Using Terraform to manage multiple AWS regions

Question

Could someone please give me an example of how to programmatically create Terraform provider aliases based on a variable map? This is what I've tried, but I'm receiving the following error:

variable "aws_regions" {
  default = [
    {
      region = "us-east-1"
      alias  = "default"
    },
    {
      region = "us-east-2"
      alias  = "useast2"
    },
    {
      region = "us-west-1"
      alias  = "uswest1"
    },
    {
      region = "us-west-2"
      alias  = "uswest2"
    },
    {
      region = "eu-central-1"
      alias  = "eucent1"
    }
  ]
}

provider "aws" {
  count  = "${length(var.aws_regions)}"
  region = "${lookup(var.aws_regions[count.index], "region")}"
  alias  = "${lookup(var.aws_regions[count.index], "alias")}"
}

# CloudWatch Log Groups
resource "aws_cloudwatch_log_group" "linux" {
  count    = "${length(var.aws_regions)}"
  provider = "aws.${lookup(var.aws_regions[count.index], "alias")}"

  name = "Linux"
}

Error:

$ terraform plan
* provider.aws.${lookup(var.aws_regions[count.index], "alias")}: count.index: count.index is only valid within resources

Answer 1

It turns out that Terraform provider processing takes place very early and the current version (v.0.11.3) doesn't currently support variable interpolation for providers. I did discover a workaround that isn't too terrible, but it requires a lot of code duplication.

main.tf

# Default Region
provider "aws" {
  region  = "us-east-1"
  version = "~> 1.8"
}

provider "aws" {
  alias  = "us-east-1"
  region = "us-east-1"
}

provider "aws" {
  alias  = "us-east-2"
  region = "us-east-2"
}

provider "aws" {
  alias  = "us-west-1"
  region = "us-west-1"
}

provider "aws" {
  alias  = "us-west-2"
  region = "us-west-2"
}

provider "aws" {
  alias  = "eu-central-1"
  region = "eu-central-1"
}

# CloudTrail Setup in Default Region
module "cloudtrail" {
  source = "./cloudtrail"
}

# CloudWatch Setup per Region
module "us-east-1_cloudwatch" {
  source = "./cloudwatch"
  providers = {
    "aws.region" = "aws.us-east-1"
  }
}

module "us-east-2_cloudwatch" {
  source = "./cloudwatch"
  providers = {
    "aws.region" = "aws.us-east-2"
  }
}

module "us-west-1_cloudwatch" {
  source = "./cloudwatch"
  providers = {
    "aws.region" = "aws.us-west-1"
  }
}

module "us-west-2_cloudwatch" {
  source = "./cloudwatch"
  providers = {
    "aws.region" = "aws.us-west-2"
  }
}

module "eu-central-1_cloudwatch" {
  source = "./cloudwatch"
  providers = {
    "aws.region" = "aws.eu-central-1"
  }
}

cloudwatch/main.tf

provider "aws" {
  alias = "region"
}

# CloudWatch Log Groups
resource "aws_cloudwatch_log_group" "linux" {
  name     = "Linux"
  provider = "aws.region"

  tags {
    OS = "Linux"
  }
}