弹性不是「加机器」，是让容量跟着业务曲线走

resource "aws_autoscaling_group" "web" {
  name                = "web-asg"
  vpc_zone_identifier = var.private_subnet_ids
  min_size            = 4     # 基线容量，由 Savings Plans 覆盖
  max_size            = 40
  desired_capacity    = 6

  health_check_type         = "ELB"
  health_check_grace_period = 90
  default_instance_warmup   = 60

  mixed_instances_policy {
    instances_distribution {
      # 基线 4 台走按需，超出部分 75% 用 Spot
      on_demand_base_capacity                  = 4
      on_demand_percentage_above_base_capacity = 25
      spot_allocation_strategy                 = "price-capacity-optimized"
      spot_max_price                           = "  # 留空表示不超过按需价
    }

    launch_template {
      launch_template_specification {
        launch_template_id = aws_launch_template.web.id
        version            = "$Latest"
      }
      # 多类型分散中断风险，注意保持相近的 vCPU/内存比
      override { instance_type = "m7i.large" }
      override { instance_type = "m7a.large" }
      override { instance_type = "m6i.large" }
      override { instance_type = "m6a.large" }
      override { instance_type = "m5.large"  }
    }
  }

  # 优雅退出：Spot 中断通知触发连接排空
  capacity_rebalance = true

  tag {
    key                 = "CostCenter"
    value               = "web-frontend"
    propagate_at_launch = true
  }
}

resource "aws_autoscaling_policy" "target_tracking" {
  name                   = "web-req-per-target"
  autoscaling_group_name = aws_autoscaling_group.web.name
  policy_type            = "TargetTrackingScaling"

  target_tracking_configuration {
    predefined_metric_specification {
      predefined_metric_type = "ALBRequestCountPerTarget"
      resource_label         = "${var.alb_arn_suffix}/${var.tg_arn_suffix}"
    }
    target_value = 800   # 由压测拐点反推，而非拍脑袋
  }
}