Migrating Interdependent Terraform Configurations

This guide addresses the specific challenges of migrating systems built from multiple interdependent Terraform configurations that share data using the terraform_remote_state data source.

If you're migrating a single, standalone Terraform configuration, refer to the Migration Guide instead.

When to use this guide

Use this guide if you meet any of the following criteria:

- **Data sharing between root modules** using `terraform_remote_state`
- **Dependencies between root modules** (one root module consumes outputs from the other)

Understanding terraform_remote_state

What is terraform_remote_state?

The terraform_remote_state data source allows one configuration to read outputs from another configuration's state file. This creates a dependency relationship between configurations.

For more details, see the terraform_remote_state documentation

Example:

# Configuration A: Creates a VPC
output "vpc_id" {
  value = aws_vpc.main.id
}

# Configuration B: Consumes the VPC ID
data "terraform_remote_state" "vpc" {
  backend = "s3"
  config = {
    bucket = "my-terraform-state"
    key    = "vpc/terraform.tfstate"
  }
}

resource "aws_subnet" "main" {
  vpc_id = data.terraform_remote_state.vpc.outputs.vpc_id
  //...
}

In this example, Configuration B depends on Configuration A. Configuration B reads A's state file to get the VPC ID.

Why this matters for migration

When migrating to OpenTofu, the order in which you migrate configurations is critical because:

1. Dependent configurations must read state files written by their dependencies
2. State file compatibility between Terraform and OpenTofu determines which migration orders are safe
3. Mixed environments (some configs on Terraform, others on OpenTofu) may exist during migration

Understanding your dependency graph

Before migrating a config, map out your configuration dependencies.

Simple dependency example

Configuration A (VPC)
        ↑
        |
Configuration B (Subnets)

Configuration B depends on A (B reads A's state).

Complex dependency example

        A
        ↑
        |
   +----+----+
   |    |    |
   B    C    D
   ↑    ↑    ↑
   |    +----+
   |         |
   |         E
   |         ↑
   |         |
   F         |
   ↑         |
   +----+----+
        |
        G

In this graph:

• G depends on F and E (reads their state)
• E depends on C and D (reads their state)
• F depends on B (reads its state)
• B, C, and D depend on A (read its state)

To create your dependency graph:

1. List all your Terraform configurations
2. Identify which configurations use terraform_remote_state
3. Note which state files each configuration reads
4. Draw the dependency relationships

Migration order: Bottom-up approach

Critical principle: Migrate dependent configurations before their dependencies.

Why bottom-up?

OpenTofu maintains state file compatibility with Terraform 1.x, which means:

• OpenTofu can read Terraform state files

• Terraform may not reliably read OpenTofu state files (especially if OpenTofu-specific features are used)

This compatibility pattern makes bottom-up migration safe:

• Configurations migrated to OpenTofu can safely use terraform_remote_state on state from configurations that are still on Terraform
• You can migrate gradually without breaking the dependency chain
• Mixed Terraform/OpenTofu environments work during the transition

Migration order example

For the complex dependency graph above, the safe migration order is:

Phase 1: Migrate the leaf node (no dependents)

G → (migrate first)

Phase 2: Migrate G's direct dependencies

F, E → (migrate after G)

Phase 3: Continue up the tree

B, C, D → (migrate after F and E)

Phase 4: Migrate the root

A → (migrate last)

Why top-down is risky

If you migrate A first (top-down):

A (OpenTofu) → B, C, D (Terraform)

Configurations B, C, and D (still on Terraform) would try to read state written by A (now on OpenTofu). This could cause issues if:

• OpenTofu writes state in a format Terraform doesn't recognize
• OpenTofu uses features not present in Terraform
• Compatibility breaks in future versions

Bottom-up migration avoids this risk entirely.

Step-by-step migration process

Step 1: Map your dependencies

Create a visual representation of your configuration dependencies using terraform_remote_state data sources by:

• Searching your codebase for terraform_remote_state
• Document which configurations read from which backends/state files
• Draw the dependency graph

Step 2: Identify leaf nodes

Find configurations that:

• Have no other configurations depending on them
• May themselves depend on other configurations

These are your starting points for migration.

Step 3: Back up everything

Before migrating any configuration:

1. Back up all state files (for all configurations, not just the one you're migrating)
2. Commit all configuration files to version control
3. Document the current state of each configuration (Terraform version, provider versions)

Step 4: Migrate leaf configurations first

For each leaf configuration:

1. Follow the standard Migration Guide
2. Verify the migration with tofu plan
3. Apply with tofu apply
4. Test the migration: For any non leaf configuration, ensure that dependent configuration(s) can still read this configuration's state

Step 5: Verify dependencies still work

After migrating a configuration, verify that:

• Any configurations that depend on it still work correctly by running terraform plan (or tofu plan if already migrated) on those dependent configs
• Confirm no unexpected changes appear

Step 6: Continue up the dependency tree

Repeat steps 4-5 for each level of your dependency graph, moving from leaf nodes toward root nodes.

Step 7: Migrate root configurations last

Once all dependent configurations are migrated, migrate the root configurations (those that have no dependencies themselves but are depended upon by others).

State compatibility notes

What OpenTofu guarantees

• OpenTofu reads Terraform 1.x state files without modification
• State format is compatible during migration
• Mixed environments work (some configs on Terraform, others on OpenTofu)

Important considerations

• Avoid OpenTofu-specific features during migration to maintain maximum compatibility
• Test thoroughly after each configuration migration
• Keep backups of all state files throughout the migration process
• Document which configurations are on Terraform vs. OpenTofu during the transition

Rolling back

If you need to roll back a specific configuration:

1. Stop using OpenTofu for that configuration
2. Restore from backups if state changes were made
3. Run terraform init and terraform plan
4. Verify no unexpected changes
5. Continue with Terraform for that configuration

The bottom-up approach makes rollback safer because:

• Only leaf nodes are migrated early (fewer dependents affected)
• Root configurations migrate last (after dependents are stable)

Handling version differences

Your configurations may be using different Terraform versions (1.6.x, 1.7.x, 1.8.x).

Recommended approach: Before migrating to OpenTofu, upgrade all configurations to the same Terraform version. This narrows down variables and makes troubleshooting easier if you encounter any issues during migration.

Getting Help

If you encounter issues during migration:

• Join the <a href="/slack/">OpenTofu Slack</a>
• Ask on GitHub Discussions
• Report bugs on GitHub Issues