I recently was setting up a couple of AWS environments for a client. This client had a typical web application which talked to an RDS database. There was DNS, a CDN and other components involved. We wanted to use Terraform to maintain traceability and replicability, and have the same configuration for production and staging, with perhaps small differences like ec2 instance size. We also wanted to separate out the components into their own Terraform workspaces to limit the blast radius (so if one component had changes that caused issues or Terraform corruption, it wouldn’t affect others). Finally, we wanted each environment to have its own Terraform backend, again to separate the environments.

I wasn’t able to complete this project due to external factors (I left the position before testing could be completed), but wanted to share the concepts. Obviously I can’t share the working code, but I set up an example project which is simpler. That’s the project I’ll be examining in this post. I also want to be clear that while I’ve tested this as much as I could and have validated the ideas with others who have more Terraform experience, this hasn’t been run in production. You have been warned. (Here’s the Terraform docs about setting up modules, workspaces and repositories.)

Using a tool like Terraform is great for a number of reasons, but my favorite is that it lets you track changes to cloud infrastructure. More than once I’ve wandered into an AWS account and wondered why certain resources were set up in the way they were, and what might break if I changed them. There are occasionally comments, but it is far better to examine a commit. Even better to review the set of commits and see the customer request or bug tied to it. (Bonus link: learn more about Terraform and other cloudy tools in this podcast episode with the creator of Terraform.)

So this simpler example project has a lambda that writes to an SQS queue. For now, it just writes the date of invocation, but obviously you could have it reach out to an external API, read from a database, or do some kind of calculation. The SQS queue could then be read from by an EC2 instance, which processes the message and perhaps updates a database. You have three components of the system:

• The lambda function
• The SQS queue
• The EC2 instance (implementation of which is left as an exercise for the reader)

The SQS queue is shared infrastructure and needs to be accessed by both of the other systems. However, the SQS system doesn’t need to know about either the lambda or the EC2 instance. Using Terraform, we can create each of these components as their own workspace. Each of the subsidiary systems can evolve or change (for instance, the EC2 instance could be replaced with an autoscaling group) with minimal impact on other systems. They could be managed by different teams as well if that made sense.

To enforce this separation, set up each component as a separate Terraform workspace. (All code is on github here.) I use remote state so that more than one person can manage the terraform state, and use the S3/dynamodb backend because we are targetting AWS and want a free scalable solution. This post assumes you know how to set up Terraform using s3/dynamodb as a remote state storage.

Here’s the outputs of the SQS system:

output "queue_url" {
  value = "${aws_sqs_queue.myqueue.id}"
}

output "queue_arn" {
  value = "${aws_sqs_queue.myqueue.arn}"
}

I explicitly define the output variables so I can pull them in from the lambda and EC2 workspaces. This is how you can do that.

...
data "terraform_remote_state" "sqs" {
  backend = "s3"
  config = {
    bucket = "${var.terraform_bucket}"
    key = "sqs/terraform.tfstate"
    encrypt = true
    dynamodb_table = "terraform-remote-state-locks"
    profile = "${var.aws_profile}"
    region = "us-east-2"
  }
}
...
resource "aws_lambda_function" "mylambda" {
...
  environment {
    variables = {
      sqs_url = "${data.terraform_remote_state.sqs.outputs.queue_url}"
    }
  }
}

The terraform_remote_state block defines the location of the previously defined sqs workspace, and the ${data.terraform_remote_state.sqs.outputs.queue_url} references that url. That is then injected as an environment variable into the lambda, which reads it and uses the url to create an SQS client. It can then post whatever message it wants.

You can see how this would work with any number of configuration parameters. If you have typical three tier database driven application with a separate caching layer you can create each of these major components and inject the values into either the environment (for lambda) or the userdata (for EC2). I’m not sure I’d use this with a microservices architecture because using a services registry might be more appropriate.

Note that the lambda component has a rudimentary lambda function (you have to define something). It also uses Terraform to deploy the lambda code. That’s fine for the toy example, but for production you will want to use a real CI/CD system to deploy your lambdas.

Now, suppose you want to run production and staging environments, because you are ready to launch. Here are the constraints you’d want:

• Production and staging run the same config (except when staging is changing, of course)
• Production and staging may differ in a few details (the size of the EC2 instance, for example)
• Production and staging execute in different AWS accounts to limit access and issues (charity link). You don’t want an error in staging to affect production. This is handled by creating different profiles which have access to different accounts.
• Production and staging execute in different Terraform backends for the same reason as the separate AWS accounts.

Staging and production can use the same git repository, but when pulled down they are kept in two places on the filesystem. This is because you need to specify the profile and the bucket when using terraform init. So you end up running something like these two commands:

git clone git@github.com:mooreds/terraform-remote-state-example.git # staging
git clone git@github.com:mooreds/terraform-remote-state-example.git production-terraform-remote-state-example # production

I set up the project so that staging can be managed by normal terraform commands (since that will happen more often), and that production uses either special incantations or a script. For the initialization of the production Terraform environment, this looks like: terraform init -backend-config="profile=trsproduction" -backend-config="bucket=". For staging, it’s just terraform init. I didn’t have a lot of luck switching between these two Terraform backends in the same filesystem locations, so that having two trees was a straightforward workaround.

Any changes between production and staging are each pulled out to a variable, with the staging value as the default. Then each workspace has a script which applies the Terraform configuration to the production environment. The script sets variables to be the correct value for production. Here’s an example for the lambda workspace:

terraform apply -var aws_profile=trsproduction -var terraform_bucket="mooreds-terraform-remote-state-example-production" -var env_indicator="production" -var lambda_memory_size=256

We pass in the production terraform_bucket in case any references need to be made to the remote state (to pull in the SQS queue url, for example). We also pass in an increased lambda memory size because, hey, it’s production. Other things that might vary between environments: for example, VPC or subnet ids, API endpoints, and S3 bucket names.

For simplicity, we just use two profiles for staging and production (in ~/.aws/credentials), but any way of getting credentials that works with Terraform will work:

[trsstaging]
aws_access_key_id = ...
aws_secret_access_key = ...

[trsproduction]
aws_access_key_id = ...
aws_secret_access_key = ...

This lets us separate out who has production access. Some users can have both staging and production profiles (perhaps operations), and others can have only staging profiles (perhaps developers). You can pass region values in via variables as well.

Using this system, the workflow for a change would be:

• Check out the terraform git repository
• Create a feature branch (including an issue identifier)
• Pull request and approval
• Run terraform apply to apply to staging
• Run any additional tests
• Merge to master
• Run prodapply.sh

Again, I want to be clear that I’ve implemented this partially, but I didn’t get a chance to run this fully in production. I tested all these concepts with the simple system mentioned above (and you can stand up your own using the code on github). There will be issues that I haven’t experienced. But I hope that this post helps illuminate the complexity of managing multiple workspaces and environments within a single Terraform github repository.