Mastering Terraform Module Input Variables and Output Values

Terraform modules are the building blocks of reusable infrastructure code. Effectively managing how data flows into and out of these modules is crucial for creating flexible, maintainable, and scalable infrastructure. This module delves into the essential concepts of module input variables and output values, empowering you to design robust Terraform configurations.

Understanding Module Input Variables

Input variables allow you to parameterize your modules, making them adaptable to different environments and requirements. Instead of hardcoding values, you define variables that can be set when the module is called. This promotes reusability and reduces duplication.

Input variables make Terraform modules configurable and reusable.

Input variables act as placeholders within a module. When you use a module, you provide values for these variables, customizing the module's behavior without altering its source code. This is fundamental for creating generic modules that can be applied across various projects or environments.

In Terraform, input variables are declared within a module using the variable block. Each variable can have a type (e.g., string, number, bool, list, map, object), a description, and optionally a default value. If a default value is provided, the variable becomes optional. When calling a module, you pass values to these variables using the module block's argument syntax. This allows for dynamic configuration, enabling a single module to provision resources with different settings, such as varying instance sizes, region names, or security group rules.

What is the primary benefit of using input variables in Terraform modules?

Input variables make Terraform modules configurable and reusable by allowing users to parameterize them.

Defining and Using Input Variables

Let's explore how to define and utilize input variables within a Terraform module.

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In the diagram above, the module source defines variables using

code

variable

blocks. These variables can include type constraints, descriptive text, and default values. When another configuration calls this module, it uses a

code

module

block to specify the source and then passes values to the defined input variables.

Understanding Module Output Values

Output values expose information about the resources created by a module. This allows calling configurations to access important attributes, such as IP addresses, ARNs, or resource IDs, for use in subsequent resources or for informational purposes.

Output values provide access to module-created resource attributes.

Output values are defined within a module using the output block. They allow the module to communicate essential information back to the calling configuration. This is vital for chaining resources together or for providing users with key details about the deployed infrastructure.

Within a module, you define output values using the output block. Each output block specifies a name, a value (which is typically an attribute of a resource managed by the module), and an optional description. When a module is called, its outputs can be accessed using the module.<module_name>.<output_name> syntax. This enables the calling configuration to reference these outputs as inputs to other resources, or to display them after terraform apply.

How does a calling configuration access information from a Terraform module's outputs?

By using the syntax module.<module_name>.<output_name>.

Defining and Using Output Values

Here's how output values are defined and consumed.

Consider a module that provisions an AWS EC2 instance. We define an output for the instance's public IP address. The output block specifies the value as aws_instance.example.public_ip. In the calling configuration, we can then reference this output, for example, to configure a DNS record or to display it to the user. This demonstrates the flow of information from the module's internal state to the external configuration.

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Best Practices for Variables and Outputs

Adhering to best practices ensures your modules are robust and easy to manage.

Aspect	Input Variables	Output Values
Purpose	Parameterize module behavior	Expose module-created resource attributes
Declaration	`variable` block	`output` block
Usage in Module	Accessed directly by resource configurations	Defined using resource attributes
Usage in Calling Config	Passed as arguments in `module` block	Accessed via `module.<module_name>.<output_name>`
Default Values	Optional, makes variable optional	Not applicable
Description	Highly recommended for clarity	Highly recommended for clarity

Always provide clear and concise descriptions for both input variables and output values. This significantly improves the usability and understanding of your modules for yourself and others.

Advanced Concepts and Considerations

Explore more nuanced aspects of variables and outputs for advanced module development.

Variable validation rules can be added to ensure that provided input values meet specific criteria, preventing errors before Terraform even plans. For outputs, consider sensitive values using the

code

sensitive = true

argument, which prevents Terraform from displaying their values in the output, enhancing security.

Learning Resources

Terraform Input Variables Documentation(documentation)

The official HashiCorp documentation on input variables, covering their definition, types, and usage.

Terraform Output Values Documentation(documentation)

Official documentation detailing how to define and use output values to expose information from Terraform configurations.

Terraform Modules: Input Variables(documentation)

Specific documentation on how input variables are used within the context of Terraform modules.

Terraform Modules: Output Values(documentation)

Official guide on how output values function when working with Terraform modules.

Terraform Variable Validation Rules(documentation)

Learn how to add validation rules to input variables to ensure data integrity.

Terraform Sensitive Values(documentation)

Understand how to mark output values as sensitive to protect confidential information.

Terraform Modules: Best Practices(blog)

A blog post from HashiCorp outlining best practices for creating and using Terraform modules, including variable and output management.

Creating Reusable Terraform Modules(blog)

An article discussing the principles of module development, with a focus on effective variable and output design.

Terraform Module Development Tutorial(video)

A video tutorial demonstrating the creation and usage of Terraform modules, including input and output configuration.

Terraform Module Patterns: Input Variables and Outputs(blog)

A blog post from Spacelift exploring advanced patterns for input variables and output values in Terraform modules.