AWS Application Auto-scaling for ECS with Terraform

Update: Target tracking scaling is now available for ECS services.

I’ve been working on setting up autoscaling settings for ECS services recently, and here are a couple notes from managing auto-scaling for ECS services using Terraform.

Creating multiple scheduled actions at once


Terraform will perform the following actions:

  + aws_appautoscaling_scheduled_action.green_evening
      id:                                    
      arn:                                   
      name:                                  "ecs"
      resource_id:                           "service/default-production/green"
      scalable_dimension:                    "ecs:service:DesiredCount"
      scalable_target_action.#:              "1"
      scalable_target_action.0.max_capacity: "20"
      scalable_target_action.0.min_capacity: "2"
      schedule:                              "cron(0 15 * * ? *)"
      service_namespace:                     "ecs"

  + aws_appautoscaling_scheduled_action.wapi_green_morning
      id:                                    
      arn:                                   
      name:                                  "ecs"
      resource_id:                           "service/default-production/green"
      scalable_dimension:                    "ecs:service:DesiredCount"
      scalable_target_action.#:              "1"
      scalable_target_action.0.max_capacity: "20"
      scalable_target_action.0.min_capacity: "3"
      schedule:                              "cron(0 23 * * ? *)"
      service_namespace:                     "ecs"

This fails with:


* aws_appautoscaling_scheduled_action.green_evening: ConcurrentUpdateException: You already have a pending update to an Auto Scaling resource.

To fix, the scheduled actions need to be executed serially.


resource "aws_appautoscaling_scheduled_action" "green_morning" {
  name               = "ecs"
  service_namespace  = "${module.green-autoscaling.service_namespace}"
  resource_id        = "${module.green-autoscaling.resource_id}"
  scalable_dimension = "${module.green-autoscaling.scalable_dimension}"
  schedule           = "cron(0 23 * * ? *)"

  scalable_target_action {
    min_capacity = 3
    max_capacity = 20
  }
}

resource "aws_appautoscaling_scheduled_action" "green_evening" {
  name               = "ecs"
  service_namespace  = "${module.green-autoscaling.service_namespace}"
  resource_id        = "${module.green-autoscaling.resource_id}"
  scalable_dimension = "${module.green-autoscaling.scalable_dimension}"
  schedule           = "cron(0 15 * * ? *)"

  scalable_target_action {
    min_capacity = 2
    max_capacity = 20
  }

  # Application AutoScaling actions need to be executed serially
  depends_on = ["aws_appautoscaling_scheduled_action.green_morning"]
}

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