Specifying multiple metrics in EC2 Auto Scaling Group Target Tracking thru Terraform

Question

I'm trying to implement the EC2 Scaling on SQS queue size example from the AWS docs, in Terraform. Using the autoscaling module from terraform-aws-modules. I have successfully created the ASG (auto-scaling group) without the policy.

I tried adding the three metrics specified in the example:

  scaling_policies = {
    scale_on_sqs_queue_policy = {
      policy_type = "TargetTrackingScaling"
      target_tracking_configuration = {
        target_value = 100
        metric_data_queries = {
          expression = "m1 / m2"
          id = "e1"
          label = "Calculate the backlog per instance"
          return_data = true
        }
        customized_metric_specification = {
          metric_dimension = {
            name = "QueueName"
            value = "my-sqs-queue"
          }
          metric_name = "ApproximateNumberOfMessagesVisible"
          namespace = "AWS/SQS"
          statistic = "Sum"
        }
        customized_metric_specification = {
          metric_dimension = {
            name = "AutoScalingGroupName"
            value = "my_autoscaling_group_name"
          }
          metric_name = "GroupInServiceInstances"
          namespace = "AWS/Autoscaling"
          statistic = "Average"
        }
      }
    }
  }

However, only the first customized_metric_specification is added by Terraform:

module.my_autoscaling_group.aws_autoscaling_policy.this["scale_on_sqs_queue_policy"] will be created

resource "aws_autoscaling_policy" "this" {    
    arn                     = (known after apply)    
    autoscaling_group_name  = "my_autoscaling_group_name"    
    enabled                 = true    
    id                      = (known after apply)    
    metric_aggregation_type = (known after apply)    
    name                    = "scale_on_sqs_queue_policy"    
    policy_type             = "TargetTrackingScaling"    

    target_tracking_configuration {
        disable_scale_in = false
        target_value     = 100

        customized_metric_specification {
            metric_name = "GroupInServiceInstances"
            namespace   = "AWS/Autoscaling"
            statistic   = "Average"

            metric_dimension {
                name  = "AutoScalingGroupName"
                value = "my_autoscaling_group_name"
            }
        }
    }
}

Plan: 1 to add, 0 to change, 0 to destroy.

The metric math query is completely ignored too. This is also reflected in the AWS console:

My question is, how do I use multiple metrics when defining a dynamic policy for an EC2 ASG in Terraform? It seems to be in the spec as multiple TargetTrackingMetricDataQuery objects.

I've also tried making customized_metric_specification an array or an object, which results in errors like this:

// [...]
target_tracking_configuration = {
  target_value = 100
  customized_metric_specification = {
    m1 = {
      metric_dimension = {
        name  = "QueueName"
        value = "my-sqs-queue"
      }
      metric_name = "ApproximateNumberOfMessagesVisible"
      namespace   = "AWS/SQS"
      statistic   = "Sum"
    },
    m2 = {
      metric_dimension = {
        name  = "AutoScalingGroupName"
        value = "my_autoscaling_group_name"
      }
      metric_name = "GroupInServiceInstances"
      namespace   = "AWS/Autoscaling"
      statistic   = "Average"
    }
  }
}
// [...]

Error: Unsupported attribute
on .terraform/modules/my_autoscaling_group/main.tf line 723, in resource "aws_autoscaling_policy" "this":
723:
metric_name = customized_metric_specification.value.metric_name
customized_metric_specification.value is object with 2 attributes
This object does not have an attribute named "metric_name".

I consulted these sources before posting:

• The module's GitHub repo - I looked at the code for the block and for any reported issues
• The Terraform docs for the aws_autoscaling_policy docs which seem to allow this
• Searched StackOverflow for "[terraform] multiple metrics autoscaling"
• This question, however I haven't misspelt the key(s)

Answer 1

Update: This functionality is available in v4.57.0 of the Terraform AWS Provider:

resource/aws_autoscaling_policy: Add metrics to the target_tracking_configuration.customized_metric_specification configuration block in support of metric math

(source)

See docs for customized_metric_specification > metrics > expression

---

This functionality isn't supported in the underlying Terraform resources yet, which are made for Application Auto-scaling, not EC2 ASG. It's proposed to be added in this PR however.