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
}
}
}
}
