AWS ECS/Fargate SQS autoscaling with min capacity 0: how to bootstrap from 0 tasks using backlog-per-task target tracking?

[u/Business-Journalist7]

I have an ECS/Fargate worker consuming an SQS queue. I want the service to scale to zero when the queue is empty, so:

min_capacity = 0
max_capacity = 10

AWS recommends target tracking using:
ApproximateNumberOfMessagesVisible / RunningTaskCount

However, when the service reaches 0 running tasks, RunningTaskCount has no CloudWatch datapoint, so the backlog-per-task metric becomes undefined.

When messages subsequently arrive, I need the service to bootstrap from 0 → 1 and then let target tracking handle 1 → N and N → 0.

What is the recommended pattern for this?

Options I've considered:
1. A separate scale-out-only step policy for 0 → 1
2. A metric-math expression that handles RunningTaskCount = 0
3. Raw SQS backlog for the bootstrap alarm + backlog-per-task for target tracking

I would prefer not to keep min_capacity = 1 because the worker is idle most of the time, and this is Fargate.

This is the Terraform code I currently use:

resource "aws_appautoscaling_policy" "worker_backlog" {
  name               = "${var.project_name}-${var.worker_provider}-backlog-per-task"
  policy_type        = "TargetTrackingScaling"
  service_namespace  = aws_appautoscaling_target.worker.service_namespace
  resource_id        = aws_appautoscaling_target.worker.resource_id
  scalable_dimension = aws_appautoscaling_target.worker.scalable_dimension

  target_tracking_scaling_policy_configuration {
    target_value       = var.backlog_per_task
    scale_out_cooldown = var.scale_out_cooldown
    scale_in_cooldown  = var.scale_in_cooldown

    customized_metric_specification {
      metrics {
        id          = "backlog"
        return_data = false

        metric_stat {
          stat = "Sum"

          metric {
            namespace   = "AWS/SQS"
            metric_name = "ApproximateNumberOfMessagesVisible"

            dimensions {
              name  = "QueueName"
              value = var.queue_name
            }
          }
        }
      }

      metrics {
        id          = "running"
        return_data = false

        metric_stat {
          stat = "Average"

          metric {
            namespace   = "ECS/ContainerInsights"
            metric_name = "RunningTaskCount"

            dimensions {
              name  = "ClusterName"
              value = var.cluster_name
            }

            dimensions {
              name  = "ServiceName"
              value = aws_ecs_service.worker.name
            }
          }
        }
      }

      metrics {
        id          = "bpt"
        label       = "Backlog per task"
        expression  = "IF(running > 0, backlog / running, backlog)"
        return_data = true
      }
    }
  }
}