Terraform data sources provide information on existing resources. This is handy for configuration dependencies that exist across Terraform plans.

In my scenario, I wanted to attach an Amazon Web Services (AWS) Virtual Private Cloud (VPC) to an existing Transit Gateway. I needed to use a data source to pull the Transit Gateway’s id value. Using the assigned AWS tags seemed like a viable solution.

Below is an example of my configuration in AWS:

I could not find a clear description of how to filter a data source by the AWS tag key/value pairs. However, the aws_instance data source provided me a clue.

The key/value pair can be provided using the syntax below:

• Key: The name parameter uses the syntax tag:<key> to provide the AWS key name.
• Value: The values parameter provides the AWS key value.

The full working Terraform code snippet is shown below:

data "aws_ec2_transit_gateway" "tgw" {
  filter {
    name   = "tag:Name"
    values = ["wahlnetwork-tgw-prod"]
  }
}

Now that I had the Transit Gateway’s id value, I was able to supply it to a resource object further along in the plan.

resource "aws_ec2_transit_gateway_vpc_attachment" "vpc-tgw-attachment" {
  subnet_ids          = [aws_subnet.priv01.id, aws_subnet.priv02.id]
  transit_gateway_id  = data.aws_ec2_transit_gateway.tgw.id
  vpc_id              = aws_vpc.vpc.id
  }
}

At least now I have written down the filter syntax for an AWS tag for the next time I need it!

Next Steps

Please accept a crisp high five for reaching this point in the post!

If you’d like to learn more about Infrastructure as Code, or other modern technology approaches, head over to the Guided Learning page.

If there’s anything I missed, please reach out to me on Twitter. Cheers! 🙂