能否通过Terraform为Auto Scaling组的同一CloudWatch告警设置多条件？

你需要为Auto Scaling组的GroupMinSize指标设置两个触发条件：大于阈值1、小于阈值1，但CloudWatch单条指标告警不支持直接配置互斥的多条件触发。不过可以通过以下两种方式实现需求：

方式一：创建两条独立的指标告警 #

通过Terraform的count批量生成两条告警，分别对应两个条件：

resource "aws_cloudwatch_metric_alarm" "GroupMinSize" {
  count               = 2
  alarm_name          = "Autoscaling_Group_Min_Size_${count.index == 0 ? "Above" : "Below"}_Threshold"
  comparison_operator = count.index == 0 ? "GreaterThanThreshold" : "LessThanThreshold"
  evaluation_periods  = "2"
  metric_name         = "GroupMinSize"
  namespace           = "AWS/AutoScaling"
  statistic           = "Minimum"
  period              = "120"
  threshold           = 1

  dimensions = {
    AutoScalingGroupName = aws_autoscaling_group.asg[count.index].name
  }

  alarm_description = count.index == 0 ? "Auto Scaling组最小容量超过阈值1" : "Auto Scaling组最小容量低于阈值1"
}

这种方式会生成两条独立告警，当GroupMinSize大于1或小于1时，对应告警会分别触发。

方式二：使用复合告警合并条件 #

先创建两个单条件的基础告警，再通过CloudWatch复合告警将它们合并，任意一个基础告警触发时，复合告警就会触发：

# 基础告警1：最小容量大于1
resource "aws_cloudwatch_metric_alarm" "GroupMinSize_Above" {
  alarm_name          = "Autoscaling_Group_Min_Size_Above_Threshold"
  comparison_operator = "GreaterThanThreshold"
  evaluation_periods  = "2"
  metric_name         = "GroupMinSize"
  namespace           = "AWS/AutoScaling"
  statistic           = "Minimum"
  period              = "120"
  threshold           = 1

  dimensions = {
    AutoScalingGroupName = aws_autoscaling_group.asg[0].name
  }

  alarm_description = "Auto Scaling组最小容量超过阈值1"
}

# 基础告警2：最小容量小于1
resource "aws_cloudwatch_metric_alarm" "GroupMinSize_Below" {
  alarm_name          = "Autoscaling_Group_Min_Size_Below_Threshold"
  comparison_operator = "LessThanThreshold"
  evaluation_periods  = "2"
  metric_name         = "GroupMinSize"
  namespace           = "AWS/AutoScaling"
  statistic           = "Minimum"
  period              = "120"
  threshold           = 1

  dimensions = {
    AutoScalingGroupName = aws_autoscaling_group.asg[0].name
  }

  alarm_description = "Auto Scaling组最小容量低于阈值1"
}

# 复合告警：任意一个基础告警触发时触发
resource "aws_cloudwatch_composite_alarm" "GroupMinSize_Deviation" {
  alarm_name        = "Autoscaling_Group_Min_Size_Deviation"
  alarm_description = "Auto Scaling组最小容量偏离阈值1"
  alarm_rule        = "ALARM(${aws_cloudwatch_metric_alarm.GroupMinSize_Above.alarm_name}) OR ALARM(${aws_cloudwatch_metric_alarm.GroupMinSize_Below.alarm_name})"
}

复合告警适合需要统一接收告警通知的场景，只需配置一次通知规则即可覆盖两个触发条件。

内容的提问来源于stack exchange，提问作者Silvia Rotari