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Terraform

Terraform Modules: Output from for_each Loops

By Filip on 10/06/2024

Learn how to effectively use Terraform modules with for_each loops and output variables for scalable and dynamic infrastructure management.

Terraform Modules: Output from for_each Loops

Table of Contents

Introduction

When working with Terraform, using modules with for_each to create multiple resources requires an understanding of how to access their outputs, which often involves working with maps and lists of maps. This guide will explain how to dynamically access outputs from module instances created using for_each. We will cover a practical example using a module that creates an AWS EC2 instance and outputs its ID, demonstrating how to use for_each to create multiple instances and then access their IDs through a structured output. Finally, we will touch upon important considerations regarding data structures and output formatting when working with for_each and module outputs.

Step-by-Step Guide

When using Terraform modules with for_each to create multiple resources and access their outputs, you need to understand how to work with maps and potentially lists of maps. Here's a breakdown:

Understanding the Challenge

  • Modules and Outputs: Terraform modules often use outputs to expose values from the resources they create.
  • for_each and Dynamic Blocks: The for_each meta-argument allows you to create multiple instances of a resource or module based on a map or set of strings.
  • Accessing Outputs Dynamically: The challenge lies in accessing the outputs of these dynamically created module instances.

Solution

  1. Module Structure (Example):

    Let's assume you have a module named "my_module" that creates a resource (e.g., an AWS EC2 instance) and outputs its ID:

    # my_module/main.tf
resource "aws_instance" "example" {
  # ... instance configuration ...
}

output "instance_id" {
  value = aws_instance.example.id
}

  2. Using for_each with the Module:

    In your main Terraform file, you use for_each to create multiple instances of "my_module":

    module "my_instances" {
  source = "./my_module"
  for_each = {
    "instance1" = { type = "t2.micro" }
    "instance2" = { type = "t3.small" }
  }

  # Pass instance type to the module
  instance_type = each.value.type 
}

  3. Accessing Module Outputs:

    The key is that module.my_instances now represents a map of modules, where the keys are your instance names ("instance1", "instance2") and the values are the module instances. You access the outputs like this:

    output "instance_ids" {
  value = {
    for key, instance in module.my_instances : key => instance.outputs.instance_id
  }
}

Explanation

  • module.my_instances: This expression now refers to a map of module instances created by for_each.
  • for key, instance in module.my_instances: This loop iterates over each key-value pair in the module.my_instances map.
    • key: Holds the instance name (e.g., "instance1").
    • instance: Represents the actual module instance and allows you to access its outputs.
  • instance.outputs.instance_id: This retrieves the "instance_id" output from the specific module instance.
  • Result: The output "instance_ids" will be a map where the keys are your instance names and the values are the corresponding instance IDs.

Important Considerations

  • Data Structures: Understand whether your for_each is creating a map or a list of maps. Adjust your output access accordingly.
  • Output Formatting: Use for expressions or other Terraform functions to format the output data structure as needed for other parts of your infrastructure.

By following these steps, you can effectively create multiple resources using modules with for_each and access their outputs in a structured and organized way.

Code Example

The code demonstrates Terraform modules and for_each. A module "my_module" defines an EC2 instance and outputs its ID. The main file uses this module with for_each to create two instances with different types. It then retrieves and outputs the IDs of both instances using a for expression, showcasing dynamic access to module outputs.

This example demonstrates how to use Terraform modules with for_each to create multiple AWS EC2 instances and access their outputs.

1. Module: my_module

This module defines a single EC2 instance and outputs its ID.

# my_module/variables.tf
variable "instance_type" {
  type = string
}

# my_module/main.tf
resource "aws_instance" "example" {
  ami           = "ami-0c55b159c09b05a0a" # Replace with your desired AMI
  instance_type = var.instance_type
  tags = {
    Name = "Created by Terraform Module"
  }
}

output "instance_id" {
  value = aws_instance.example.id
}

2. Main Terraform File

This file uses the my_module with for_each to create two EC2 instances and then accesses their IDs.

