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Terraform

Access Terraform Module State in Remote State File?

By Ondřej DolanskĆ½ on 01/05/2025

Learn how to access the state of individual Terraform modules within a remote state file and streamline your infrastructure management.

Access Terraform Module State in Remote State File?

Table of Contents

Introduction

In Terraform, accessing values from another module's state file is exclusively achieved using the terraform_remote_state data source. This data source allows you to retrieve output values defined within the remote state. As demonstrated in the provided code snippet, you can fetch the subnet_id output from the network module's state file. However, it's crucial to understand that terraform_remote_state establishes an implicit dependency on the remote state's output values.

Step-by-Step Guide

You can only access the output values of a module using terraform_remote_state.

data "terraform_remote_state" "network" {
  backend = "local"
  config = {
    path = "../network/terraform.tfstate"
  }
}

resource "aws_instance" "app_server" {
  ami           = "ami-0c94855ba95c574c9"
  instance_type = "t2.micro"
  subnet_id     = data.terraform_remote_state.network.outputs.subnet_id
}

It's important to note that terraform_remote_state creates an implicit dependency. This means that changes to the output values in the remote state could break your current configuration.

If you need to access resources that are not defined as outputs in the remote state, you need to modify the module that manages those resources and define the necessary outputs.

Alternatively, you can refactor your code to avoid using remote state for accessing resources directly. This can involve restructuring your modules or using alternative approaches like shared modules or data sources for specific use cases.

Code Example

This code demonstrates using terraform_remote_state to access outputs from a separate Terraform module's state file. The first module provisions network resources and outputs the subnet ID. The second module accesses this output to launch an EC2 instance in the correct subnet. However, it highlights a limitation: you can only access output values, not resources directly. Attempting to reference resources like the VPC ID from the remote state results in an error. Solutions include defining necessary outputs in the first module or refactoring code to avoid direct resource access, such as using data sources to fetch information based on tags. Over-reliance on remote state can lead to tight coupling and complexity, so careful consideration is advised.

This example demonstrates how to access output values from a remote state file using terraform_remote_state and highlights the limitations of this approach.

network/main.tf (Module to manage network resources):

resource "aws_vpc" "example" {
  cidr_block = "10.0.0.0/16"

  tags = {
    Name = "Example VPC"
  }
}

resource "aws_subnet" "example" {
  vpc_id     = aws_vpc.example.id
  cidr_block = "10.0.1.0/24"

  tags = {
    Name = "Example Subnet"
  }
}

# Output the subnet ID
output "subnet_id" {
  value = aws_subnet.example.id
}

app/main.tf (Module to deploy an EC2 instance using the remote state):

data "terraform_remote_state" "network" {
  backend = "local"
  config = {
    path = "../network/terraform.tfstate"
  }
}

resource "aws_instance" "app_server" {
  ami           = "ami-0c94855ba95c574c9"
  instance_type = "t2.micro"
  subnet_id     = data.terraform_remote_state.network.outputs.subnet_id
}

# This will cause an error because aws_vpc.example is not an output
# resource "aws_security_group_rule" "allow_ssh" {
#   type              = "ingress"
#   from_port        = 22
#   to_port          = 22
#   protocol         = "tcp"
#   cidr_blocks      = ["0.0.0.0/0"]
#   security_group_id = data.terraform_remote_state.network.aws_vpc.example.id
# }

Explanation:

  1. network/main.tf: This module defines a VPC and a subnet. It also defines an output variable subnet_id which exposes the subnet ID.
  2. app/main.tf: This module uses terraform_remote_state to access the state file generated by the network module. It retrieves the subnet_id output value and uses it to provision an EC2 instance in the correct subnet.
  3. Limitations: The commented-out code demonstrates that you can only access output values from the remote state. Attempting to access resources directly (e.g., data.terraform_remote_state.network.aws_vpc.example.id) will result in an error.

Solutions:

  • Define necessary outputs: Modify the network module to include an output for the VPC ID if you need to access it in other modules.
  • Refactor code: Consider restructuring your modules or using alternative approaches like shared modules or data sources to avoid directly accessing resources from remote states. For example, you could use a data source to fetch the VPC information based on tags or other criteria.

Remember that relying heavily on remote state can lead to tight coupling between modules and make your infrastructure harder to manage. Carefully consider the trade-offs before using this approach.

Additional Notes

  • Implicit Dependencies: Using terraform_remote_state creates an implicit dependency between your configurations. This means Terraform will try to ensure that the remote state is updated before applying changes to the configuration that depends on it. While this enforces a relationship, it can lead to unexpected issues if the remote state changes unexpectedly.
  • Limited Access: You can only access output values defined within the remote state. Direct access to resources within the remote state is not possible. This emphasizes the importance of carefully planning and defining outputs in modules intended for remote state sharing.
  • Tight Coupling: Over-reliance on terraform_remote_state can lead to tight coupling between modules, making your infrastructure less modular and harder to refactor. Changes in one module's outputs could potentially break other modules that depend on them.
  • Alternatives to Consider:
    • Shared Modules: For common infrastructure components, consider using shared modules stored in a central repository. This promotes reusability and consistency across your projects.
    • Data Sources: Instead of directly accessing resources from remote state, utilize data sources to fetch information based on tags, names, or other criteria. This decouples your configurations and makes them more resilient to changes.
  • Best Practices:
    • Minimize Remote State Usage: Only use terraform_remote_state when absolutely necessary. Strive for modularity and loose coupling between your configurations.
    • Clearly Define Outputs: Ensure that modules intended for remote state sharing have well-defined and documented outputs.
    • Version Your Modules: Version control your Terraform modules to track changes and prevent unexpected breakages due to remote state modifications.

By understanding the implications and limitations of terraform_remote_state, you can make informed decisions about its usage and design more robust and maintainable infrastructure.

Summary

This article explains how to access resources managed by a different Terraform module using terraform_remote_state.

Key Points:

  • Accessing Outputs: You can only access the output values defined in the remote state file using data.terraform_remote_state.RESOURCE_NAME.outputs.OUTPUT_NAME.
  • Implicit Dependency: Using terraform_remote_state creates an implicit dependency on the remote state. Changes to the remote state's outputs can impact your current configuration.
  • Accessing Non-Output Resources: To access resources not defined as outputs in the remote state, you need to:
    • Modify the Remote Module: Add the necessary outputs to the module managing those resources.
    • Refactor Your Code: Consider restructuring your modules or using alternative approaches like shared modules or data sources.

Example:

The provided code snippet demonstrates accessing the subnet_id output from a remote state named "network" and using it to provision an EC2 instance.

Recommendation:

While terraform_remote_state can be useful, be mindful of the implicit dependency it creates. Consider refactoring your code to minimize dependencies and improve maintainability when possible.

Conclusion

In conclusion, terraform_remote_state provides a mechanism to access output values from another Terraform module's state. While useful for sharing information between modules, it's crucial to understand its limitations. You can only access output values, not resources directly, and it creates an implicit dependency that can lead to tight coupling. Over-reliance on terraform_remote_state can impact modularity and maintainability. Consider alternative approaches like shared modules or data sources to minimize dependencies and enhance your infrastructure's robustness. Remember to carefully define outputs in modules intended for remote state sharing and version your modules to track changes effectively. By understanding these aspects, you can leverage terraform_remote_state appropriately and build more manageable and scalable infrastructure.

References

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