Resource Stack Migration | AWS CFT

The process you described for migrating resources between CloudFormation stacks is generally correct, but there have been some changes and best practices that might help resolve the "resource already exists" error you're encountering. Here's an updated approach to migrate resources between stacks using CloudFormation templates:

1. In the source stack (Stack 1):

   • Add a DeletionPolicy of Retain to the resource you want to migrate. This ensures the resource isn't deleted when removed from the stack.
   • Update Stack 1 to apply this change.

2. Remove the resource, along with its related parameters and outputs, from the source template (Stack 1).

   • Update Stack 1 again. This will remove the resource from the stack but not delete it from AWS.

3. In the target stack (Stack 2):

   • Add the resource to the template, including all necessary properties and configurations.
   • Instead of a standard create operation, you need to use the resource import feature.
   • In the template, add the DeletionPolicy attribute and set it to Retain for the imported resource.

4. Create an import operation for Stack 2:

   • Use the AWS Management Console or AWS CLI to create a change set for import.
   • Identify the resource you're importing by its type and unique identifier (like ARN or name).
   • Review and execute the change set to import the resource into Stack 2.

The key difference in this updated approach is the use of the resource import feature, which allows you to bring existing resources into a CloudFormation stack. This should help avoid the "resource already exists" error you're encountering.

Remember that not all resources support import operations, so make sure the resource you're trying to migrate is supported for import. Also, ensure that the resource configurations in your template match the actual resource settings to avoid drift issues.

If you continue to face issues, you may need to check for any dependencies or references to the migrated resource in other parts of your templates or stacks, as these could potentially cause conflicts during the migration process.

Sources
Moving resources between stacks - AWS CloudFormation
Creating a stack from existing resources - AWS CloudFormation
Update your stack template - AWS CloudFormation

Additional Tips Check Resource Properties: Ensure that all properties match between the resource as currently configured and the CloudFormation template to avoid validation errors.

Limitations: Not all resources support import operations. If your resource type is unsupported, consider alternative migration methods or consult AWS support.

Testing: Before migrating critical resources, consider testing the process on a similar resource in a staging environment to ensure compatibility.

This approach leverages the import functionality, which is a newer CloudFormation feature and works well for resources that cannot be re-created without downtime