# main.tf
module "my_instances" {
  source = "./my_module"
  for_each = {
    "instance1" = { type = "t2.micro" }
    "instance2" = { type = "t3.small" }
  }

  instance_type = each.value.type
}

output "instance_ids" {
  value = {
    for key, instance in module.my_instances : key => instance.outputs.instance_id
  }
}

Explanation:

  • module "my_instances": This block uses the my_module and creates two instances using for_each.
  • for_each: This argument takes a map where keys are instance names and values are objects containing instance types.
  • instance_type = each.value.type: This line passes the instance type from the for_each map to the module variable.
  • output "instance_ids": This output uses a for expression to iterate over the module.my_instances map.
  • key => instance.outputs.instance_id: This part creates a new map entry with the instance name as the key and the corresponding instance ID as the value.

Running the Code:

  1. Save the module code in a directory named my_module.
  2. Save the main Terraform code in the same directory as my_module.
  3. Run terraform init to initialize the project.
  4. Run terraform apply to create the resources.

After applying, you'll see an output similar to this:

Outputs:

instance_ids = {
  "instance1" = "i-0a1b2c3d4e5f6g7h8i"
  "instance2" = "i-9j8k7l6m5n4o3p2q1r"
}

This output shows a map where keys are the instance names ("instance1", "instance2") and values are their respective IDs. This demonstrates how to effectively use modules with for_each and access their outputs dynamically.

Additional Notes

  • Real-world applications: This pattern is extremely useful for managing collections of similar resources like creating multiple EC2 instances with varying types, deploying a fleet of load balancers across different availability zones, or setting up identical databases for development, staging, and production environments.
  • Error handling: When using for_each with complex data structures, ensure your map keys are unique to avoid errors. Consider using the try() function to handle potential errors gracefully, especially when accessing nested outputs.
  • Alternative approaches: While for_each is powerful, consider using count for simpler scenarios where you need to create a fixed number of instances.
  • Refactoring for clarity: For very complex scenarios, break down your code into smaller, reusable modules to improve readability and maintainability.
  • Terraform state: Remember that each module instance created with for_each will have its own representation in the Terraform state file.
  • Dynamically accessing other outputs: The same principle applies to accessing other outputs from your modules. Just replace instance.outputs.instance_id with the desired output name.
  • Passing complex data structures: You can pass more complex data structures (like lists or maps) to your modules within the for_each map. This allows for greater flexibility in configuring your resources.
  • Keep it organized: Using clear and descriptive naming conventions for your module instances and outputs will make your code easier to understand and maintain.

By understanding these concepts and applying the techniques described, you can leverage the full power of Terraform modules and for_each to manage your infrastructure efficiently and effectively.

Summary

This document summarizes how to access outputs from Terraform modules used with the for_each meta-argument.

Challenge:

When using for_each to create multiple module instances, accessing their individual outputs requires understanding how Terraform represents these instances as a map.

Solution:

  1. Module Definition: Define your module with outputs representing the desired values from created resources.

    # my_module/main.tf
# ... resource creation ...

output "instance_id" {
  value = aws_instance.example.id
}

  2. for_each Usage: In your main file, use for_each to create multiple module instances based on a map.

    module "my_instances" {
  source = "./my_module"
  for_each = {
    "instance1" = { type = "t2.micro" }
    "instance2" = { type = "t3.small" }
  }
  # ... pass values to the module ...
}

  3. Accessing Outputs: Access outputs using a for loop to iterate over the map of module instances.

    output "instance_ids" {
  value = {
    for key, instance in module.my_instances : key => instance.outputs.instance_id
  }
}

Explanation:

  • module.my_instances becomes a map, with keys from your for_each map and values representing individual module instances.
  • The for loop iterates over this map, providing access to each instance's outputs via instance.outputs.

Key Points:

  • Understand the data structure (map or list(map)) created by your for_each usage.
  • Use for expressions or other functions to format output data as needed.

By following these steps, you can effectively manage and access outputs from multiple module instances created with for_each.

Conclusion

By understanding how Terraform uses maps to represent module instances created with for_each, you can effectively access and utilize the outputs of these instances. This involves defining clear outputs in your modules, using for_each to create multiple instances with varying configurations, and then employing for loops or other Terraform functions to access and structure the output data as needed. This approach enables you to write more efficient and reusable Terraform code, especially when dealing with collections of similar resources with varying configurations. Remember to consider data structures, error handling, and output formatting for optimal results. By mastering these techniques, you can significantly enhance your infrastructure management capabilities with Terraform.

References

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