Anyone who has worked with Terraform knows the pain associated with managing Terraform state. Things get even more interesting when factoring in disaster recovery (DR) for Terraform state. If that rings true for you, read on to find a relatively simple approach to DR with Terraform state.
Credit: HashiCorp Terraform
What is Terraform?
Terraform is Infrastructure as Code (IaC) that allows you to [mostly] declaratively define infrastructure. The idea is that infrastructure definitions can be quickly and easily defined, managed as composable units and versioned using common source control tools (Git, Subversion, Perforce, etc). In a nutshell, it allows you to treat infrastructure similarly to how you might treat other source code.
A common use case for Terraform is cloud-based infrastructure. And to that end, Terraform can manage resources in AWS, Azure, Google Cloud and more. Spoiler: the example provided in this post will be in the context of AWS.
Terraform uses something called state (commonly in the form of state files) to manage, well, the state of Terraform configurations. Every Terraform root module has an associated state, which is representative of the infrastructure following its last apply. And when an apply command is issued against a root module that has an existing state attached, Terraform first refreshes its state to reconcile the actual infrastructure state with the root module’s last known state. This allows for configuration drift detection. It also enables Terraform to best determine how to apply configuration changes that were made since the last apply.
The Problem With Multiple Regions
Let’s say you have a situation where you have resources from multiple regions bundled together in a single Terraform root module. Let’s further assume that when the root module is applied, all of the regions associated with those resources are 100% available. In this scenario, the Terraform apply works as expected: the state is updated and all of the resources are provisioned and wired up just as described in the root module configuration.
Now, let’s say there is a partial region outage in one of the regions where resources were targeted in that root module. Even assuming that access still exists to the associated state file, any future Terraform apply executed against that root module would fail. Can you see why?
Above, we said that on apply, Terraform first tries to refresh its state. If there is an outage that affects one or more resources in the root module, a state refresh would not be possible and the configuration along with its associated state would be in a non-working state.
An S3 Example
S3 Replication; Created using Cloudcraft
A common deployment scenario with S3 is to have a primary S3 bucket in one region that replicates its data to another bucket in a different region. This is called S3 cross-region replication. In this scenario, if there was an availability problem with either region, the data would still be available. However, Terraform would not be able apply future root module configuration updates.
If the source bucket’s region was lost, then you might want to spin up a different bucket in an available region and switch the replicated bucket to be your new source bucket. Or, if the destination bucket’s region was lost, then you might want to spin up a new bucket in a different region and change the source bucket to replicate to that new bucket. But… if you provisioned both the source bucket and the destination bucket from the same root module configuration, then you might be in trouble.
Terraform Modules to The Rescue
If the S3 destination bucket were in one root module and the S3 source bucket were in a different root module, then there wouldn’t be a problem since each root module would have its state confined to only one region. But this is, quite frankly, a pain in the ass. Because now you have to do Terraform apply twice (once for each root module) and you have to manually sequence the order in which resources in different regions are provisioned - that’s supposed to be Terraform’s job!
What we can do, however, is break the provisioning of each S3 bucket into its own submodule and have a root module orchestrate the provisioning of both.
S3 Replication Using Submodules; Credit: Cloudcraft
This solves the problem of having to manually sequence two Terraform apply operations to effectively accomplish a single operation, albeit across more than one region. But we still have the problem with state from multiple regions being combined into a single state file… or do we?
To Import or Not to Import?
By breaking down the submodules by region, some level of isolation through abstraction is achieved. It’s not quite enough to solve the resources across multiple regions stored in a single Terraform state file problem. But it feels like we are closer. And indeed, we are.
To finish the solution, it’s necessary to understand that the only major differences between a Terraform submodule and a Terraform root module are that a Terraform root module has state attached to it and Terraform commands can be directly executed against a Terraform root module. Of course, there are other subtle differences. But in most cases, there is nothing stopping us from using a Terraform submodule as a Terraform root module.
Fortunately, Terraform provides a mechanism to do exactly that: Terraform import. Terraform import allows Terraform to bring existing infrastructure under the management of new Terraform state. Using Terraform import, we could initialize one (or both) of the S3 submodules as a root module and then import the existing corresponding S3 bucket, which would then allow the s3 submodule to manage the S3 bucket independently of the other S3 bucket (that resides in a different region).
What About More Robust Configurations?
The problem with import is that it must be executed on a per-resource basis. This is OK for the example above since each module only deals with a single resource. But what about more robust configurations that contain more than one resource per module? That’s where Terraform state mv comes into play.
- Availability outages can cause trouble for Terraform configurations - this must be a consideration when designing Terraform configurations.
- Break down Terraform configuration region dependencies into submodules.
- Orchestrate submodules using a root module.
- During an outage, Terraform import or Terraform state mv can be used to allow submodules to manage the state of their resources.