ForceNew Route Table ID In AWS Route Table Association: Discussion

Hey everyone! Today, we're diving deep into an interesting discussion surrounding the aws_route_table_association resource within the Terraform AWS provider. Specifically, we're looking at a proposal to mark the route_table_id attribute as ForceNew. What does this mean? Why is it important? Let's break it down and explore the implications.

Understanding the Current Behavior

Currently, the aws_route_table_association resource exhibits some non-standard update behavior. This non-standard update behavior effectively allows the id of the resource to change mid-apply. For those not deeply entrenched in Terraform, this might sound like technical jargon, but it has real-world implications. Imagine you're managing your AWS infrastructure as code, relying on Terraform to keep everything consistent. Now, picture a scenario where you change the route table associated with a subnet. Ideally, you'd expect Terraform to handle this gracefully, updating the association without causing any surprises. However, the current behavior can lead to the resource's id changing unexpectedly, which can create headaches when you're trying to track and manage your infrastructure.

When implementing resource identity, we've had to mark the identity as mutable in order to work around this unique behavior. In essence, we've had to tell Terraform, "Hey, this resource's id might change, so be prepared for it." This isn't ideal, as it adds complexity and can potentially lead to unexpected behavior. To truly understand why this happens, we need to delve a bit deeper into how AWS route table associations work and how Terraform interacts with them. When you associate a route table with a subnet in AWS, an association object is created. This object has its own unique ID. If you then change the route table associated with that subnet, AWS creates a new association object, hence the change in id. Terraform, in its current implementation, reflects this behavior.

The Proposal: Marking route_table_id as ForceNew

So, what's the proposed solution? The idea is to mark the route_table_id attribute as ForceNew in a future major version of the Terraform AWS provider. What does ForceNew mean? In Terraform terms, it essentially tells Terraform that any change to this attribute should trigger a destroy-then-create operation. In other words, if you change the route_table_id, Terraform will first destroy the existing aws_route_table_association resource and then create a brand new one with the new route_table_id.

This ForceNew approach offers a more predictable and consistent way to manage route table associations. Instead of the id changing mid-apply, you'll know that changing the route_table_id will always result in a replacement. This aligns better with how many other Terraform resources behave and simplifies reasoning about your infrastructure code. Think of it like this: instead of trying to swap out the tires on a moving car (the current behavior), you're stopping the car, changing the tires, and then starting again (the ForceNew behavior). It's a bit more disruptive in the short term, but it's much safer and easier to manage in the long run.

Benefits of ForceNew

• Predictability: Changes to route_table_id will always result in a destroy-then-create, making the behavior consistent and easier to understand. You'll always know what to expect when you make this type of change.
• Resource Identity: The id of the aws_route_table_association resource will remain stable as long as the route_table_id doesn't change. This simplifies resource tracking and management.
• Reduced Complexity: By aligning with the standard Terraform behavior, we can reduce the need for workarounds and special handling in our code.

Potential Drawbacks of ForceNew

• Disruption: Destroying and recreating the association can briefly disrupt network traffic if not handled carefully. It's crucial to consider the impact on your applications and services during this transition.
• Increased Apply Time: Destroy-then-create operations generally take longer than in-place updates, potentially increasing your Terraform apply times. However, this is often a worthwhile trade-off for the added stability and predictability.

Impact on Users

For existing users of the aws_route_table_association resource, this change would be a breaking change. This means that upgrading to a version of the Terraform AWS provider with this change would require careful planning and potentially some code modifications. Terraform would detect the change and propose to destroy the existing resource before creating the new resource, so users would need to acknowledge and approve this action.

It's important to note that this change would likely be introduced in a major version of the Terraform AWS provider, giving users ample time to prepare and adjust their configurations. Provider teams typically provide clear upgrade paths and documentation to help users navigate breaking changes. To minimize disruption, it's best practice to stage these types of changes in a non-production environment before applying them to production.

Alternatives Considered

Before proposing ForceNew, other alternatives were likely considered. These might have included attempting to handle the route table swap in-place or introducing a new resource attribute to control the behavior. However, the ForceNew approach was ultimately deemed the most straightforward and aligned with Terraform's design principles. In-place updates can be complex to implement and can lead to unexpected side effects. Adding a new attribute might introduce confusion and not fully address the underlying issue.

Affected Resource(s) or Data Source(s)

As the discussion clearly states, the primary resource affected by this change is:

• aws_route_table_association

This is the resource that manages the association between a route table and a subnet in AWS. Any Terraform configurations that use this resource would be impacted by the proposed change.

Related Issues and Discussions

The original discussion references a related issue: #43843. Following these references provides valuable context and can help you understand the broader discussion surrounding this topic. Often, these linked issues contain detailed explanations, alternative proposals, and user feedback. Engaging with these discussions is a great way to stay informed and contribute to the evolution of the Terraform AWS provider.

Conclusion: A Step Towards Predictable Infrastructure

Marking route_table_id as ForceNew in the aws_route_table_association resource represents a significant step towards more predictable and manageable infrastructure. While it introduces a breaking change, the benefits of ForceNew, such as improved resource identity and reduced complexity, outweigh the drawbacks. By adopting this change, we can ensure that Terraform behaves in a consistent and intuitive manner, making it easier to reason about and manage our AWS environments. What are your thoughts on this proposal? Let us know in the comments below!