Migrating from CloudFormation/CDK to Terraform: Lessons Learned

Migrating your infrastructure as code (IaC) from AWS CloudFormation to Terraform can be a significant step towards increasing the agility and flexibility of your cloud resources management. This guide provides a detailed roadmap for teams looking to make this transition, covering key areas such as migration strategy, state import, testing, rollback plans, team training, and managing the transition. We will also share practical scripts and lessons learned from real migrations to help you navigate common challenges.

The first step in your migration strategy should involve a thorough assessment of your existing AWS CloudFormation templates. Identify resources, dependencies, and any custom resources or nested stacks. This assessment will help you understand the scope and complexity of your migration.

- **Incremental vs. Big-Bang**: Decide whether to migrate your infrastructure incrementally (service by service) or all at once. An incremental approach is recommended to reduce risk. - **Resource Prioritization**: Based on dependencies and business priorities, determine the order in which resources will be migrated. - **Timeline and Milestones**: Establish a realistic timeline and set milestones to track progress.

Terraform maintains state about your managed infrastructure and configuration. When migrating from CloudFormation, you will need to import the state of your existing resources into Terraform.

1. **Identify Resources**: List all resources managed by CloudFormation that you intend to manage with Terraform. 2. **Create Terraform Configuration**: Write Terraform code that represents your existing infrastructure. Tools like `cdktf` can help generate Terraform configurations from CloudFormation templates.

For each resource, you will use the Terraform `import` command. The general syntax is:

terraform import [options] ADDRESS ID

- **ADDRESS**: The address of the resource in your Terraform configuration. - **ID**: The unique identifier of the resource in AWS.

Suppose you have an AWS S3 bucket defined in CloudFormation and you've written corresponding Terraform configuration:

resource "aws_s3_bucket" "my_bucket" {
  bucket = "my-bucket-name"
  acl    = "private"
}

To import this S3 bucket into Terraform state, you would run:

terraform import aws_s3_bucket.my_bucket my-bucket-name

While manual import is feasible for a small number of resources, larger migrations may benefit from automation. Scripting the generation of import commands based on your resource inventory can save time. Unfortunately, due to the variability in resource types and identifiers, a universal script is beyond the scope of this guide, but here's a simple example for S3 buckets:

import boto3

s3 = boto3.client('s3')
response = s3.list_buckets()

for bucket in response['Buckets']:
    print(f"terraform import aws_s3_bucket.{bucket['Name']} {bucket['Name']}")

Testing is a critical component of any migration strategy. For migrating from CloudFormation to Terraform, consider the following testing stages:

- **Static Analysis**: Use tools like `terraform validate` and `tflint` to catch syntax and best practices errors. - **Resource Testing**: Employ Terraform modules and resources testing using frameworks like `terratest`.

- **Dry Runs**: Leverage `terraform plan` to understand the changes Terraform intends to make without applying them. - **Partial Applies**: Apply changes to a non-production or isolated environment to validate the migration steps.

- **Validation**: Confirm that the migrated resources function as expected within the application or service context. - **Performance Benchmarking**: Compare the performance of services before and after migration to ensure there are no regressions.

Despite thorough testing, things can go wrong. Having a rollback plan is essential. For each phase of your migration, define a rollback strategy:

- **Backup CloudFormation State**: Ensure you have a backup of your CloudFormation stacks and resources. - **Terraform State Snapshots**: Regularly back up your Terraform state file during the migration process. - **Incremental Migration Rollback**: If a specific service migration fails, you should be able to revert changes for just that service without affecting others.

Migrating to a new IaC tool requires upskilling your team. Key areas include:

- **Terraform Basics**: Ensure everyone understands Terraform's syntax, resource management, and state management. - **Advanced Concepts**: Dive into more complex topics like module creation, state backends, and workspace management. - **Best Practices**: Cover Terraform best practices, including code organization, versioning, and security considerations.

Keep all stakeholders informed throughout the migration process. Regular updates, milestones achievement, and any encountered challenges should be transparently shared.

Update your project's documentation to reflect the new Terraform-based infrastructure. Include guides on running Terraform commands, understanding the new state management, and troubleshooting common issues.

Integrate Terraform into your CI/CD pipelines. Automation not only speeds up the deployment process but also helps maintain consistency and reduces manual errors.

- **Start Small**: Begin with less complex, non-critical infrastructure. This approach helps you learn Terraform's nuances without risking significant systems. - **Embrace Modularization**: Use Terraform modules to encapsulate common patterns. This practice enhances code reusability and maintainability. - **Automate Where Possible**: Invest time in automating repetitive tasks, including state import and resource mapping. It pays off in the long run. - **Prioritize State Management**: Proper Terraform state management is crucial. Consider using remote state backends like AWS S3 with state locking via DynamoDB to ensure consistency. - **Plan for Downtime**: While many resources can be imported into Terraform without downtime, some scenarios might require it. Plan these migrations carefully to minimize impact.

Migrating from AWS CloudFormation to Terraform is a journey that requires careful planning, testing, and execution. By following the steps outlined in this guide, you can ensure a smooth transition that leverages Terraform's capabilities to enhance your cloud infrastructure management. Remember, the key to a successful migration is not just technical execution but also effective communication, training, and change management throughout your organization.