module', an 'EC2 Instance Module', an 'RDS Database Module', and a 'Load Balancer Module'. The composite module would expose inputs like instance_type, database_size, and domain_name, and its outputs might include the load_balancer_dns and database_endpoint. This structure allows a user to provision a complete, functional web app by configuring just a few variables in the composite module.

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When designing, consider the scope of your composite module. It should represent a cohesive unit of infrastructure that makes sense to deploy together. Avoid making composite modules too monolithic, as this can negate the benefits of modularity.

What is the primary purpose of a composite module in Terraform?

To orchestrate and combine multiple smaller, focused modules into a higher-level, reusable infrastructure pattern.

Key Considerations for Composite Modules

When building composite modules, keep these points in mind:

Define clear input variables for your composite module. These should represent the configurable aspects of the infrastructure pattern you're abstracting. Well-defined inputs make the module easy to use.

Manage dependencies carefully. Ensure that the modules called within your composite module are compatible and that their outputs are correctly passed as inputs to subsequent modules.

Document your composite modules thoroughly. Explain what infrastructure they provision, what inputs are required, and what outputs are provided. This is crucial for adoption by other teams or for your future self.

Test your composite modules rigorously. Ensure they deploy as expected and that all internal dependencies are met. Versioning your modules is also a best practice.

Why is thorough documentation important for composite modules?

It clarifies what the module does, its inputs, and outputs, making it easier for others (or your future self) to use effectively.

Learning Resources

Terraform Modules: The Official Guide(documentation)

The official HashiCorp documentation on Terraform modules, covering their structure, usage, and best practices.

Terraform Module Best Practices(documentation)

Learn about developing reusable Terraform modules, including advice on composition and structure.

Terraform Registry: Find and Use Modules(documentation)

Explore the Terraform Registry to discover and utilize a vast collection of community and official modules, which can serve as building blocks for composite modules.

Creating Reusable Terraform Modules(video)

A video tutorial demonstrating how to create and structure reusable Terraform modules, including concepts applicable to composite modules.

Terraform Composite Modules: A Practical Approach(blog)

A blog post detailing a practical approach to building composite modules in Terraform, offering insights into design patterns.

Advanced Terraform Module Patterns(blog)

This article discusses advanced patterns for Terraform modules, including composition strategies for building complex infrastructure.

Terraform Module Composition Example: Web Server(documentation)

A practical example from HashiCorp's learning resources showcasing a composite module for a web server setup.

Terraform Module Inputs and Outputs(documentation)

Understand how to define and manage input variables and output values, which are critical for composing modules effectively.

Terraform Module Sources(documentation)

Learn about the different sources from which Terraform modules can be loaded, essential for referencing modules within a composite module.

Structuring Terraform Projects(video)

A video that covers best practices for structuring Terraform projects, including advice on module organization and composition.