AWS CloudFormation 的新功能「AWS CloudFormation adds Rollback Triggers Feature」開始可以 Rollback
CloudFormation 原本就能作為 CD (Continuous Delivery) 使用，現在加上可以 rollback trigger 更完整
這個 rollback trigger 一樣是依據 CloudWatch alarm 做的
By default, CloudFormation only rolls back stack operations if an alarm goes to ALERT state, not INSUFFICIENT_DATA state. To have CloudFormation roll back the stack operation if an alarm goes to INSUFFICIENT_DATA state as well, edit the CloudWatch alarm to treat missing data as breaching. For more information, see Configuring How CloudWatch Alarms Treats Missing Data.
預設要 Alarm 才會 rollback，但 INSUFFICIENT_DATA 也能做，但要另外設定。
